Abstract:
A technique for utilizing resources in a virtual machine operating system. The virtual machine operating system comprises a multiplicity of virtual machines. A share of resources is allocated to each of the virtual machines. Utilization by one of the virtual machines of the resources allocated to the one virtual machine is automatically monitored. If the one virtual machine needs additional resources, the one virtual machine is automatically cloned. The clone is allocated a share of the resources taken from the shares of other of the virtual machines, such that the resultant shares allocated to the one virtual machine and the clone together are greater than the share allocated to the one virtual machine before the one virtual machine was cloned. The clone performs work with its resources that would have been performed by the one virtual machine if not for the existence of said clone.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This is a continuation of patent application Ser. No. 10/425,470 filed on Apr. 29, 2003, now U.S. Pat. No. 7,299,468. 

   FIELD OF THE INVENTION 
   The invention relates generally to computer systems, and deals more particularly with management of virtual machines to effectively allocate and utilize virtual resources. 
   BACKGROUND OF THE INVENTION 
   A virtual machine operating system is well known today, and includes a common base portion and separate user portions formed by the common base portion. In an IBM z/VM operating system, the common base portion is called the “Control Program” or “CP” and each user portion is called a “virtual machine” or “guest”. A virtual machine or guest is a virtual sharing/partitioning of real resources such as real memory, CPU and I/O. Examples of I/O devices are DASD, network cards, printers and displays. A guest operating system executes/runs on each virtual machine, and one or more applications run on the guest operating system. Each application and guest operating system behave as if they are running on their own private, real computer. 
   Typically, each virtual machine is allocated a finite amount of resources, such as private virtual memory, real CPU and real I/O. The amounts allocated are intended to accommodate maximum needs of the virtual machine during most operating conditions. However, during operation of the virtual machine, the virtual machine has varying needs for each of these resources. During some periods, the virtual machine may be executing applications requiring complex arithmetic computations which are CPU intensive and during other periods the virtual machine may be executing applications such as data backup applications which hardly use the CPU. Likewise, during some periods the virtual machine may be executing applications such as data base searching, reading and writing applications which require much private memory and during other periods the virtual machine may be executing applications such as text editing applications which require little private memory. Likewise, during some periods the virtual machine may be executing applications such as data backup applications which require substantial I/O activity and during other periods the virtual machine may be executing applications such as arithmetic computation applications which require little I/O activity. During some of the periods of operation, the virtual machine may need more of a virtual resource than has been allocated, in which case the finite virtual resource allocation may constrain the operation of the virtual machine. During other periods of operation, the virtual machine does not utilize its full allocation of one or more virtual resources, so these virtual resources may be wasted in the sense that another virtual machine could have used the excess allocation. 
   The following is an example of how a known virtual machine utilizes its CPU to perform work items. Each virtual machine has its own dispatch function which consists of its synchronization or lock function, work queue assignment function, work scheduler and associated work queue of work items or tasks assigned by and to the virtual machine. The synchronization or lock function, work queue assignment function, work scheduler and the work queue are all private to the virtual machine. The synchronization or lock function manages locks for a work queue to control which work items must run sequentially and which tasks can run in parallel. A work queue assignment function is a program function within the virtual machine which adds work items to the work queue of the virtual machine when generated by the virtual machine. The work items are added to the queue at a position based on an assignment algorithm. The assignment algorithm may consider such factors as relative priority level of each work item and the order in which work items were created, i.e. first in first out. Each work item on the queue includes information indicating its type, and therefore, which function within the virtual machine is best suited to handle it. A “work scheduler” is a program function which schedules each of the work items on its queue for execution. Generally, the work scheduler removes work items from an end of the queue. The work scheduler passes the work items to the appropriate function within the virtual machine for execution by the virtual CPU. If the work items on the work queue are CPU intensive and the allocation of virtual CPU is inadequate, the work queue may grow in length as the existing work items on the queue are removed more slowly than new work items are added to the queue. In such a case, the virtual machine will fall behind in its work. 
