Abstract:
The virtual machine system according to the present invention comprises: physical processors which each include an interrupt controller including an interrupt timer; and a virtualization unit which allocates computer resources including the physical processors and physical memories to a plurality of virtual machines. The virtualization unit allocates, to the plurality of virtual machines, virtual processors that are created by virtualizing the physical processors, sets one of the physical processors into either a shared processor mode or an exclusive processor mode, and then sets the interrupt timer of the physical processor that has been set into either the shared processor mode or the exclusive processor mode, into either a shared timer mode or an exclusive timer mode.

Description:
BACKGROUND 
       [0001]    This invention relates to a virtual machine system configured to exclusively dynamically allocate an interrupt timer of a physical CPU to a virtual CPU. 
         [0002]    There have been increases in the number of CPU sockets mounted to a computer and the number of CPU cores for each of the CPU sockets. A virtual machine system is available as one of methods of effectively using a computer with such an increased number of CPU cores. The virtual machine system is a platform that enables a virtual machine monitor (or hypervisor) to build a plurality of virtual machines through use of resources included in a physical computer, and enables an operating system to operate independently on each of the virtual machines. Employment of the virtual machine system has become widespread because of advantages that the resources of the physical computer can be effectively utilized, the management and maintenance of software can be facilitated, and the like. A program operating on the virtual machine is specifically referred to as “guest”, and the virtual machine system (for example, virtual machine monitor) is specifically referred to as “host”. 
         [0003]    In the virtual machine system, the virtual machine monitor has a shared CPU mode and an exclusive CPU mode as a scheduling mode for the physical CPU. When a small number of physical CPUs of the physical computer were used, it was often the case that the shared CPU mode that allows time-sharing of the physical CPUs by virtual CPUs was used. However, as the number of physical CPUs of the physical computer or the number of CPU cores increases, the exclusive CPU mode that allows each virtual CPU to occupy one specific physical CPU core has become more widespread. The exclusive CPU mode has begun to be used for applications having a strict latency (processing latency) requirement, specifically, an in-memory database, real-time control, and the like. 
         [0004]    Those applications frequently use an interrupt timer of a CPU. The interrupt timer of a CPU herein represents a timer included in an interrupt controller built into a CPU motherboard. The timer includes a remaining-time counter configured to decrement a value at a fixed frequency, and has an interval timer function of causing the remaining-time counter to start the decrementing when a value is written to the remaining-time counter and causing the interrupt controller to generate a timer interrupt in the CPU to which the interrupt controller belongs when the value becomes zero and to stop the operation until the next value is written to the remaining-time counter. 
         [0005]    In JP 2008-171293 A, there is described a basic method of virtually achieving a timer for a virtual machine through sharing of a timer of a physical computer in a virtual machine system. 
         [0006]    In U.S. Pat. No. 7,475,002 B2 and U.S. Pat. No. 8,533,709 B2, there are disclosed methods of virtualizing a timer for catching up with an actual time when a timer of a virtual machine lags behind an actual time in a virtual machine system. 
         [0007]    In U.S. Pat. No. 7,707,578 B2, there is disclosed a method of scheduling context to a specific CPU based on how a memory or a cache is accessed. 
         [0008]    In U.S. Pat. No. 8,489,699 B2, there is disclosed a method of migrating an I/O exclusively allocated to a virtual machine to be migrated when the virtual machine is migrated between physical computers. 
       SUMMARY 
       [0009]    However, in such related-art virtual machine systems as described in JP 2008-171293 A and disclosed in U.S. Pat. No. 7,475,002 B2, U.S. Pat. No. 8,533,709 B2, U.S. Pat. No. 7,707,578 B2, and U.S. Pat. No. 8,489,699 B2, an interrupt timer of a virtual CPU of a virtual machine is emulated through use of an interrupt timer of a physical CPU, but the overhead of the emulation is large, which may affect the operation of the above-mentioned applications. 
         [0010]    In other words, according to the above-mentioned related-art virtual machine systems, when a specific physical CPU is occupied to a given virtual CPU, the virtual CPU is allowed to use a hardware resource included in the physical CPU, but the interrupt timer of the physical CPU is not occupied. 
         [0011]    Therefore, the interrupt timer of the virtual CPU is emulated through the use of the interrupt timer of the physical CPU, but the overhead of the emulation is large, which may affect an application operating on the virtual CPU. 
         [0012]    The interrupt timer of the physical CPU is also used to, for example, activate host processing, detect a timeout, and serve as a clock tick timer of a fixed cycle period irrespective of whether or not an application is operating. The interrupt timer not only consumes CPU time but also causes data eviction from a translation lookaside buffer (TLB) or a cache, which may lead to deterioration in performance with the above-mentioned applications having a strict requirement in regard to the TLB or the cache. 
         [0013]    Loads on the applications operating on the virtual machine dynamically change, which necessitates dynamic switching between the exclusive CPU mode and the shared CPU mode. However, there is a problem in that, in regard to the interrupt timer of the physical CPU, it is not possible to dynamically cause the interrupt timer of one specific virtual CPU to start or end the occupancy without erroneously executing each function that uses the interrupt timer of the physical CPU. 
         [0014]    Therefore, this invention has an object to control occupancy of an interrupt timer of a physical CPU by an interrupt timer of a virtual CPU in a virtual machine system in conjunction with dynamic switching of a scheduling mode for the physical CPU. 
         [0015]    A representative aspect of the present disclosure is as follows. A method of controlling a virtual machine system, the virtual machine system comprising: an interrupt controller comprising an interrupt timer; a physical processor comprising the interrupt controller; a physical computer comprising the physical processor and a physical memory; and a virtualization module configured to allocate a computer resource of the physical computer to a plurality of virtual machines, the virtualization module being configured to control allocation of the physical processor, the method comprising: a first step of generating, by the virtualization module, the plurality of virtual machines by generating: a virtual processor comprising a virtual interrupt timer obtained by virtualizing the interrupt timer and a virtual interrupt controller obtained by virtualizing the interrupt controller, to which the computer resource of the physical processor is allocated; and a virtual memory to which the computer resource of the physical memory is allocated; a second step of setting, by the virtualization module, a processor scheduling mode to any one of: a shared processor mode in which one of the physical processors allocated to the plurality of virtual machines is shared among a plurality of the virtual processors; and an exclusive processor mode in which one of the physical processors allocated to the plurality of virtual machines is occupied by a specific virtual processor; and a third step of setting, by the virtualization module, in conjunction with the processor scheduling mode, a timer scheduling mode to any one of: a shared timer mode in which the interrupt timer of the physical processor for which either one of the shared processor mode or the exclusive processor mode is set is shared among timer events for the plurality of virtual machines; and an exclusive timer mode in which the interrupt timer is occupied by a virtual timer of the specific virtual processor. 
         [0016]    According to this invention, when a virtual CPU occupies a specific physical CPU, the interrupt timer of the virtual CPU is caused to occupy the interrupt timer of the specific physical CPU in conjunction with the occupancy of the virtual CPU. With this configuration, the overhead of the emulation of the interrupt timer of the virtual CPU can be minimized, and data eviction from a TLB or a cache by a timer function for a host, which has occurred irrespective of whether or not a program on the virtual CPU is operating, can be suppressed to ensure the performance of the physical CPU. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a block diagram for illustrating an example of a virtual machine system to which this invention is applied according to an embodiment of this invention. 
           [0018]      FIG. 2  is a block diagram for illustrating a detailed configuration relating to the scheduler and the timer management module according to the embodiment of this invention. 
           [0019]      FIG. 3A  is a diagram for illustrating a state of the scheduling conducted when all the physical CPUs operate in the shared CPU mode according to the embodiment of this invention. 
           [0020]      FIG. 3B  is a diagram for illustrating a state of the scheduling conducted when the physical CPU  0  and the physical CPU  1  operate in the exclusive CPU mode according to the embodiment of this invention. 
           [0021]      FIG. 4A  is a diagram for illustrating the state of the timer management module  120  and the respective timers in the shared timer mode according to the embodiment of this invention. 
           [0022]      FIG. 4B  is a diagram for illustrating the state of the timer management module and the respective timers in the shared timer mode and the exclusive timer mode according to the embodiment of this invention. 
           [0023]      FIG. 5  is a diagram for illustrating a state of the scheduler and the timer management module according to the embodiment of this invention. 
           [0024]      FIG. 6  is a graph for showing a relationship between the expiration time of the timer event and the remaining-time counter of the interrupt timer of the physical CPU in the shared timer mode according to the embodiment of this invention. 
           [0025]      FIG. 7  is a graph for showing a relationship between the expiration time of the timer event and the remaining-time counter of the interrupt timer of the physical CPU in the shared timer mode according to the embodiment of this invention. 
           [0026]      FIG. 8A  is a flowchart for illustrating an example of processing for updating the remaining-time counter of the interrupt timer of the virtual CPU on the physical CPU in the shared timer mode according to the embodiment of this invention. 
