Abstract:
A system and a method dynamically adjusts the quality of service guarantees for virtual servers based upon the resource demands experienced by the virtual servers. Virtual server resource denials are monitored to determine if a virtual server is overloaded based upon the resource denials. Virtual server resources are modified dynamically to respond to the changing resource requirements of each virtual server. Occasionally, a physical host housing a virtual server may not have additional resources to allocate to a virtual server requiring increased resources. In this instance, a virtual server hosted by the overloaded physical host is transferred to another physical host with sufficient resources.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is related to U.S. patent Ser. No. 09/499,098, entitled “Selective Interception of System Calls,” by Borislav D. Deianov et al., filed Feb. 4, 2000, now U.S. Pat. No. 6,546,546 and commonly assigned with the present application. The subject matter of this related application is incorporated by reference herein in its entirety. 
   BACKGROUND 
   1. Field of Invention 
   The present invention relates generally to resource allocation for a virtual server, and more particularly, to monitoring and dynamically modifying the resource allocation for a virtual server based upon usage. 
   2. Background of the Invention 
   Networked computer resources are growing more popular as the benefits of sharing computing resources become evident. One of the fastest-growing segments of the Internet is the network market. Network systems contain common elements, generally including a dedicated local server to maintain the shared network data, and a communications system for providing data communication services between devices on the network. Data communications services and servers are not easy to configure, manage, and maintain. Thus, there is an incentive for Internet Service Providers (ISPs) to provide such network services and servers, thereby relieving corporations of the burden of providing these services directly. 
   It is not economically feasible for an ISP to remotely manage servers located on a customer&#39;s premises, and support many different customers in this fashion. Rather, an ISP would prefer to offer network services to multiple customers while keeping all of the server host computers within a central location of the ISP for ease of management. Accordingly, ISPs typically dedicate one or more physical host computers as each individual customer&#39;s server(s), and maintain each host computer in the centralized facility. This means the ISP will have to own and maintain potentially large numbers of physical host computers, at least one for each customer&#39;s server or private network. However, most customers will neither require nor be amenable to paying for the user of an entire host computer. Generally, only a fraction of the processing power, storage, and other resources of a host computer will be required to meet the needs of an individual customer. 
   Different customers have different virtual server needs. For example, a company A providing large quantities of data and information to its employees and customers will want to ensure that its virtual servers are always available to perform a large number of tasks. Company A may be willing to pay a premium for a guaranteed high quality of service, with high server availability and large amounts of processing power always on-call. By contrast, a small individual B who merely uses his virtual server for back-up file storage space has very different quality of service requirements. Customer B needs (and wishes to pay for) only a limited amount of storage space to be available on an intermittent basis. 
   When servicing the needs of multiple customers having different needs, it is desirable to provide a virtual server that is dynamic, not static, in its allocation of resources. A customer&#39;s virtual server is typically assigned a fixed level of resources, corresponding to either a fixed percentage of the capacity of a particular physical host (for example, the operating system may be instructed to allocate twenty percent of the central processing unit cycles to process A and two percent to process B) or a fixed number of units (for example, the operating system may be instructed to allocate X cycles per second to process A and Y cycles per second to process B). However, customers may be unable to anticipate the exact amount of resources they will require, and a static assignment of a particular resource allocation limit may not allow the virtual server system to adapt to changing customer needs. 
   Instead of requiring customers to select a static level of resources, a better resource allocation model is structured along the lines of electricity pricing—a customer receives what he needs, and he pays for what he receives. Referring back to a previous example, small customer B may initially request a very low level of resources. However, should his new home business suddenly expand, he may quickly bump up against the limit of the server resources he originally requested. In this case, it would be preferable if customer B&#39;s virtual server resources were able to automatically, dynamically adjust to his increased resource needs. 
   Thus it is desirable to provide a system and method for a virtual server capable of providing quality of service guarantees for a customer, which is also capable of adjusting the quality of service based upon changing customer demand. It is desirable for such a system to dynamically adjust the physical host resources allocated to a virtual server. 
   SUMMARY OF THE INVENTION 
   The present invention dynamically adjusts the quality of service guarantees for virtual servers based upon the resource demands experienced by the virtual servers. Virtual servers having individual quality of service guarantees are distributed among a group of physical hosts. Each physical host&#39;s resources are allocated among the physical host&#39;s resident virtual servers. The resources allocated to a particular virtual server may be dynamically adjusted in response to changing virtual server resource needs. 
   Occasionally, a physical host executing a virtual server may not have additional resources to allocate to a virtual server requiring increased resources. In this instance, a virtual server hosted by the overloaded physical host is transferred to another physical host with sufficient resources. 
   In one embodiment, a dynamic resource configuration module monitors resource denials received by virtual servers and determines if a virtual server is overloaded based upon the resource denials. A resource denial may refer to any request by the virtual server that cannot be immediately serviced, such as a denial of a request to create a file or a network packet delay. If the resource denials received by a particular virtual server exceed a pre-specified limit, the virtual server is considered overloaded and a request is made for additional resources. 
   The resource usage of the physical hosts within the system is monitored. A load-balancing function is performed to select the appropriate physical host when a virtual server transfer becomes necessary. A virtual server is transferred between physical hosts with minimal impact upon the operation of the virtual server. 
   The features and advantages described in the specification are not all-inclusive, and particularly, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims hereof. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and may not have been selected to delineate or circumscribe the inventive subject matter, resort to the claims being necessary to determine such inventive subject matter. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an illustration of a system for dynamically modifying the resources of a virtual server. 
       FIG. 2A  is a flowchart of a process for dynamically modifying the resources of a virtual server. 
       FIG. 2B  is a flowchart of another process for dynamically modifying the resources of a virtual server. 
       FIG. 3  is a block diagram of a process for determining whether an individual resource in a virtual server has reached its limit. 
       FIG. 4  is a flowchart of a process for determining when to increase or decrease a virtual server resource allocation. 
       FIG. 5  is a block diagram of one process for performing resource load balancing among physical hosts. 
       FIG. 6  is a flowchart of one process for transferring a virtual server from one physical host to another physical host. 
   