   It was also known for multiple virtual machines to share a work queue to distribute the work items amongst the virtual machines and their respective shares of real CPUs. A server virtual machine was utilized for the purpose of “hosting” this shared work queue for the other, “working” virtual machines. The shared work queue resides in memory private to the server virtual machine. When a working virtual machine creates a new work item, and the work queue assignment function for this working virtual machine decides to send this new work item to the server virtual machine, it uses a communication protocol (e.g. TCP/IP) and a virtual I/O device driver to send that work item to this server virtual machine. Then, the server virtual machine places the new work item on the shared work queue in an order determined by the server virtual machine. When the virtual CPU within a working virtual machine is available to execute a work item on the shared work queue, the work scheduler within this working virtual machine uses a communication protocol and virtual I/O device driver to make that request to the server virtual machine. In response, the server virtual machine uses a communication protocol to send a work item to the working virtual machine that made the request. While this arrangement provides a shared work queue, it requires a high overhead communication protocol to both send a work item to the work queue and obtain a work item from the work queue. Furthermore, the server virtual machine attempts to balance the load among the working virtual machines by monitoring the working virtual machines and estimating which working virtual machine will be able to handle the work item most expeditiously. The server virtual machine must also be able to re-balance the load among working virtual machines when working virtual machines are dynamically added and/or deleted. Still further, the server virtual machine must synchronize the work items as determined by its private synchronization function. All these server virtual machine functions require considerable communication and “overhead” between the server virtual machine and the working virtual machines. 
   An object of the present invention is to provide functionality in a virtual machine operating system which helps to match the needs of the application(s) running on each virtual machine to the available resources. 
   Another object of the present invention is to provide functionality in a virtual machine operating system of the foregoing type which operates dynamically to help match the changing needs of the application(s) running on each virtual machine to the available resources. 
   Another object of the present invention is to provide functionality of the foregoing type which also considers limits set by the system administrator for resources available to the application(s) running on each virtual machine. 
   SUMMARY OF THE INVENTION 
   The invention resides in a system, computer program product and method for utilizing resources in a virtual machine operating system. The virtual machine operating system comprises a multiplicity of virtual machines. A share of resources is allocated to each of the virtual machines. Utilization by one of the virtual machines of the resources allocated to the one virtual machine is automatically monitored. If the one virtual machine needs additional resources, the one virtual machine is automatically cloned. The clone is allocated a share of the resources taken from the shares of other of the virtual machines, such that the resultant shares allocated to the one virtual machine and the clone together are greater than the share allocated to the one virtual machine before the one virtual machine was cloned. 
   According to one feature of the present invention, the clone performs work with its resources that would have been performed by the one virtual machine if not for the existence of said clone. 
   According to another feature of the present invention, the one virtual machine and the clone share a work queue, such that both the one virtual machine with its resources and the clone with its resources perform work items on the shared work queue. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIG. 1  is a block diagram of a computer system with multiple virtual machines in one state according to the present invention. 
       FIG. 2  is a block diagram of the computer system of  FIG. 1  with multiple virtual machines in another state according to the present invention 
       FIG. 3  is a block diagram of a synchronization or lock function that allows a virtual machine and its clone(s) to be collectively synchronized. 
       FIG. 4  is a flow chart illustrating the synchronization function of  FIG. 3  and associated operation of the virtual machine and its clone(s). 
       FIGS. 5(   a ) and  5 ( b ) form a flow chart illustrating operation of a resource manager within the computer system of  FIG. 1  to create and delete clones of a virtual machine. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring now to the drawings in detail wherein like reference numbers indicate like elements throughout,  FIG. 1  illustrates a computer system generally designated  10  in accordance with the present invention. Computer system  10  includes a physical computer  20  (which includes a CPU  23 ) and a virtual machine operating system  11 . By way of example, the virtual machine operating system can be IBM z/VM version 4.2.0 or 4.3.0 modified to include the present invention. The details of z/VM 4.2.0 are disclosed in IBM publication “z/VM 4.2.0 General Information” (Document Number: GC24-5991-03) which is available from International Business Machines Corp. at PO Box 29570, IBM Publications, Raleigh, N.C. 27626-0570 or on the WWW at the website of IBM selecting/shop/publications/order. This publication is hereby incorporated by reference as part of the present disclosure. Operating system  11  executes in the physical computer  10  such as an IBM zSeries mainframe although the present invention can be implemented in other server computers or personal computers as well. Operating system  11  includes a common base portion  21  (called “CP” in the z/VM operating system). 