           [0027]      FIG. 8B  is the first half of the flowchart for illustrating an example of the processing for delivering the timer interrupt of the virtual CPU on the physical CPU in the shared timer mode according to the embodiment of this invention. 
           [0028]      FIG. 8C  is the second half of the flowchart for illustrating an example of the processing for delivering the timer interrupt of the virtual CPU on the physical CPU in the shared timer mode according to the embodiment of this invention. 
           [0029]      FIG. 9A  is a flowchart for illustrating an example of the processing for updating the remaining-time counter of the interrupt timer of the virtual CPU on the physical CPU in the exclusive timer mode according to the embodiment of this invention. 
           [0030]      FIG. 9B  is a flowchart for illustrating an example of the processing for delivering the timer interrupt of the interrupt timer of the virtual CPU on the physical CPU  10   a  in the exclusive timer mode according to the embodiment of this invention. 
           [0031]      FIG. 10A  is the first half of the flowchart for illustrating an example of the processing for dynamically changing the timer scheduling mode for the interrupt timer of the physical CPU according to the embodiment of this invention. 
           [0032]      FIG. 10B  is the second half of the flowchart for illustrating an example of the processing for dynamically changing the timer scheduling mode for the interrupt timer of the physical CPU according to the embodiment of this invention. 
           [0033]      FIG. 10C  is a flowchart for illustrating the processing for dynamically changing the timer scheduling mode for the interrupt timer of the physical CPU. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0034]    Now, one embodiment to which this invention is applied is described in detail with reference to the accompanying drawings. 
         [0035]      FIG. 1  is a block diagram for illustrating an example of a virtual machine system to which this invention is applied. The virtual machine system includes a physical computer  1 , a virtual machine monitor  100 , and virtual machines  200   a  to  200   c  ( 1  to  3 ). In the following description, a generic virtual machine is represented by reference symbol  200  without an alphabetical suffix, while individual virtual machines are distinguished by reference symbols  200   a  to  200   c  with alphabetical suffixes. The same applies to other components, that is, the type of component is identified by a reference symbol without an alphabetical suffix, while individual components are distinguished by reference symbols with alphabetical suffixes. 
         [0036]    The physical computer  1  includes physical CPUs  10   a  to  10   d  ( 0  to  3 ), a memory  50 , an I/O interrupt controller  60 , at least one I/O device  70 , and a system time  80 . The physical CPUs  10   a  to  10   d  ( 0  to  3 ) may be formed of processor cores of a multicore processor. 
         [0037]    The physical CPUs  10   a  to  10   d  ( 0  to  3 ) respectively include interrupt controllers  30   a  to  30   d  built into a CPU motherboard. The interrupt controller  30   a  provides a function of receiving an external interrupt from a generation source of the interrupt and delivering the external interrupt to the physical CPU  0  ( 10   a ), and a function of delivering and receiving an intra-processor interrupt to/from another physical CPU  10  in a computer system including a plurality of physical CPUs  10 . The interrupt controller  30   a  also includes an interrupt timer  40   a . The interrupt controller  30   a  is implemented in the physical CPU  10   a  as, for example, a local advanced programmable interrupt controller (APIC). The interrupt timer  40   a  is implemented in the interrupt controller  30   a  as, for example, a local APIC timer. 
         [0038]    The physical CPU  0  ( 10   a ) includes an external interrupt mask  20   a . The external interrupt mask  20   a  provides a function of being closed to ignore the external interrupt delivered to the physical CPU  0  ( 10   a ) while being held. A program, for example, an operating system, uses an external interrupt mask between the start and the end of processing that cannot guarantee a correct operation when an external interrupt occurs during the execution of the program. The same applies to the other physical CPUs  1  to  3  ( 10   b  to  10   d ), that is, the physical CPUs  1  to  3  ( 10   b  to  10   d ) each include the interrupt controller  30 , the interrupt timer  40 , and the external interrupt mask  20 . 
         [0039]    The physical CPU includes a CPU virtualization assisting function as standard, but in this invention, it is irrelevant whether or not the physical CPU includes the CPU virtualization assisting function, and hence the CPU virtualization assisting function is omitted in  FIG. 1  for the sake of brevity of description. 
         [0040]    The memory  50  is used as a volatile storage device configured to store a program and data. 
         [0041]    The I/O interrupt controller  60  provides a function of delivering a command completion notification, which is issued from the I/O device  70  to the physical CPU  10 , or the like to the interrupt controller  30  of the physical CPU  10  as the external interrupt. 
         [0042]    The I/O device  70  includes a disk controller configured to operate a disk drive and a network interface configured to communicate to/from an external computer system through a network. 
         [0043]    The system time  80  provides a common time to be shared among all the physical CPUs  10  and to be used by the virtual machine monitor  100  to acquire a current time and an elapsed time period. As the system time  80 , a real time clock (RTC), a high precision event timer (HPET), or the like can be used. 
         [0044]    The virtual machine monitor  100  is configured to control computer resources included in the physical computer  1  to generate at least one virtual machine  200  in which a program typified by a general-purpose operating system provides a program execution environment to be recognized as one computer. The virtual machine monitor  100  is a control program for providing a role of operating a program on the virtual machine  200  in a manner equivalent to the physical computer  1  except for slight performance degradation and a slight difference in timing. The virtual machine monitor  100  includes a scheduler  110  and a timer management module  120 . A virtualization module, for example, a hypervisor, may be employed in place of the virtual machine monitor  100 . 
         [0045]    The virtual machine monitor  100  controls the computer resources included in the physical computer  1  to build the virtual machines  200   a  to  200   c  ( 1  to  3 ) each including a virtual CPU  0  ( 210   a ), a virtual CPU  1  ( 210   b ), a virtual memory  250 , a virtual I/O device  260 , and a virtual I/O interrupt controller  270 . 
         [0046]    The virtual CPU  0  ( 210   a ) includes an interrupt controller  220   a  obtained by virtualizing the interrupt controller  30   a . The interrupt controller (virtual interrupt controller)  220   a  includes an interrupt timer (virtual interrupt timer)  230   a  obtained by virtualizing the interrupt timer  40   a  in the same manner as the interrupt controller  30  of the physical CPU  10 . The virtual CPU  1  ( 210   b ) has the same configuration. 
         [0047]    A general-purpose operating system  201  operates on each of the virtual machines  200   a  to  200   c . The general-purpose operating system  201  is specifically referred to as “guest operating system  201 ”. 
         [0048]    The configurations of the virtual machine  2  ( 200   b ) and the virtual machine  3  ( 200   c ) are the same as the above-mentioned configuration. 
         [0049]    The virtual machine monitor  100  includes processes dedicated to conducting pieces of processing for controlling the virtual machine system, for example, terminal control and virtual machine system management. Those pieces of processing are each specifically referred to as “host processing”, and a process dedicated to providing the host processing is referred to as “host process”. In the embodiment, the virtual machine monitor  100  operates four host processes in total, which include a host process  1  ( 150   a ), a host process  2  ( 150   b ), a host process  3  ( 150   c ), and a host process  4  ( 150   d ). 
         [0050]    The respective functional modules of the scheduler  110  and the timer management module  120  of the virtual machine monitor  100  are loaded into the memory  50  as programs. 
         [0051]    The CPU  10  is configured to run as a functional module for providing a predetermined function by performing processing in accordance with the program of each functional module. For example, the CPU  10  is configured to function as the timer management module  120  by performing processing in accordance with a timer management program. The same also applies for the other programs. The CPU  10  is also configured to run as a functional module for providing each function of the plurality of processes executed by the various programs. A computer and a computer system are an apparatus and a system including those functional modules, respectively. 
         [0052]    Programs, tables, and other information for achieving the individual functions of the virtual machine monitor  100  may be stored in a storage subsystem, in a storage device such as a nonvolatile semiconductor memory, a hard disk drive, or a solid state drive (SSD), or in a non-transitory computer readable data storage medium, such as an IC card, an SD card, or a DVD. 
         [0053]      FIG. 2  is a block diagram for illustrating a detailed configuration relating to the scheduler  110  and the timer management module  120  according to the embodiment. In  FIG. 2 , only the physical CPU  0  ( 10   a ), the host process  1  ( 150   a ), the virtual CPU  0  ( 210   a ) of the virtual machine  1  ( 200   a ), and components directly involved with those components are extracted. The same configuration of  FIG. 2  applies to another physical CPU  10 , another virtual CPU  210 , and another host process  150 . 
         [0054]    The interrupt timer  40   a  of the physical CPU  0  ( 10   a ) includes a timer frequency  42   a  and a remaining-time counter  41   a  configured to decrement a set value at a cycle period corresponding to the frequency  42   a . When a value is written to the remaining-time counter  41   a , the remaining-time counter  41   a  starts the decrementing. When the value becomes zero, the interrupt controller  30   a  generates a timer interrupt in the physical CPU  0  ( 10   a ). After generating the timer interrupt, the interrupt controller  30   a  including the remaining-time counter  41   a  stops the operation until the next value is written to the remaining-time counter  41   a . Such a function is referred to as “interval timer function”. 