   The figures depict a preferred embodiment of the present invention for purposes of illustration only. One skilled in the art will readily recognize from the following discussion that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the invention described herein. 
   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Reference will now be made in detail to several embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Wherever practicable, the same reference numbers will be used throughout the drawings to refer to the same or like parts. The term “virtual server” as used herein refers to a virtual server capable of receiving a quality of service guarantee from a physical host. Multiple virtual servers may reside in a single physical host, and different virtual servers on the same physical host may receive different quality of service guarantee. 
     FIG. 1  shows an embodiment of a system for dynamic resource configuration in virtual servers. A dynamic resource configuration module  100  is coupled via a network to a group of physical host machines  160  ( 160 A,  160 B and  160 C), or may be resident on any of these hosts  160 . The physical host machines  160  may be any kind of computer adapted to support virtual servers. The module  100  may be implemented in a software driver. It is to be understood that the dynamic resource configuration module  100  will typically support more than one physical host machine  160 . However, in one embodiment, the dynamic resource configuration module  100  may support a single physical host  160 . 
   The group of physical hosts  160  contains a group of virtual servers  162 . Physical host  160 A contains virtual servers  162 A and  162 B; physical host  160 B contains virtual servers  162 C,  162 D and  162 E; and physical host  160 C contains virtual servers  162 F and  162 G. 
   In one embodiment, each individual virtual server  162  has a different quality of service guarantee. Different quality of service guarantees are implemented by allocating different amounts of the resources of each physical host machine  160  to servicing each of the virtual servers  162 . Physical host  160  resources may be allocated as percentages of the resources of a particular physical host  160 , or as a particular number of units within a physical host  160  (for example, the operating system may be instructed to allocate X cycles per second to process A and Y cycles per second to process B). In the embodiment shown in  FIG. 1 , physical host  160  resources are allocated to individual virtual servers  162  as percentages of each physical host  160 . Table 1 lists the resource allocations of each virtual server  162  as shown in  FIG. 1 : 
   
     
       
             
           
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               Virtual Server Resource Allocation in FIG. 1 
             
           
        
         
             
                 
               Virtual Server 
               Resource Allocation 
             
             
                 
                 
             
             
                 
               162A 
               15% of physical host 160A 
             
             
                 
               162B 
               60% of physical host 160A 
             
             
                 
               162C 
               10% of physical host 160B 
             
             
                 
               162D 
               10% of physical host 160B 
             
             
                 
               162E 
               10% of physical host 160B 
             
             
                 
               162F 
               20% of physical host 160C 
             
             
                 
               162G 
               30% of physical host 160C 
             
             
                 
                 
             
           
        
       
     