   The systems administrator, during installation of the virtual machine operating system  11 , defines user virtual machines  12 ,  14 ,  16  and resource manager virtual machine  17  in a directory  191 . The directory  191  contains a name or identifier of each virtual machine, a “soft” limit for allocation of virtual and real resources to each virtual machine and a specification of a number of permitted clones of each virtual machine. The systems administrator can also specify in the directory  191  an initial virtual and real resource allocation for each virtual machine. In the absence of such a specification, each of the virtual machines shares equally in the total available virtual and real resources. Based on the “default” allocation, each of the virtual machines  12 ,  14 ,  16  and  17  initially has one quarter of the total virtual and real resources. In response to these definitions, common base portion  21  logically partitions the resources (including the CPU, I/O and memory) of the physical computer to form user portions  12 ,  14  and  16  and resource manager portion  17  (called “virtual machines” or “guests virtual machines” in the z/VM operating system). The resource manager virtual machine  17  is responsible for creating and deleting virtual machine clones, as described below. The common base portion also performs functions such as virtualizing memory, virtualizing I/O devices and virtualizing CPU. 
   Guest operating systems  22 ,  24  and  26  execute on user portions  12 ,  14  and  16 , respectively, and applications  32 ,  34  and  36  execute on guest operating systems  22 ,  24  and  26  respectively. There may be multiple applications executing on each operating system. By way of example, guest operating systems  22  and  24  are the Linux (TM of Linus Torvalds) operating system and operating system  26  is an IBM CMS operating system. Other guest operating systems executing on user portions are also feasible such as Microsoft Windows™ operating system, Unix™ operating system, Sun Microsystems Solaris™ operating system or Hewlett Packard HP UX operating system. By way of example, applications  32 ,  34  and  36  can be IBM DB2 data base management application, IBM WebSphere application, communications applications, etc. The nature of applications  32 ,  34  and  36  form no part of the present invention, except that they may generate work items. 
   Each virtual machine has its own private memory for its private data, applications and operating system functions such as Work Queue Assignment Functions  62 ,  64  and  66  (“WQAFs”) and work schedulers  42 ,  44  and  46  in user virtual machines  12 ,  14  and  16 , respectively. Consequently, each virtual machine is afforded a measure of privacy from the other virtual partitions as in separate physical computers. The logical partition between virtual machines is also provided by the allocation of a share of real CPU, a share of real I/O and virtual private memory to each virtual machine. A share of real CPU is a time share of the total system&#39;s real CPU(s). The CPU share appears to the guest operating system as its own CPU. Likewise, a share of real I/O is a time share of the system&#39;s total real I/O capability. The I/O resources comprise the processing power devoted to I/O, e.g. “Channels” on an IBM zSeries mainframe. Virtual memory is a series of virtual addresses assigned to a virtual machine, which are translated by CP into real addresses of real memory. As explained in more detail below, each WQAF  62 ,  64  and  66  assigns each work item created by its own virtual machine to a proper location in its respective work queue  52 ,  54  or  56  in shared memory based on its assignment algorithm. Examples of work items are to read or write data, execute an application, make a request to an application, etc. The work items are initiated by a user of the application and passed via the application to the guest operating system for handling. The assignment algorithm may be based on priority level of each work item, and/or first in first out, etc. If the assignment algorithm is simply first in first out, then the WQAF assigns each new work item to the beginning of the work queue, so it is last to be removed. If the assignment algorithm is based on priority level, then the WQAF assigns each new work item to a position within the work queue before other work items of lower priority and after work items of the same priority (to prevent work items from becoming stale) or higher priority. Each WQAF also monitors and updates a status of the respective virtual machine as “idle” or “not idle” as described below. Each scheduler schedules the execution of work items from its virtual machines&#39; work queue, and generally removes work items from the end of the work queue. 