         [0055]    The system time  80  includes a system time frequency  82  and a system time counter  81  configured to monotonously increment a value at the cycle period of the system time frequency  82 . The value read out from the system time counter  81  does not necessarily have an hour-minute-second format. 
         [0056]    When the scheduler  110  of the virtual machine monitor  100  allocates the virtual CPU  210  to any one of the physical CPUs  10 , the program on the relevant virtual CPU  210  is executed. When the scheduler  110  allocates the host process  150  to any one of the physical CPUs  10 , the program on the relevant host process  150  is executed. 
         [0057]    The scheduler  110  is provided with a CPU scheduling mode  111   a  for setting a binary value of an exclusive CPU mode or a shared CPU mode for the physical CPU  10   a . A run queue  112   a  or the like for executing the host process  150  is also set in the scheduler  110 . The CPU scheduling mode  111   a  can be provided for each of the physical CPUs  10   a  to  10   d , and as described later with reference to  FIG. 3A , the CPU scheduling mode  111   a  and CPU scheduling modes  111   b  to  111   d  are provided for the physical CPUs  10   a  to  10   d , respectively. The run queue  112   a  can also be provided for each of the physical CPUs  10   a  to  10   d.    
         [0058]    The exclusive CPU mode is a mode in which one virtual CPU  210  occupies one physical CPU  10 , and the shared CPU mode is a mode in which a plurality of virtual CPUs  210  share one physical CPU  10 . 
         [0059]    The scheduler  110  also includes a timer queue  122   a  configured to hold a timer event for the physical CPU  10   a . The timer queue can be provided for each of the physical CPUs  10   a  to  10   d , and as described later with reference to  FIG. 4A , the timer queue  122   a  and timer queues  122   b  to  122   d  are set for the physical CPUs  10   a  to  10   d , respectively. 
         [0060]    The timer management module  120  of the virtual machine monitor  100  provides roles of, through use of the interrupt timer  40   a  of the physical CPU  10   a , virtually realizing the interrupt timer  230   a  of the virtual CPU  210   a , dispatching the host process  150  at a designated time, detecting a timeout of the program in operation, measuring a CPU usage amount of each piece of processing, and updating statistical information. 
         [0061]    In order to achieve each of the above-mentioned functions, the timer management module  120  provides each of the components with a data structure relating to the timer. 
         [0062]    The timer management module  120  provides the interrupt timer  40   a  of the physical CPU  0  ( 10   a ) with a timer scheduling mode  121   a  formed of a binary value of an exclusive timer mode or a shared timer mode. One timer scheduling mode  121   a  can be provided to each of the physical CPUs  10   a  to  10   d , and as described later with reference to  FIG. 4A , the timer scheduling mode  121   a  and timer scheduling modes  121   b  to  121   d  are provided for the physical CPUs  10   a  to  10   d , respectively. 
         [0063]    The exclusive timer mode is a mode in which the interrupt timer  230  of one virtual CPU  210  occupies the interrupt timer  40  of one physical CPU  10 . The shared timer mode is a mode in which the timer events of a plurality of the virtual CPUs  210  share the interrupt timer  40  of one physical CPU  10 . 
         [0064]    In order to time-divisionally share the physical CPU  0  ( 10   a ) among the plurality of virtual CPUs  210 , the timer management module  120  sets a timer event  130   a  for a clock tick timer configured to generate a timer interrupt at a fixed cycle period. The timer event  130   a  for the clock tick timer includes a start time  131   a  using the system time  80  as a time axis, an expiration time  132   a , a cycle period  133   a , and an enabled or disabled status  134   a . The clock tick timer may be a system timer. The timer event  130  for the clock tick timer is set for each of the physical CPUs  10 . For example, as described later with reference to  FIG. 4A , the timer event  130   a  for the clock tick timer and timer events  130   b  to  130   d  for the clock tick timer can be provided. 
         [0065]    In order to activate the host process  1  ( 150   a ) at the designated time and detect a timeout of the relevant host process  1  ( 150   a ), the timer management module  120  associates a timer event  160   a  with the host process  1  ( 150   a ). The timer event  160   a  includes an expiration time  161   a  using the system time  80  as the time axis and an enabled or disabled status  162   a.    
         [0066]    The interrupt timer  230   a  of the virtual CPU  0  ( 210   a ) of the virtual machine  1  ( 200   a ) includes a remaining-time counter  231   a  and a timer frequency  232   a  in the same manner as the interrupt timer  40   a  of the physical CPU  0  ( 10   a ). 
         [0067]    In order to assist the emulation of the interrupt timer  230   a , the timer management module  120  associates a timer event  240   a  with the virtual CPU  0  ( 210   a ). The timer event  240   a  includes a remaining-time counter update time  241   a  using the system time  80  as the time axis, an expiration time  242   a , and an enabled or disabled status  243   a.    
         [0068]    In this embodiment, in order to simplify the description, the frequency of the interrupt timer  40   a  of the physical CPU  10   a  and the frequency of the interrupt timer  230   a  of the virtual CPU  0  ( 210   a ) are set equal to each other, but may be different from each other. 
         [0069]    The CPU scheduling mode  111  for the physical CPU  10  is described in detail. When a given virtual CPU  210  occupies a specific physical CPU  10 , the relevant physical CPU  10  operates in the exclusive CPU mode, and the physical CPU  10  that is not occupied by one specific virtual CPU  210  operates in the shared CPU mode. 
         [0070]      FIG. 3A  is a diagram for illustrating a state of the scheduling conducted when all the physical CPUs operate in the shared CPU mode.  FIG. 3B  is a diagram for illustrating a state of the scheduling conducted when the physical CPU  0  ( 10   a ) and the physical CPU  1  ( 10   b ) operate in the exclusive CPU mode. 
         [0071]    In  FIG. 3A  and  FIG. 3B , an arbitrary virtual CPU  210  and an arbitrary host process  150  can operate on the physical CPUs  10  in the shared CPU mode. The scheduling of the physical CPUs  10  in the shared CPU mode is basically classified into scheduling conducted independently for each of the physical CPUs  10  and scheduling based on load balancing corresponding to the situation of loads. 
         [0072]    The scheduling conducted independently for each of the physical CPUs  10  is enabled by associating the virtual CPU  210  or the host process  150  with the run queue  112  for one physical CPU  10  at one point in time. 
         [0073]    Meanwhile, the load balancing is conducted by dynamically changing the association between the run queue  112  for each physical CPU  10  and the virtual CPU  210  or the host process  150 . As a method for the load balancing, any publicly known or well-known method may be employed, and therefore the method is not described in detail herein. 
         [0074]    In  FIG. 3A , for example, the physical CPU  0  ( 10   a ) is time-divisionally shared among the host process  1  ( 150   a ), the virtual CPU  0  ( 210   a ) of the virtual machine  1  ( 200   a ), and the virtual CPU  0  ( 210   a ) of the virtual machine  2  ( 200   b ). The same applies to the physical CPU  1  ( 10   b ), the physical CPU  2  ( 10   c ), and the physical CPU  3  ( 10   d ). 
         [0075]    On the physical CPU  10  in the exclusive CPU mode, only the virtual CPU  210  occupying the relevant physical CPU  10  is allowed to operate. The virtual CPU  210  and the host process  150  that were operating when the relevant physical CPU  10  was in the shared CPU mode are migrated to another physical CPU  10  in the shared CPU mode when the relevant physical CPU  10  starts the exclusive CPU mode. 
         [0076]    In  FIG. 3B , for example, the physical CPU  0  ( 10   a ) is occupied by the virtual CPU  0  ( 210   a ) of the virtual machine  1  ( 200   a ). Both the virtual CPU  0  ( 210   a ) of the virtual machine  2  ( 200   b ) and the host process  1  ( 150   a ), which were operating on the physical CPU  0  ( 10   a ) in  FIG. 3A , are migrated to and operate on the physical CPU  2  ( 10   c ) in the shared CPU mode. 
         [0077]    As the switching of the CPU scheduling mode  111   a  for the physical CPU  10 , the switching from the shared CPU mode to the exclusive CPU mode or the switching from the exclusive CPU mode to the shared CPU mode can both be conducted dynamically. 
         [0078]    When the switching is conducted from  FIG. 3A  to  FIG. 3B , specifically, when the physical CPU  0  ( 10   a ) and the physical CPU  1  ( 10   b ) are dynamically switched from the shared CPU mode to the exclusive CPU mode, the following processing is conducted. 
         [0079]    When the physical CPU  0  ( 10   a ) is taken as an example, the virtual machine monitor  100  migrates both the virtual CPU  0  ( 210   a ) of the virtual machine  2  ( 200   b ) and the host process  1  ( 150   a ), which were operating on the physical CPU  0  ( 10   a ), to the physical CPU  2  ( 10   c ). 