   Computer  10  also includes a memory area  25  which is shared by all of the virtual machines  12 ,  14  and  16 . Being “shared” each virtual machine can directly access the shared memory  25  and the data and data structures (including lock structures) stored in the shared memory by appropriate address, when it knows the address. The work queues  52 ,  54  and  56  for the WQAFs  62 ,  64  and  66  and respective schedulers  42 ,  44  and  46  are located in shared memory (even though the WQAFs and schedulers are all in the private memory of the respective virtual machines). Consequently, each WQAF can access all the work queues to add a work item to any of the work queues, when it knows the address of the work queues. In the preferred embodiment of the present invention, each WQAF is programmed to add a work item only to the work queue dedicated to its virtual machine and its clones, and each scheduler is programmed to remove work items only from the work queue dedicated to its virtual machine and its clones. Work queue  52  is dedicated to virtual machine  12  and its clones, work queue  54  is dedicated to virtual machine  14  and its clones, and work queue  56  is dedicated to virtual machine  16  and its clones. 
   In the state illustrated in  FIG. 1 , work queue  52  has three work items  70 ,  71  and  73  acquired from virtual machine  12  by action of WQAF  62 , work queue  54  is empty, and work queue  56  has three work items  72 ,  74  and  76  acquired from virtual machine  16 . Also in the state illustrated in  FIG. 1 , virtual machine  12  will acquire work item  73  from work queue  52  by action of scheduler  42 , and virtual machine  16  will acquire work item  76  from work queue  56  by action of scheduler  46 . A control block  58  indicates the current state, “idle” or “non idle”, of the virtual machines and which, if any, of the work items from each work queue is the next to be scheduled. In the state illustrated in  FIG. 1 , virtual machine  12  is idle with its work queue pointer in scheduler  42  pointing to work item  73 . Also in the state illustrated in  FIG. 1 , virtual machine  14  is idle with its work queue pointer in scheduler  44  indicating “null” because there are currently no work items in work queue  54 . Also in the state illustrated in  FIG. 1 , virtual machine  16  is “not idle”; it is currently performing a work item previously acquired from one of the work queues. The work queue pointer of scheduler  46  within virtual machine  16  is currently indicating work item  76 , so this work item has not yet been removed by scheduler  46  for execution by virtual machine  16 . 
   After each virtual machine completes a work item or receives an interrupt, it alerts its scheduler to checks control block  58  to determine if the respective work queue contains a work item. If so, the scheduler can remove the work item indicated by the respective work queue pointer, parse it to determine the appropriate function within the virtual machine for handling, and then pass it to that function for handling. Some of the work items are CPU intensive, others are I/O device intensive and still others require substantial private memory of the virtual machine. Conversely, some of the work items require little CPU activity, others require little or no I/O activity and still others require little private memory of the virtual machine. It is often the case that work items executed during certain periods of time are virtual resource intensive and work items executed during other periods of time require little virtual resources. For example, if virtual machine  12  is executing an application that gets heavy use at a certain time of day, then virtual machine  12  may require substantial virtual resources then. Conversely, the same application may be substantially idle at another time of day and require little virtual resources then. In the state illustrated in  FIG. 1 , there are four virtual machines, and by default, each has been allocated twenty five percent of the total virtual resources available. 
   A resource monitor function  84  within the common base portion routinely monitors the utilization level of each virtual and real resource by each virtual machine and the system&#39;s total real resource utilization. Resource monitor  84  stores these levels in memory as usage records  85 . The real CPU utilization of each virtual machine is measured by how much processor time each virtual machine uses when it is operated. The real I/O utilization by each virtual machine is measured by counting the number of I/O accesses and the time utilized during each such access. The virtual private memory utilization is measured by counting the number of pages referenced by a virtual machine during a time period. A resource manager function  217  within virtual machine  17  monitors the level of virtual and real resources being used by each of the user virtual machines  12 ,  14  and  16  by reading records  85 . As described in more detail below, resource manager  217  compares these usage levels to needs by the virtual machines for virtual resources and soft limits set by the system administrator for virtual and real resources for each virtual machine. 
     FIG. 2  illustrates the state of computer system  10  a short time after that of  FIG. 1 . The following is a general explanation of how computer system  10  changed from the state of  FIG. 1  to the state of  FIG. 2 . Between states, the resource manager  217  determined the current virtual and real resource utilization of virtual machine  12  from the usage records  85  (step  83 ). The virtual and real resource utilization comprises the current amount of real CPU utilization, current amount of real I/O utilization and current amount of virtual private memory utilization. In step  83 , the resource manager  217  determined that virtual machine  12  was entitled to or should get more virtual and real resources than currently has, i.e. more than the (default) one quarter of the total virtual and real resources available. The need for additional virtual resources by virtual machine  12  can be based on additional need for one or more of the virtual or real resources. According to the present invention, resource manager  217  then created a single virtual machine clone  12 A of virtual machine  12 . (Virtual machine clone  12 B was subsequently created as described below.) Resource manager  217  created virtual machine clone  12 A by calling an activate function  86  within the common base portion, specifying the name or identifier of virtual machine  12  and requesting that another virtual machine identical to virtual machine  12  be created (step  87 ). Activate function  86  created virtual machine clone  12 A by reading the directory  191  to determine the guest operating system of virtual machine  12  and the attributes of virtual machine  12 , i.e. virtual and real resource allocation and operating privileges. Guest operating system  22  within virtual machine  12  includes a list of the applications currently running on virtual machine  12  and how to fetch and start a copy of them. So, when virtual machine  12 A begins operating, it fetches and starts a copy  32 A of application  32 . With the default of equal allocation of virtual resources amongst all virtual machines, virtual machines  12  and  12 A together have forty percent of the total virtual resources available, virtual machine  14  has twenty percent, virtual machine  16  has twenty percent and virtual machine  17  has twenty percent. 
   To collectively utilize the virtual resources of virtual machine  12  and its virtual machine clone  12 A, the resource manager  212  grants to the virtual machine clone  12 A access to work queue  52  (step  89 ). This access is “granted” by the resource manager  212  furnishing to the virtual machine clone  12 A an authorization to access a portion or segment of the shared memory containing the work queue  52  of virtual machine  12 . The beginning of the shared memory segment may contain the address of the shared work queue  52  and control block  58 , or the resource manager can provide these addresses separately to the WQAF  62 A and the scheduler  42 A. The shared access by virtual machines  12  and  12 A to work queue  52  also requires possession of a lock  90  described in more detail below with reference to  FIGS. 3 and 4 . 
   In one embodiment of the present invention, all the work items are created by users of application  32  on virtual machine  12  and not by users of application  32 A on virtual machine  12 A, i.e. no users were assigned to application  32 A. In such a case, WQAF  62 A does not have any work items to assign to the work queue  52 . However, scheduler  42 A obtains work items from the work queue  52  along with scheduler  42 , so that the virtual resources of both virtual machines  12  and  12 A are used to perform the work items on work queue  52 . In this embodiment, clone  12 A need not include a copy of application  32  if the copy is not needed to perform the work items generated by application  32  on virtual machine  12 . Consider now an alternate embodiment of the present invention where the work items are created by users of application  32  on virtual machine  12  and users of application  32 A on virtual machine  12 A. In this case, the users of application  32 A could have been reassigned from application  32  on virtual machine  12  or new users assigned to application  32 A. In either case, both WQAFs  62  and  62 A have work items to assign to the work queue  52 , and both schedulers  42  and  42 A obtain work items from the work queue  52 . So, the virtual resources of both virtual machines  12  and  12 A are used to perform the work items on work queue  52 . (In this alternate embodiment, it is also possible to create a separate work queue for virtual machine  12 A, so that virtual machine  12 A does not share work queue  52 .) 
   Virtual machine  14  and its interaction with work queue  54  remains unchanged by the creation of the virtual machine clone  12 A except for the diminished virtual and real resources available to virtual machine  14  to execute the work items on work queue  54 . Likewise, virtual machine  16  and its interaction with work queue  56  remains unchanged by the creation of the virtual machine clone  12 A except for the diminished virtual and real resources available to virtual machine  16  to execute the work items on work queue  56 . 