         [0080]    Then, the virtual machine monitor  100  changes the CPU scheduling mode  111   a  for the physical CPU  0  ( 10   a ) to the exclusive CPU mode, and starts the occupancy by the virtual CPU  0  ( 210   a ) of the virtual machine  1  ( 200   a ). The same applies to the physical CPU  1  ( 10   b ). 
         [0081]    When the switching is conducted from  FIG. 3B  to  FIG. 3A , specifically, when the physical CPU  0  ( 10   a ) and the physical CPU  1  ( 10   b ) are dynamically switched from the exclusive CPU mode to the shared CPU mode, the following processing is conducted. 
         [0082]    When the physical CPU  0  ( 10   a ) is taken as an example, the virtual machine monitor  100  first switches the physical CPU  0  ( 10   a ) from the exclusive CPU mode to the shared CPU mode. Then, the virtual machine monitor  100  detects that the load on the physical CPU  0  ( 10   a ) is lower than the loads on the physical CPU  2  ( 10   c ) and the physical CPU  3  ( 10   d ). 
         [0083]    The virtual machine monitor  100  migrates the virtual CPU  0  ( 210   a ) of the virtual machine  2  ( 200   b ) and the host process  1  ( 150   a ) from the physical CPU  2  ( 10   c ) to the physical CPU  0  ( 10   a ) based on the load balancing. The patterns of migrating the virtual CPU  210  and the host process  150  in conjunction with the dynamic switching of the CPU scheduling mode  111   a  for the physical CPU  10   a  depend on occasions, and are not determined in advance. The same applies to the physical CPU  1  ( 10   b ). 
         [0084]    The virtual machine monitor  100  sets the timer scheduling mode  121   a  for the interrupt timer  40  of each physical CPU  10  in conjunction with the corresponding CPU scheduling mode  111   a  for the physical CPU  10 . 
         [0085]    For example, the virtual machine monitor  100  sets the timer scheduling mode  121   a  for the interrupt timer  230   a  for the physical CPU  0  ( 10   a ) in conjunction with the CPU scheduling mode  111   a  for the physical CPU  0  ( 10   a ). 
         [0086]    The virtual machine monitor  100  sets the timer scheduling mode  121   a  for the interrupt timer  40   a  to the shared timer mode when the physical CPU  0  ( 10   a ) is in the shared CPU mode, and sets the timer scheduling mode  121   a  for the interrupt timer  40   a  to the exclusive timer mode when the physical CPU  0  ( 10   a ) is in the exclusive CPU mode. 
         [0087]    In other words, the timer scheduling mode  121   a  for the interrupt timer  40   a  can be dynamically switched. The above-mentioned switching can be conducted when the timer management module  120  of the virtual machine monitor  100  receives a scheduling mode switching request. 
         [0088]    The virtual machine monitor  100  sets a scheduling mode switching request  123   a  for the physical CPU  0  ( 10   a ) in order to dynamically switch the CPU scheduling mode  111  and the timer scheduling mode  121 . The timer management module  120  sets the received scheduling switching request in the scheduling mode switching request  123   a . In the same manner, scheduling mode switching requests  123   b  to  123   d  are set for the physical CPUs  1  to  3  ( 10   b  to  10   d ), respectively. 
         [0089]      FIG. 4A  and  FIG. 4B  are conceptual diagrams of the states of timers exhibited when the states of the scheduling of the physical CPUs  10  are those illustrated in  FIG. 3A  and  FIG. 3B , respectively.  FIG. 4A  is a diagram for illustrating the state of the timer management module  120  and the respective timers in the shared timer mode.  FIG. 4B  is a diagram for illustrating the state of the timer management module  120  and the respective timers in the shared timer mode and the exclusive timer mode. 
         [0090]      FIG. 4A  is an illustration of the state in which the interrupt timers  40  of all the physical CPUs  10  are in the shared timer mode.  FIG. 4B  is an illustration of the state in which the interrupt timer  40   a  of the physical CPU  0  ( 10   a ) and the interrupt timer  40   b  of the physical CPU  1  ( 10   b ) are in the exclusive timer mode and the interrupt timer  40   c  of the physical CPU  2  ( 10   c ) and the interrupt timer  40   d  of the physical CPU  3  ( 10   d ) are in the shared timer mode. 
         [0091]    On the physical CPU  10  having the interrupt timer  40  in the shared timer mode, the interrupt timer  40  of the relevant physical CPU  10  is shared among the timer event  240  for the interrupt timer  230  of the virtual CPU  210 , the timer event  160  of the host process  150 , and the timer event  130  for the clock tick timer of the relevant physical CPU  10 , which share the relevant physical CPU  10 . 
         [0092]    In  FIG. 4A , for example, the timer event  240   a  for the virtual CPU  0  ( 210   a ) of the virtual machine  1  ( 200   a ), the timer event for the virtual CPU  0  ( 210   a ) of the virtual machine  2  ( 200   b ), and the timer event  160   a  for the host process  1  ( 150   a ) are managed on the interrupt timer  40   a  of the physical CPU  0  ( 10   a ) by being inserted into the timer queue  122   a . In the figures, the virtual machine  1  is represented by “VM  1 ”, the virtual CPU  0  is represented by “VCPU  0 ”, and the physical CPU  0  is represented by “PCPU  0 ”. 
         [0093]    The timer event  130   a  for the clock tick timer of the physical CPU  0  ( 10   a ) is managed without being inserted into the timer queue  122   a . The timer event  130   a  for the clock tick timer is not inserted into the timer queue  122   a  because the timer event  130   a  for the clock tick timer is not migrated between the physical CPUs  10 . 
         [0094]    On the physical CPU  10  having the interrupt timer  40  in the exclusive timer mode, the interrupt timer  40  of the relevant physical CPU  10  is occupied by the interrupt timer  230  of the virtual CPU  210  occupying the relevant physical CPU  10 . 
         [0095]    As illustrated in  FIG. 4B , in the exclusive timer mode, the timer event  240   a  for the interrupt timer  230   a  of the virtual CPU  210   a  is not used and is disabled, and the timer event  130   a  for the clock tick timer of the relevant physical CPU  10   a  is also disabled. The timer event that was inserted in the timer queue  122   a  for the relevant physical CPU  10   a  is migrated to the timer queue for another physical CPU  10   c  in the shared timer mode. 
         [0096]    In  FIG. 4B , for example, the interrupt timer  40   a  of the physical CPU  0  ( 10   a ) in the exclusive timer mode is occupied by the interrupt timer  230   a  of the virtual CPU  0  ( 210   a ) of the virtual machine  1  ( 200   a ). Further, the timer event  240   a  for the virtual CPU  0  ( 210   a ) of the virtual machine  1  ( 200   a ) is extracted from the timer queue  122   a  and is disabled, and the timer event  130   a  for the clock tick timer is also disabled. 
         [0097]    In  FIG. 4B , the timer event  160   a  for the host process  1  ( 150   a ) and the timer event  240   c  for the virtual CPU  0  ( 210   a ) of the virtual machine  2  ( 200   b ), which were inserted in the timer queue  122   a  in  FIG. 4A , are both inserted in the timer queue  122   c  for the physical CPU  2  ( 10   c ). 
         [0098]    Now, a description is made of the updating of the interrupt timer  230   a  of the virtual CPU  210  on the physical CPU  10  in the shared timer mode and processing for delivering the timer interrupt to the virtual CPU  210  in conjunction with the timer interrupt issued from the interrupt controller  30  of the relevant physical CPU  10 . 
         [0099]    Then, a description is made of the updating of the interrupt timer  230   a  of the virtual CPU  210  on the physical CPU  10  in the exclusive timer mode and processing for delivering the timer interrupt to the virtual CPU  210  in conjunction with the timer interrupt issued from the interrupt controller  30   a  of the relevant physical CPU  10 . Finally, a description is made of processing for dynamically switching the timer scheduling mode  121   a  for the interrupt timer  40  of the physical CPU  10 . 
         [0100]    First, the interrupt timer  40   a  of the physical CPU  0  ( 10   a ) of  FIG. 4A  is taken as an example to describe the updating of the interrupt timer  230   a  of the virtual CPU  210   a  on the physical CPU  10   a  in the shared timer mode and the processing for delivering the timer interrupt to the virtual CPU  210  in conjunction with the timer interrupt issued from the interrupt controller  30   a  of the relevant physical CPU  10 . 
         [0101]      FIG. 5  is a diagram for illustrating a state of the scheduler  110  and the timer management module  120  exhibited immediately before the remaining-time counter  231   a  of the interrupt timer  230   a  of the virtual CPU  0  ( 210   a ) of the virtual machine  1  ( 200   a ) is updated.  FIG. 6  is a graph for showing a relationship between the expiration time of the timer event and the remaining-time counter  41   a  of the interrupt timer of the physical CPU  10  in the shared timer mode. 
         [0102]    In the graph of  FIG. 6 , the relationship between the timer event  160   a  for the host process  1  exemplified as an enabled timer event and the remaining-time counter  41   a  of the interrupt timer  40   a  of the physical CPU  0  ( 10   a ) is shown in time series. 