   Then, resource manager  212  repeated the foregoing analysis and determined that virtual machines  12  and  12 A still have insufficient resources. So, resource manager  212  created another virtual machine clone  12 B including clone application  32 B. Consequently, virtual machine  12  and its virtual machine clones  12 A and  12 B together have fifty percent of the virtual and real resource total, virtual machine  14  has 16.7 percent of the virtual and real resource total, virtual machine  16  has 16.7 percent of the virtual and real resource total, and virtual machine  17  has 16.7 percent of the virtual and real resource total. To collectively utilize the virtual and real resources of virtual machine  12  and its virtual machine clones  12 A and  12 B, the resource manager  212  now grants to virtual machine clone  12 B access to work queue  52  (step  89 ). (Virtual machine clone  12 A retains its earlier granted access to work queue  52 .) This access to virtual machine clone  12 B is “granted” by the resource manager  212  furnishing to the virtual machine clone  12 B an authorization to access a portion or segment of the shared memory containing the work queue  52  of virtual machine  12 . The beginning of the shared memory segment may contain the address of the shared work queue  52  and control block  58 , or the resource manager can provide these addresses separately to the WQAF  62 B and the scheduler  42 B. The shared access by virtual machines  12 ,  12 A and  12 B to work queue  52  also requires possession of the lock  90  described in more detail below. 
   In one embodiment of the present invention, all the work items are created by users of application  32  on virtual machine  12  and not by users of applications  32 A or  32 B on virtual machines  12 A and  12 B, respectively. In such a case, WQAFs  62 A and  62 B do not have any work items to assign to the work queue  52 . However, schedulers  42 A and  42 B obtain work items from the work queue  52 . In the state illustrated in  FIG. 2 , virtual machine  12  obtains and executes work item  73 , virtual machine  12 A obtains and executes work item  71  and virtual machine  12 B obtains and executes work item  70 . Thus, the virtual and real resources of all three virtual machines  12 ,  12 A and  12 B are collectively used to execute the work items on work queue  52 . In an alternate embodiment of the present invention, the work items are created by users of application  32  on virtual machine  12 , users of application  32 A on virtual machine  12 A and users of application  32 B on virtual machine  12 B. The users of applications  32 A and  32 B could have been reassigned from application  32  on virtual machine  12  or they can be new users. In such a case, WQAFs  62 ,  62 A and  62 B all have work items to assign to the work queue  52 , and schedulers  42 ,  42 A and  42 B all obtain work items from the work queue  52 . So, the virtual and real resources of virtual machines  12 ,  12 A and  12 B are all used to perform the work items on work queue  52 . (In this alternate embodiment, it is also possible to create a separate work queue for virtual machine  12 A and a separate work queue for virtual machine  12 B, so that virtual machines  12 A and  12 B do not share work queue  52 .) 
   Virtual machine  14  and its interaction with work queue  54  remains unchanged by the creation of the virtual machine clones  12 A and  12 B except for the diminished virtual and real resources available to virtual machine  14  to execute the work items on work queue  54 . Likewise, virtual machine  16  and its interaction with work queue  56  remains unchanged by the creation of the virtual machine clones  12 A and  12 B except for the diminished virtual and real resources available to virtual machine  16  to execute the work items on work queue  56 . 
   Other distributions of the virtual and real resources are available depending on which virtual machines are cloned, and how many clones are created. The resource manager periodically determines the virtual and real resource utilization of the virtual machines, and the need to create or delete a clone. 
     FIG. 3  figuratively illustrates a synchronization or lock structure generally designated  90  within the shared memory  25  of computer system  10 . A lock is required for any work queue which is shared by more than one virtual machine. This will be the case when a virtual machine has one or more clones which share a work queue, such as work queue  52  shared by virtual machines  12 ,  12 A and  12 B illustrated in  FIG. 2 . When there are no clones for a virtual machine, then the lock structure can be bypassed or the virtual machine can continuously hold the lock. ( FIG. 3  does not illustrate virtual machines  14  or  16  or their work queues  54  or  56 , respectively.) In the illustrated example, virtual machine  12 A holds lock  91 , virtual machine  12  has a place holder  92  waiting for the lock from virtual machine  12 A, and virtual machine  12 B has a place holder  93  waiting for the lock from virtual machine  12 . This is actually recorded in control block  58  which indicates that virtual machine  12 A holds the lock and virtual machines  12  and  12 B are currently waiting for the lock. The “waiter list”  95  of control block  58  indicates the order of the waiters, i.e. virtual machine  12  is first in line waiting for the lock and virtual machine  12 B will attempt to obtain the lock after virtual machine  12  obtains the lock. In the example, virtual machine  12 A holds lock  91  exclusively, that is, no other virtual machine may concurrently hold this lock. Virtual machine  12  and  12 B are waiting for the lock and willing to hold the lock shared, that is, they may concurrently hold the lock with each other. 