         [0103]    As illustrated in  FIG. 5 , the virtual CPU  0  ( 210   a ) of the virtual machine  1  ( 200   a ) is operating on the physical CPU  0  ( 10   a ). The remaining-time counters  231   a  of both the interrupt timer  230   a  of the virtual CPU  0  ( 210   a ) of the virtual machine  1  ( 200   a ) and the interrupt timer  230   a  of the virtual CPU  0  ( 210   a ) of the virtual machine  2  ( 200   b ) have a value of zero, and the status  243   a  and the status  243   c  of the corresponding timer events  240   a  and  240   c , respectively, are both disabled. 
         [0104]    The timer event  160   a  of the host process  1  ( 150   a ) has a value set in the expiration time  161   a  with the status  162   a  being enabled, and is inserted in the timer queue  122   a  for the physical CPU  0  ( 10   a ). The timer event  130   a  for the clock tick timer of the physical CPU  0  ( 10   a ) has a value set in the expiration time  132   a  with the status  134   a  being enabled. 
         [0105]    In  FIG. 6 , in comparison between the expiration time  132   a  of the timer event  130   a  for the clock tick timer and the expiration time  161   a  of the timer event  160   a  of the host process  1  ( 150   a ), the expiration time  132   a  of the timer event  130   a  for the clock tick timer arrives first. Therefore, the expiration time  132   a  of the timer event  130   a  for the clock tick timer is set in the remaining-time counter  41   a  of the interrupt timer  40   a  of the physical CPU  0  ( 10   a ). 
         [0106]    With reference to  FIG. 8A , a description is made of processing conducted after the guest operating system  201  operating on the virtual CPU  0  ( 210   a ) writes a value to the remaining-time counter  231   a  of the interrupt timer  230   a  until the state of  FIG. 7  is finally obtained. 
         [0107]      FIG. 7  is a graph for showing a relationship between the expiration time of the timer event and the remaining-time counter  41  of the interrupt timer  40  of the physical CPU  10  in the shared timer mode.  FIG. 8A  is a flowchart for illustrating an example of processing for updating the remaining-time counter of the interrupt timer  230   a  of the virtual CPU  210   a  on the physical CPU  10   a  in the shared timer mode. 
         [0108]    In  FIG. 8A , the guest operating system  201  operating on the virtual CPU  0  ( 210   a ) requests the remaining-time counter  231   a  of the interrupt timer  230   a  to update the value (Step  601 ). 
         [0109]    The virtual machine monitor  100  traps the update request issued by the remaining-time counter  231   a  of the virtual CPU  210   a , and transfers control to the timer management module  120  (Step  602 ). The virtual machine monitor  100  may activate the timer management module  120  at this point in time. 
         [0110]    The timer management module  120  sets the value requested to be updated in the remaining-time counter  231   a  of the interrupt timer  230   a  of the virtual CPU  210   a  (Step  603 ). 
         [0111]    The timer management module  120  reads the value of the system time counter  81  of the system time  80  to acquire the current time. Then, the timer management module  120  sets the acquired current time in the remaining-time counter update time  241   a  of the timer event  240   a  associated with the virtual CPU  0  ( 210   a ), and updates the expiration time  242   a  of the timer event  240   a  based on Expression A described below (Step  604 ). 
         [0000]      (Expiration time (242 a ) of timer event (240 a ))=(current time)+((remaining-time counter (231 a ) of interrupt timer (230 a ))×(system time frequency (82)))/(interrupt timer frequency (232 a ))  (Expression A)
 
         [0112]    When the frequency of the interrupt timer  40   a  of the physical CPU  10   a  and the frequency  232   a  of the interrupt timer  230   a  of the virtual CPU  0  ( 210   a ) are equal to each other in (Expression A), the system time frequency  82  may be omitted in the calculation of Expression A or the like. The same applies to Expressions B and D described later. 
         [0113]    Incidentally, in a computer system in which the virtual machine  200  is migrated among a plurality of physical computers  1 , it is premised that the virtual CPU  210  operates with the same frequency before and after the migration. Meanwhile, the frequency of the physical CPU  10  may differ before and after the migration because the frequency depends on the physical computer  1 . 
         [0114]    Therefore, in the computer system in which the virtual machine  200  is migrated among the plurality of physical computers  1 , the timer frequency  42  of the interrupt timer  40  of the physical CPU  10  and the timer frequency  232  of the interrupt timer  230  of the virtual CPU  210  may often differ from each other. According to the embodiment of this invention, the virtual CPU  210  is allowed to dynamically occupy the interrupt timer  40  of the physical CPU  10  based on Expression A or the like in consideration of such a difference in timer frequency as described above. 
         [0115]    Then, the timer management module  120  sets the status  243   a  of the timer event  240   a  to “enabled”, and inserts the timer event  240   a  into the timer queue  122   a  (Step  605 ). The timer management module  120  compares the expiration times of the timer events within the timer queue  122   a  with each other, and sorts the timer events in ascending order of the time axis of the system time  80  (namely, chronological order of the expiration time). 
         [0116]    In the situation of  FIG. 7 , when the timer management module  120  inserts the timer event  240   a  into the timer queue  122   a , the timer event  160   a  of the host process  1  ( 150   a ) is already inserted in the timer queue  122   a.    
         [0117]    The timer management module  120  compares the expiration times within the timer queue  122   a  with each other as described above, determines that the expiration time  242   a  of the timer event  240   a  is closer than the expiration time  161   a  of the timer event  160   a , and migrates the timer event  240   a  to the head of the timer queue  122   a.    
         [0118]    After finishing inserting the timer event  160   a  into the timer queue  122   a , the timer management module  120  compares the expiration time  242   a  of the timer event  240   a  at the head of the timer queue  122   a  and the expiration time  132   a  of the timer event  130   a  for the clock tick timer. The timer management module  120  acquires the closer expiration time of the timer event as the nearest expiration time (Step  606 ). 
         [0119]    The timer management module  120  reads the system time counter  81  of the system time  80  to acquire the current time, and subtracts the current time from the nearest expiration time to calculate the remaining time. Then, the timer management module  120  converts the remaining time before the expiration time into a count number (remaining-time count) of the interrupt timer  40   a  based on Expression B described below. 
         [0120]    The timer management module  120  sets the value (remaining-time count) calculated by Expression B in the remaining-time counter  41   a  of the interrupt timer  40   a  of the physical CPU  0  ( 10   a ) (Step  607 ). 
         [0000]      (Remaining-time count)=((remaining time)×(interrupt timer frequency 42 a ))/(system time frequency 82 a )  (Expression B)
 
         [0121]    When the flowchart of  FIG. 8A  is executed by the above-mentioned processing in the states of  FIG. 5  and  FIG. 6 , a value corresponding to the expiration time  242   a  of the timer event  240   a  migrated to the head of the timer queue  122   a  is set in the remaining-time counter  41   a  of the interrupt timer  40   a  in which the remaining-time count of the timer event  160   a  for the host process  1  was set. Thus, the state of  FIG. 7  is obtained. 
         [0122]    In a related-art virtual machine system, the timer event  130   a  for the clock tick timer is always enabled except in a temporary case where the expiration time  132   a  of the timer event  130   a  is updated. Therefore, even when the timer queue  122   a  is empty, the interrupt timer  40   a  of the physical CPU  0  ( 10   a ) keeps operating, and the interrupt controller  30   a  keeps generating the timer interrupt at least at a fixed cycle period. 
         [0123]    The above-mentioned processing also allows the guest OS  201  to determine that the updating processing relating to the remaining-time counter  231   a  of the interrupt timer  230   a  of the virtual CPU  210   a  has been completed. 
         [0124]    Next, the current time progresses from the state of  FIG. 7 , and the timer interrupt is generated by the interrupt controller  30   a  when the interrupt timer  40   a  of the physical CPU  0  ( 10   a ) finishes the counting. With reference to the flowchart of  FIG. 8B  and  FIG. 8C , a description is made of processing conducted before the timer interrupt is delivered to the virtual CPU  0  ( 210   a ) of the virtual machine  1  ( 200   a ). 
         [0125]      FIG. 8B  is the first half of the flowchart for illustrating an example of the processing for delivering the timer interrupt of the virtual CPU  210  on the physical CPU  10  in the shared timer mode.  FIG. 8C  is the second half of the flowchart for illustrating the example of the above-mentioned transmission processing. 
         [0126]    When the remaining-time counter  41   a  of the interrupt timer  40   a  of the physical CPU  0  ( 10   a ) is updated by the above-mentioned processing of  FIG. 8A , the remaining-time counter  41   a  starts the decrementing at the cycle period of the frequency  42   a.    