     FIG. 4  illustrates each of the synchronization or lock functions  562 ,  562 A and  562 B within virtual machines  12 ,  12 A and  12 B, respectively, and associated operation of each of the virtual machines that is trying to obtain the lock for the shared work queue  52  in shared memory. In the following example, a virtual machine (such as virtual machine  12 B) requests a lock for the shared work queue  52  (step  700 ). If the virtual machine just wants to read the shared work queue, then the virtual machine need only request a shared lock. However, if the virtual machine wants to remove a work item from the shared work queue (which is more typically the case), then the virtual machine will request an exclusive lock. Typically, the virtual machine will want to remove a work item from the work queue, so will request an exclusive lock for this objective and decision  702  leads to decision  704 . In decision  704 , the synchronization function determines if the requested lock is currently held by another virtual machine (either in a shared or exclusive manner). If so, the exclusive lock is not available to the current requester, and synchronization function updates the control block to indicate that the requesting virtual machine is idle and “waiting” for the exclusive lock (step  706 ). Also, the virtual machine enters a waiting/quiescent state (step  708 ) where it waits for an interrupt (decision  710 ). Referring again to decision  704 , if the requested lock is not currently held by anyone, then the synchronization function marks the virtual machine as “not idle” (step  712 ) and grants the lock to the virtual machine (step  714 ). This granting of the lock is accomplished by corresponding update to control block  58 . Next, the virtual machine removes the next work item from the shared work queue  52  and performs the task indicated by the work item (step  716 ). Afterwards, the virtual machine notifies the synchronization function that it has performed the work item, and it is “releasing” the lock (step  718 ). In response, the synchronization function updates the control block  58  to indicate that the lock has been released. Also, the synchronization function determines from the control block  58  if any other virtual machine is currently waiting for the lock (decision  720 ). If not, the processing of the synchronization function is complete (step  722 ). If so, the synchronization function determines from the control block  58  if the first virtual machine marked “waiting” is “idle” (decision  724 ). If not, the processing of the synchronization function is complete (step  722 ) because it would be too disruptive of the non idle, waiting virtual machine to be interrupted at this time. Instead, when the non idle, waiting virtual machine completes its current work item or at some other time that is convenient for the waiting virtual machine, it will likely request the lock on its own at step  700 . Referring again to decision  724 , if the waiting virtual machine is idle, then the synchronization function issues an interrupt to the waiting virtual machine (step  728 ). This will not be wasteful to the waiting virtual machine because it is idle anyway. After receiving the interrupt, the idle virtual machine will awaken and can request the lock at step  700 . 
   Referring back again to decision  702 , if the lock requested by the virtual machine is shared and not exclusive, such as to read the shared work queue, then the synchronization function determines if the lock is currently being held in an exclusive manner (decision  740 ). If not (i.e. no lock is currently being held or only a shared lock is currently being held), then the synchronization function proceeds to step  712  and continues as described above. However, if the lock is currently being held in an exclusive manner, then the synchronization function marks in the control block  58  that the requesting virtual machine as “idle” and “waiting” for a shared lock (step  742 ). Then, the requesting virtual machine enters into a waiting/quiescent state (step  744 ), waiting for an interrupt (decision  748 ). Upon receipt of such an interrupt, it can proceed to step  700  to request the lock. 
     FIGS. 5(   a ) and  5 ( b ) illustrate the foregoing operation of resource manager  217  in more detail. The resource manager  217  performs the steps of  FIGS. 5(   a ) and  5 ( b ) for each user virtual machine  12 ,  14  and  16  to determine the need to create a clone of the virtual machine or delete a clone of the virtual machine. Initially, the resource manager  217  sets new soft limits for the real CPU, the virtual private memory and the real I/O allocated to the virtual machine or reads the original ones from the directory  191  if available there (step  100 ). The real CPU allocation for a virtual machine is the amount of CPU processing time available to the virtual machine. The virtual private memory is the amount of private memory allocated to the virtual machine; the addresses assigned for this private memory are mapped to the real memory. The real I/O is the amount of real I/O bandwidth available to the virtual machine. Next, the resource manager fetches from the usage records  85  the current utilization levels of the foregoing resources by the virtual machines (step  102 ). As noted above, the common base portion periodically monitors these virtual and real utilization levels for the resources. Then, the resource manager reads the real CPU utilization level of the virtual machine (step  104 ). Next, the resource manager determines if the current real CPU utilization by the virtual machine is more than its soft limit (decision  106 ). If not, then the resource manager determines if the virtual machine needs additional virtual resources (decision  107 ). This determination is made by monitoring the workload of the virtual machine each time it has a time slice of the real CPU. If the virtual machine usually or always has outstanding work to complete when it has access to the real CPU, then it probably needs a greater share of the real CPU. If so, then the resource manager creates a clone of the virtual machine in the manner described above (step  108 ). Also, the resource manager gives the clone access to the shared work queue in the manner described above (step  1110 ). 
   Referring again to decision  106 , if the current real CPU utilization level of the virtual machine is equal or more than the soft limit, then the resource manager determines if the total, real CPU for the system is under utilized (decision  112 ). If not, then it is likely that one or more other virtual machines are “starved” for the CPU and it is fair to give additional real CPU resource to the other virtual machines and take some back from the virtual machine currently under review. So the resource manager will proceed to delete a clone of the virtual machine as follows. The resource manager determines if the virtual machine (currently under review) is currently performing a work item (decision  114 ). If so, the resource manager waits for the virtual machine to complete its current work item (step  116 ). If not or after the resource manager completes its current work item, the resource manager deletes a clone of the virtual machine, assuming one exists (step  120 ). 
   Referring again to decision  112 , if the total, real CPU for the system is under utilized, then it is not necessary to delete a clone of the virtual machine; the other virtual machines should not be starved. However, it may be necessary to delete a clone to free up other real or virtual resources. So, the resource manager checks the virtual, private-memory utilization of the virtual machine (step  130 ). (Referring again to decision  107 , if the virtual machine does not need real CPU, then the resource manager also proceeds to step  130 .) If the virtual private-memory utilization of the virtual machine is one hundred percent of the allocation (decision  132 ), then the resource manager proceeds to step  108  as described above to create a clone of the virtual machine. However, if the virtual private-memory utilization is less than one hundred percent, then the resource manager compares the utilization level to the soft limit for virtual private memory (decision  136 ). If the current, virtual, private-memory utilization is over the soft limit (decision  138 ), then the resource manager proceeds to decision  114  and then to step  120  as described above to delete a clone of the virtual machine, if one currently exists. Referring again to decision  136 , if the current virtual private-memory utilization is not over the soft limit, then the resource manager determines if the virtual machine needs additional private virtual memory (step  140 ). This determination is made by monitoring the amount of paging required for this virtual machine. “Paging” occurs when a virtual machine has inadequate virtual private memory and must write its excess data out to disk storage. If the virtual machine needs additional private memory, then the resource manager proceeds to step  108  to create a clone. If not, then the resource manager checks the total, real I/O bandwidth for the system (step  144 ). Then, the resource manager checks the current real I/O utilization by the virtual machine (step  146 ). If the current real I/O utilization by the virtual machine is less than the soft limit (decision  148 ), then the resource manager proceeds to step  108  to create a clone. If not, the resource manager determines if the total, real I/O for the system is under utilized (decision  150 ). If not, then the resource manager proceeds to decision  114  and step  120  to delete a clone, if one exists. If so, then the resource manager loops back to step  100  to repeat the foregoing process. 
   Based on the foregoing, a computer system embodying the present invention has been disclosed. However, numerous modifications and substitutions can be made without deviating from the scope of the present invention. For example, logical partitions could be substituted for the virtual machines. Also, other virtual resource allocation techniques can be combined with the foregoing techniques. For example, a human systems administrator acting through the common base portion can manually change the virtual resource allocation to each virtual machine and the respective soft limits for such virtual machines, including the clones. After such a change, the resource manager would use the new allocations and soft limits to determine when to add or delete a clone. Therefore, the present invention has been disclosed by way of illustration and not limitation, and reference should be made to the following claims to determine the scope of the present invention.