         [0127]    When the value of the remaining-time counter  41   a  becomes zero, the interrupt controller  30   a  generates the timer interrupt, and notifies the physical CPU  0  ( 10   a ) of the timer interrupt (Step  620 ). The timer interrupt is delivered to the virtual machine monitor  100 , and the virtual machine monitor  100  activates the timer management module  120  (Step  621 ). When the timer management module  120  has already been activated, the virtual machine monitor  100  starts the control of the timer management module  120 . 
         [0128]    The timer management module  120  reads the system time counter  81  of the system time  80  to acquire the current time (Step  622 ). The timer management module  120  determines whether or not the timer queue  122   a  for the physical CPU  0  ( 10   a ) is empty (Step  623 ). When the timer queue  122   a  is not empty, the procedure advances to Step  624 , and when the timer queue  122   a  is empty, advances to Step  631  of  FIG. 8C . 
         [0129]    The timer management module  120  refers to the timer queue  122   a  to compare the expiration time of the timer event at the head and the current time (Step  624 ). Then, the timer management module  120  determines whether or not the current time has passed the expiration time of the timer event at the head of the timer queue  122   a.    
         [0130]    When the current time has passed the expiration time of the timer event at the head of the timer queue  122   a , the procedure advances to Step  625 , and when the current time has not passed the expiration time of the timer event, advances to Step  630 . 
         [0131]    In Step  625 , the current time has passed the expiration time of the timer event at the head, and hence the timer management module  120  extracts the relevant timer event from the timer queue  122   a , and sets the status  243  to “disabled”. 
         [0132]    In Step  626 , the timer management module  120  determines whether or not the timer event extracted in Step  625  is the timer event  240   a  for the virtual CPU  210   a . When the extracted timer event is the timer event  240   a  for the virtual CPU  210   a , the procedure advances to Step  627 , and otherwise advances to Step  628 . 
         [0133]    In Step  627 , the extracted timer event is the timer event  240   a  for the virtual CPU  0  ( 210   a ), and hence the timer management module  120  notifies the virtual CPU  0  ( 210   a ) of the timer interrupt. When receiving the interrupt, the virtual CPU  0  ( 210   a ) executes predetermined processing corresponding to the timer event  240   a.    
         [0134]    Meanwhile, in Step  628 , the timer management module  120  determines whether or not the timer event extracted in Step  625  is the timer event  160   a  for the host process  1  and whether or not the expiration time  161   a  has been passed (execution start). When the expiration time  161   a  has been passed, the procedure advances to Step  629 , and otherwise the procedure advances to Step  630 . 
         [0135]    In Step  629 , the processing of the host process  150   a  requested by the timer event  160   a  is executed. 
         [0136]    In Step  630 , the timer management module  120  determines whether or not the timer queue  122   a  is empty and whether or not the expiration time of the timer event at the head of the timer queue  122   a  is later than the current time. When the timer queue  122   a  is not empty or when the expiration time of the timer event at the head is earlier than the current time, the procedure returns to Step  624  to repeat the above-mentioned processing, and otherwise the procedure advances to Step  631 . 
         [0137]    In Step  624  and subsequent steps, the timer management module  120  repeats the above-mentioned processing until the timer queue  122   a  becomes empty or until the expiration time of the timer event at the head of the timer queue  122   a  becomes later than the current time. 
         [0138]    Subsequently, in Step  631  of  FIG. 8C , the timer management module  120  compares the expiration time  132   a  of the timer event  130   a  for the clock tick timer of the physical CPU  0  ( 10   a ) and the current time (Step  631 ). When the expiration time  132   a  of the timer event  130   a  has been passed, the timer management module  120  advances to Step  632 , and otherwise advances to Step  634 . 
         [0139]    In the example of  FIG. 6 , the expiration time  132   a  is later than the current time, and hence the procedure advances to Step  634 . When the current time has passed the expiration time  132   a  of the timer event  130   a , the processing requested by the clock tick timer is executed (Step  632 ). 
         [0140]    The timer management module  120  updates the expiration time  132   a  of the timer event  130   a  based on Expression C described below (Step  633 ). 
         [0000]      (Expiration time (132 a ))=(start time 131 a )+(((cycle period 133 a )+(current time)−(start time 131 a ))/(cycle period 133 a ))×(cycle period 133 a )  (Expression C)
 
         [0141]    In the same manner as when the remaining-time counter  231   a  of the interrupt timer  230   a  of the virtual CPU  0  ( 210   a ) was updated previously, the timer management module  120  compares the expiration time of the timer event at the head of the timer queue  122   a  for the physical CPU  0  ( 10   a ) and the expiration time  132   a  of the timer event  130   a  for the clock tick timer, and acquires the closer expiration time (Step  634 ). 
         [0142]    The timer management module  120  converts units of the acquired expiration time into the count number based on Expression B described above in the same manner, and sets the count number to the remaining-time counter  41   a  of the interrupt timer  40   a  of the physical CPU  0  ( 10   a ) (Step  635 ). 
         [0143]    By the above-mentioned processing, on the physical CPU  10  in the shared timer mode, the timer management module  120  can notify a computer resource corresponding to the timer event of the timer interrupt generated by the interrupt controller  30   a , and can execute the processing corresponding to the timer event. When the timer event is the timer event  240   a  for the virtual CPU  210   a , the timer management module  120  can notify the virtual CPU  210   a  of the timer interrupt, and can execute predetermined processing. 
         [0144]    Then, the timer management module  120  can prepare the next timer interrupt by setting a new expiration time in the remaining-time counter  41   a  of the interrupt timer  40   a  of the physical CPU  10 . 
         [0145]    Next, the interrupt timer  40   a  of the physical CPU  0  ( 10   a ) of  FIG. 4B  is taken as an example to describe, with reference to the flowcharts of  FIG. 9A  and  FIG. 9B , examples of the updating of the interrupt timer  230   a  of the virtual CPU  210   a  on the physical CPU  10   a  in the exclusive timer mode and the processing for delivering the timer interrupt to the virtual CPU  210   a  in conjunction with the timer interrupt issued from the interrupt controller  30   a  of the relevant physical CPU  10   a.    
         [0146]      FIG. 9A  is a flowchart for illustrating an example of the processing for updating the remaining-time counter of the interrupt timer of the virtual CPU  210   a  on the physical CPU  10   a  in the exclusive timer mode. 
         [0147]      FIG. 9B  is a flowchart for illustrating an example of the processing for delivering the timer interrupt of the interrupt timer  230   a  of the virtual CPU  210   a  on the physical CPU  10   a  in the exclusive timer mode. 
         [0148]    As illustrated in  FIG. 4B , only the virtual CPU  0  ( 210   a ) of the virtual machine  1  ( 200   a ) is operating on the physical CPU  0  ( 10   a ). When the interrupt timer  40   a  of the physical CPU  0  ( 10   a ) is the exclusive timer mode, the virtual machine monitor  100  causes the guest operating system  201  operating on the virtual CPU  0  ( 210   a ) to recognize the state of the interrupt timer  40   a  as the state of the interrupt timer  230   a  of the virtual CPU  0  ( 210   a ) as it is. 
         [0149]    In  FIG. 9A , the guest operating system  201  operating on the virtual CPU  0  issues a request to write a value to the remaining-time counter  231   a  of the interrupt timer  230   a  (Step  650 ). 
         [0150]    The timer management module  120  of the virtual machine monitor  100  traps the write request issued by the guest operating system  201 , and writes the value of the remaining-time counter  231   a  directly to the remaining-time counter  41   a  of the interrupt timer  40   a  of the physical CPU  0  ( 10   a ) (Step  651 ). 
         [0151]    As described above, in the exclusive timer mode, the value of the remaining-time counter  231   a  is written to the remaining-time counter  41   a  of the interrupt timer  40   a  of the physical CPU  0  ( 10   a ) as it is by the timer management module  120 . 
         [0152]    In  FIG. 9B , when the value is written, the remaining-time counter  41   a  starts the decrementing at the cycle period of the frequency  42   a . When the remaining-time counter  41   a  becomes zero after the repeated decrementing, the interrupt controller  30   a  generates the timer interrupt, and notifies the physical CPU  0  ( 10   a ) of the timer interrupt (Step  652 ). When receiving the timer interrupt, the virtual machine monitor  100  directly delivers the timer interrupt to the virtual CPU  0  ( 210   a ) (Step  653 ). 
         [0153]    By the above-mentioned processing, on the physical CPU  10   a  in the exclusive timer mode, the timer management module  120  can directly notify the virtual CPU  210   a  of the timer interrupt generated by the interrupt controller  30   a , and can execute the processing corresponding to the timer event. 
         [0154]    Next, a specific description is made of processing for switching the interrupt timer of the relevant physical CPU  10  from the shared timer mode to the exclusive timer mode in conjunction with the switching from the shared CPU mode to the exclusive CPU mode for the physical CPU. 
         [0155]    With reference to  FIG. 10A  and  FIG. 10B , a description is made below of an example of switching the physical CPU  0  ( 10   a ) from the shared CPU mode (shared timer mode) to the exclusive CPU mode (exclusive timer mode) in conjunction with the switching from  FIG. 4A  to  FIG. 4B . 
         [0156]      FIG. 10A  is the first half of the flowchart for illustrating an example of the processing for dynamically changing the timer scheduling mode  121  for the interrupt timer of the physical CPU  10   a .  FIG. 10B  is the second half of the flowchart for the above-mentioned processing for dynamically changing the timer scheduling mode for the interrupt timer of the physical CPU. 
         [0157]    In the shared CPU mode illustrated in  FIG. 4A , the virtual CPU  0  ( 210   a ) of the virtual machine  1  ( 200   a ), the virtual CPU  0  ( 210   a ) of the virtual machine  2  ( 200   b ), and the host process  1  ( 150   a ) are operating on the physical CPU  0  ( 10   a ). Then, in  FIG. 4B  for illustrating the state after the switching to the exclusive CPU mode, the virtual CPU  0  ( 210   a ) of the virtual machine  2  ( 200   b ) and the host process  1  ( 150   a ) are migrated to the physical CPU  2  ( 10   c ). 
         [0158]    In  FIG. 10A , when the physical CPU  0  ( 10   a ) is switched from the shared CPU mode to the exclusive CPU mode, the virtual machine monitor  100  sets the scheduling mode switching request  123   a  for the interrupt timer  40   a  (Step  660 ). 
         [0159]    When the count value of the timer event currently being counted by the interrupt timer  40   a  becomes zero, the interrupt controller  30   a  of the physical CPU  0  ( 10   a ) generates the timer interrupt. After the physical CPU  0  ( 10   a ) starts the exclusive CPU mode, the virtual machine monitor  100  receives, from the interrupt controller  30   a , the timer interrupt generated when the interrupt timer  40   a  finishes the counting (Step  661 ). 
         [0160]    A new value is not set after the occurrence of the interrupt, and hence the interrupt timer  40   a  of the physical CPU  0  ( 10   a ) stops the counting based on the above-mentioned interval timer function (Step  662 ). 
         [0161]    The virtual machine monitor  100  activates the timer management module  120 . The timer management module  120  executes the above-mentioned processing from Step  622  to Step  633  of  FIG. 8B  and  FIG. 8C . However, the timer management module  120  does not execute the processing for updating the remaining-time counter  41   a  of the interrupt timer  40   a  of the physical CPU  10   a  (Step  663 ). The timer management module  120  executes the above-mentioned processing when the interrupt timer  40   a  is in the shared timer mode. 
         [0162]    The state in which the interrupt timer  40   a  has stopped the counting is maintained through inhibition of the processing for updating the remaining-time counter  41   a  of the interrupt timer  40   a  of the physical CPU  10   a.    
         [0163]    The timer management module  120  determines whether or not the scheduling mode switching request  123   a  is set (Step  664 ). When the scheduling mode switching request  123   a  is set, the procedure advances to Step  665  of  FIG. 10B , and when the scheduling mode switching request  123   a  is not set, advances to Step  674 . 
         [0164]    The timer management module  120  first clears the scheduling mode switching request  123   a  (Step  665 ). Subsequently, the timer management module  120  changes the timer scheduling mode  121   a  for the interrupt timer  40   a  to the exclusive timer mode (Step  666 ). 
         [0165]    This inhibits the timer event from being migrated from the other physical CPUs  10   b  to  10   d  to the physical CPU  0  ( 10   a ). 
         [0166]    Subsequently, the timer management module  120  determines whether or not the timer event  240   a  for the virtual CPU  0  ( 210   a ) occupying the physical CPU  0  ( 10   a ) is inserted in the timer queue  122   a  (Step  667 ). When the timer event  240   a  is inserted in the timer queue  122   a , the timer management module  120  extracts the timer event  240   a  (Step  668 ). 
         [0167]    Subsequently, the timer management module  120  migrates all the timer events inserted in the timer queue  122   a  to the physical CPUs  10   b  to  10   d  in the shared CPU mode (Step  669 ). In  FIG. 4A , the timer event  240   c  for the virtual CPU  0  of the virtual machine  2  inserted in the timer queue  122   a  for the physical CPU  0  and the timer event  160   a  for the host process  1  are migrated to the timer queue  122   c  for the physical CPU  2  in the shared timer mode as illustrated in  FIG. 4B . 
         [0168]    Subsequently, the timer management module  120  disables the status  134   a  of the timer event  130   a  for the clock tick timer of the physical CPU  0  ( 10   a ) to stop the timer event  130   a  (Step  670 ). 
         [0169]    The timer management module  120  then refers to the status  243   a  of the timer event  240   a  for the virtual CPU  0  ( 210   a ) of the virtual machine  1  ( 200   a ), which has been extracted from the timer queue  122   a  in Step  668 , to determine whether or not the status  243   a  is enabled (Step  671 ). When the status  243   a  is enabled, the procedure advances to Step  671 , and when the status  243   a  is disabled, the processing is brought to an end. 
         [0170]    The timer management module  120  sets the status  243   a  of the timer event  240   a  to “disabled” (Step  672 ). 
         [0171]    Subsequently, the timer management module  120  acquires the current time from the system time counter  81  of the system time  80 , and converts the expiration time  242   a  of the timer event  240   a  into the remaining-time count based on Expression D described below. 
         [0000]      (Remaining-time count)=((expiration time (242 a ))−(current time))×(timer frequency (42 a ))/(system time frequency (82))  (Expression D)
 
         [0172]    Then, the timer management module  120  sets the remaining-time count calculated in the above-mentioned manner in the remaining-time counter  41   a  of the interrupt timer  40   a  (Step  673 ), and starts the occupancy of the interrupt timer  40   a  of the physical CPU  0  ( 10   a ) by the interrupt timer  230   a  of the virtual CPU  0  ( 210   a ). 
         [0173]    Meanwhile, when it is determined above in Step  664  that the scheduling mode switching request  123   a  is not set, the timer management module  120  executes the above-mentioned processing of Step  634  and Step  635  of  FIG. 8C , and brings the processing to an end (Step  674 ). Specifically, the timer management module  120  updates the remaining-time counter  41   a  of the interrupt timer  40   a  of the physical CPU  10   a , and brings the processing to an end. 
         [0174]    By the above-mentioned processing, the scheduling mode for the interrupt timer  40   a  can be dynamically switched from the shared timer mode to the exclusive timer mode in conjunction with the shift of the physical CPU  0  ( 10   a ) in the shared CPU mode to the exclusive CPU mode. 
         [0175]    In this case, the interrupt timer  40   a  has an interval function of stopping the counting when a new value is not set after the timer finishes the counting. In this embodiment, after the interrupt timer  40   a  finishes the counting and the interrupt controller  30   a  generates the timer interrupt, the scheduling mode is shifted from the shared timer mode to the exclusive timer mode with the interrupt timer  40   a  being in a stopped state. This allows the scheduling mode for the interrupt timer  40   a  to be dynamically changed after the timer event for which the counting has already been started is positively completed. 
         [0176]    In the exclusive timer mode for the interrupt timer  40   a , the timer event  240   a  for the virtual CPU  0  ( 210   a ) set in the shared mode is disabled in Step  672  described above, and hence the interrupt timer  40   a  maintains the state in which the counting is stopped. 
         [0177]    Next, a specific description is made of processing for switching the interrupt timer  40  of the relevant physical CPU  10  from the exclusive timer mode to the shared timer mode in conjunction with the switching from the exclusive CPU mode to the shared CPU mode for the physical CPU  10 . This processing is described with reference to the flowchart of  FIG. 10C  by taking the physical CPU  0  ( 10   a ) as an example in the switching from  FIG. 4B  to  FIG. 4A .  FIG. 10C  is a flowchart for illustrating the processing for dynamically changing the timer scheduling mode  121  for the interrupt timer  40  of the physical CPU  10 . 
         [0178]    When the physical CPU  0  ( 10   a ) is changed from the exclusive CPU mode to the shared CPU mode, the virtual CPU  0  ( 210   a ) of the virtual machine  1  ( 200   a ) stops the occupancy of the physical CPU  0  ( 10   a ). In the subsequent processing, an arbitrary virtual CPU  210  and an arbitrary host process  150  can operate on the physical CPU  0  ( 10   a ). 
         [0179]    The above description relates to an example in which the virtual CPU  0  ( 210   a ) of the virtual machine  2  ( 200   b ) and the host process  1  ( 150   a ) are returned to the physical CPU  0  ( 10   a ) thereafter based on the load balancing. 
         [0180]    In  FIG. 10C , when the physical CPU  0  ( 10   a ) is switched from the exclusive CPU mode to the shared CPU mode, the virtual machine monitor  100  sets the scheduling mode switching request  123   a  for the interrupt timer  40   a  (Step  680 ). 
         [0181]    When the count value of the timer event (for the remaining-time counter  41   a ) currently being counted by the interrupt timer  40   a  becomes zero, the interrupt controller  30   a  of the physical CPU  0  ( 10   a ) generates the timer interrupt. After the physical CPU  0  ( 10   a ) finishes the exclusive CPU mode, the virtual machine monitor  100  receives, from the interrupt controller  30   a , the timer interrupt generated when the interrupt timer  40   a  finishes the counting (Step  681 ). 
         [0182]    A new value is not set after the occurrence of the interrupt, and hence the interrupt timer  40   a  of the physical CPU  0  ( 10   a ) stops the counting based on the above-mentioned interval function (Step  682 ). 
         [0183]    The virtual machine monitor  100  directly delivers the timer interrupt to the virtual CPU  0  ( 210   a ) in the same manner as in Step  653  illustrated in  FIG. 9B  (Step  683 ). Subsequently, the virtual machine monitor  100  determines whether or not the scheduling mode switching request  123   a  for the interrupt timer  40   a  of the physical CPU  0  ( 10   a ) is set (Step  684 ). When the scheduling mode switching request  123   a  is set, the procedure advances to Step  685 , and when the scheduling mode switching request  123   a  is not set, brings the processing to an end. 
         [0184]    The virtual machine monitor  100  activates the timer management module  120  (Step  685 ). The timer management module  120  clears the scheduling mode switching request  123   a  (Step  686 ). 
         [0185]    Subsequently, the timer management module  120  enables the status  134   a  of the timer event  130   a  for the clock tick timer of the physical CPU  0  ( 10   a ) to restart the timer event  130   a  (Step  687 ). 
         [0186]    Then, the timer management module  120  updates the expiration time  132   a  of the timer event  130   a  based on (Expression A) described above (Step  688 ). At this point in time, the enabled timer event for the physical CPU  0  ( 10   a ) is only the timer event  130   a  for the clock tick timer. Therefore, the current time acquired by reading the system time counter  81  of the system time  80  is subtracted from the expiration time  132   a , to thereby calculate the remaining-time count. The timer management module  120  sets the value calculated from the remaining-time count by (Expression B) described above in the remaining-time counter  41   a  of the interrupt timer  40   a  of the physical CPU  0  ( 10   a ) (Step  689 ). 
         [0187]    Here, the timer event  240   a  for the virtual CPU  0  ( 210   a ) of the virtual machine  1  ( 200   a ) that was occupying the physical CPU  0  ( 10   a ) before is disabled in Step  672  described above. 
         [0188]    Finally, the timer management module  120  changes the timer scheduling mode  121   a  for the interrupt timer  40   a  of the physical CPU  0  ( 10   a ) to the shared timer mode, and finishes the occupancy of the interrupt timer  40   a  of the physical CPU  0  ( 10   a ) by the interrupt timer  230   a  of the virtual CPU  0  ( 210   a ) (Step  690 ). 
         [0189]    By the above-mentioned processing, the scheduling mode of the interrupt timer  40   a  can be dynamically switched from the exclusive timer mode to the shared timer mode in conjunction with the shift of the physical CPU  0  ( 10   a ) in the exclusive CPU mode to the shared CPU mode. 
         [0190]    In the same manner as in the above-mentioned dynamic switching from the shared timer mode to the exclusive timer mode, the interval function of the interrupt timer  40   a  is used to dynamically switch the scheduling mode from the exclusive timer mode to the shared timer mode. This allows the scheduling mode for the interrupt timer  40   a  to be dynamically changed after the timer event for which the counting has already been started is positively completed. 
         [0191]    As described above, according to this embodiment, the interrupt timer  230  of a specific virtual CPU  210  is allowed to occupy the interrupt timer  40  of the physical CPU  10  in conjunction with the occupancy of the physical CPU  10  by the virtual CPU  210 . With this configuration, the overhead of the emulation of the interrupt timer of the virtual CPU can be minimized, and data eviction from a TLB or a cache by a timer for a host, which has occurred irrespective of whether or not a program on the virtual CPU is operating, can be suppressed to ensure the performance of the physical CPU. 
         [0192]    Further, in this embodiment, the timer scheduling mode is dynamically switched between the exclusive timer mode and the shared timer mode in conjunction with the CPU scheduling mode through use of the interval function of the interrupt timer  40   a , which inhibits the operation from being conducted after the interrupt controller  30   a  generates the timer interrupt and before a value is written to the remaining-time counter of the interrupt timer  40   a.    
         [0193]    In regard to the switching of the timer scheduling mode in conjunction with the CPU scheduling mode, the shared timer mode is set in conjunction with the shared CPU mode, and the exclusive timer mode is set in conjunction with the exclusive CPU mode. 
         [0194]    In the switching from the shared timer mode to the exclusive timer mode, the clock tick timer that was using the relevant interrupt timer  40   a  is stopped, and the timer event  160   a  for the host process  1  that was using the interrupt timer of the physical CPU and the timer event for a timer for detecting a timeout or the like are migrated to another physical CPU in the shared CPU mode. Finally, the current value of the remaining-time counter  231   a  of the interrupt timer  230  of the above-mentioned virtual CPU  210  is written to the remaining-time counter  41   a  of the interrupt timer  40   a  of the physical CPU  10   a , to thereby start the occupancy of the interrupt timer  40   a  of the physical CPU  10   a  by the interrupt timer  230   a  of the virtual CPU  210 . 
         [0195]    While the interrupt timer  230   a  of the virtual CPU  210   a  occupies the interrupt timer  40   a  of the physical CPU  10   a , the program on the virtual CPU  210  writes a value to the remaining-time counter  231   a  of the interrupt timer  230   a . The relevant write request is trapped by the virtual machine monitor  100 . The virtual machine monitor  100  writes a write value directly to the remaining-time counter  41   a  of the interrupt timer  40   a  of the physical CPU  10   a . The interrupt timer  40   a  of the physical CPU  10   a  starts the decrementing after the write value is written to the remaining-time counter  41   a , and when the write value become zero, the interrupt controller  30   a  generates the timer interrupt. 
         [0196]    Further, in the exclusive timer mode, the timer event  130  for the clock tick timer and the timer event  240   a  for the virtual CPU  210   a  are disabled, and hence it is possible to ensure the performance of the physical CPU  10  by inhibiting the occurrence of the timer interrupt to prevent data eviction from the TLB or the cache. 
         [0197]    Further, in the exclusive timer mode, at least one physical CPU  10  is set to the shared timer mode. This allows the timer event  160  of the host process  150  or the timer event  130   a  for the clock tick timer to be correctly executed, and allows the timer for detecting a timeout, the clock tick timer, or the like to be continuously operated. 
         [0198]    Meanwhile, the dynamic switching of the physical CPU  10   a  from the exclusive CPU mode to the shared CPU mode also involves the use of the stopping of the counting based on the interval function after the interrupt controller  30   a  generates the timer interrupt when the interrupt timer  40   a  of the physical CPU  10   a  finishes the counting. 
         [0199]    When receiving the timer interrupt from the interrupt controller  30   a  of the physical CPU  10   a , the virtual machine monitor  100  directly delivers the timer interrupt to the virtual CPU  210 . The virtual machine monitor  100  sets the remaining-time counter  231   a  of the interrupt timer  230   a  of the virtual CPU  210  to zero to stop the interrupt timer  230   a . The virtual machine monitor  100  restarts the timer event  130   a  for the clock tick timer, which was using the interrupt timer  40   a  of the physical CPU  10   a , to allow the physical CPU  10   a  to process the timers for activating the host process  1  and detecting a timeout and the interrupt timer  40  of another virtual CPU, and finishes the occupancy of the interrupt timer  40   a  of the physical CPU  10   a  by the interrupt timer  230  of the virtual CPU  210 . 
         [0200]    In the virtual machine system, the interrupt timer  40  of at least one physical CPU  10  is inhibited from being occupied by the interrupt timer  230  of a specific virtual CPU  210 , to thereby allow the timers for activating the host process and detecting a timeout to be continuously operated through the use of the interrupt timer  40  of the physical CPU  10 . 
         [0201]    This invention is not limited to the embodiments described above, and encompasses various modification examples. For instance, the embodiments are described in detail for easier understanding of this invention, and this invention is not limited to modes that have all of the described components. Some components of one embodiment can be replaced with components of another embodiment, and components of one embodiment may be added to components of another embodiment. In each embodiment, other components may be added to, deleted from, or replace some components of the embodiment, and the addition, deletion, and the replacement may be applied alone or in combination. 
         [0202]    Some of all of the components, functions, processing units, and processing means described above may be implemented by hardware by, for example, designing the components, the functions, and the like as an integrated circuit. The components, functions, and the like described above may also be implemented by software by a processor interpreting and executing programs that implement their respective functions. Programs, tables, files, and other types of information for implementing the functions can be put in a memory, in a storage apparatus such as a hard disk, or a solid state drive (SSD), or on a recording medium such as an IC card, an SD card, or a DVD. 
         [0203]    The control lines and information lines described are lines that are deemed necessary for the description of this invention, and not all of control lines and information lines of a product are mentioned. In actuality, it can be considered that almost all components are coupled to one another.