   
   The virtual servers  162  each may consume a different amount of the resources of the physical host machines  160 . The resources of a physical host machine comprise the set of functions and features the physical host machine uses in implementing tasks for each virtual server. Examples of resources include disk space, memory, network capacity and processing cycles (CPU resources). As shown in  FIG. 1 , virtual server  162 A consumes 15% of the physical host  160 A resources. This means that 15% of physical host  160 A&#39;s disk space, memory, network bandwidth, and CPU processing will be dedicated to servicing the needs of virtual server  162 A. A variety of other types of physical host resources will be evident to one of skill in the art. 
   A resource allocation for a virtual server is specified as a “quality of service guarantee” for that particular server. Each physical host stores quality of service guarantees for the virtual servers it hosts. As a physical host performs processes associated with a particular virtual server, the physical host accesses the stored quality of service information to enable the physical host to request the correct quality of service from the operating system kernel of the physical host. 
   One implementation for storing quality of service guarantee information is a quality of service parameter table. A quality of service parameter table in each physical host  160  associates each virtual server  162  resident in the particular physical host  160  with quality of service parameters. These parameters are used to allocate physical host  160  resources for each resident virtual server  162 . For example, physical host  160 A includes a quality of service parameter table, which lists resident virtual servers  162 A and  162 B. The parameter table lists whatever virtual servers are resident in the physical host. As virtual server resource allocations are changed, and as virtual servers are transferred between physical hosts, the corresponding quality of service parameter tables are updated to reflect these changes and transfers. In another embodiment, a single master quality of service parameter table can coordinate multiple slave tables associated with each physical host. 
   Dynamic resource configuration module  100  includes a virtual server resource monitor  110 , a virtual server resource modifier  120 , a physical host load balancer  130 , a dynamic virtual server mover  140 , and a file system  150 . In one embodiment, these modules are portions of the software code implementing the dynamic resource configuration module  100 . The dynamic resource configuration module  100  is further communicatively coupled to each physical host  160 . 
   The virtual server resource monitor  110  monitors the resource usage of the virtual servers  162  to determine if they are overloaded. The virtual server resource modifier  120  dynamically modifies the resource allocations of the virtual servers  162  on an as-needed basis. The physical host load-balancer  130  periodically monitors the resource usage of the physical hosts  160 , and uses the dynamic virtual server mover  140  to transfer virtual servers  162  between physical hosts  160  as needed to balance the loads of the physical hosts  160 . The file system  150  is used for storing state information associated with a particular virtual server  162  when transferring the particular virtual server  162  to a different physical host  160 . In another embodiment, the file system  150  is not used, and state information is copied directly from one physical host to another physical host to transfer a virtual server. 
     FIG. 2A  is a flowchart of an embodiment of the overall process for dynamically modifying the resources of a virtual server. Virtual server resource denials are monitored  210 . Resource monitoring is performed using the selective interception of system calls. One embodiment of selectively intercepting system calls is disclosed in the related application, the subject matter of which is incorporated herein by reference. Each resource (e.g., disk space, memory, network bandwidth, or CPU cycles) used by a virtual server is monitored to determine the time at which the resource is fully used, that is, the point at which a request for more resources is either implicitly or explicitly denied. Examples of resource denials include a memory allocation request denial and a network packet delay signal. 
   A determination is made  220  as to whether a particular virtual server resource is overloaded. The number of times a particular resource denial is received in a time window is averaged using one of a number of well-known techniques. If the average number of denials is beyond a pre-configured threshold, the virtual server is determined  220  to be overloaded for the corresponding resource. If the virtual server is not determined to be overloaded, the method continues to monitor  210  virtual server resource denials. 
   If the virtual server is determined to be overloaded, a determination is made  230  as to whether the corresponding resource of the physical host hosting the virtual server resource is also overloaded. For example, referring to  FIG. 1 , if it was determined that a resource for virtual server  162 B was overloaded, module  100  would then check to see if that same resource was overloaded for physical host  160 A which contains virtual server  162 B. A physical host  160  resource is determined to be overloaded if the physical host  160  does not have enough of the particular resource unallocated to the resident individual virtual servers  162  to service the resource increase request. The physical host resource is overloaded if:
     Resource request&gt;Resource available; where Resource available≧0   

   For example, assume virtual server  162 B requests an additional memory allocation of 1 megabyte. If physical host  160 A has only 100 kilobytes of memory available (the rest already having been allocated to virtual servers  162 A and  162 B), then physical host  160 A cannot service virtual server  162 B&#39;s request and physical host  160 A is considered overloaded. This same principle may be extended to other types of resources. 
   If the particular physical host resource is not determined to be overloaded the virtual server resource allocation within the physical host is increased  240 . The method then continues to monitor  210  virtual server resource denials. 
   However, if the physical host is determined to be overloaded, a new physical host is selected  250  to accommodate the overloaded virtual server and its required resource increases. A variety of different fitting heuristic methods may be used to select a new physical host to execute the virtual server. For example, a first fit method may be used, wherein the first physical host  160  determined to have enough extra resources to accommodate the overloaded virtual server  162  is selected. In a best fit method, the physical host  160  with available resources most closely matching the resource needs of the overloaded virtual server  162  is selected. In an easiest fit method, the physical host  160  with the most available resources is selected to accommodate the overloaded virtual server  162 . For the following discussion, assume that physical host  160 A is overloaded, and new physical host  160 B has been selected to receive virtual server  162 B. 
   Once the new physical host  160 B has been selected  250 , the virtual server  162 B is moved  260  to the new physical host  160 B. The virtual server  162 B is also allocated its required resource increase. In one embodiment, the old overloaded physical host  160 A places state information for the virtual server  162 B being transferred into a common file system  150 , e.g. in a configuration file or other system file. The new physical host  160 B accesses the state information and restarts the virtual server  162 B as resident in the new physical host  160 B. In another embodiment, the virtual server  162 B files are copied directly from the old physical host  160 A to the new physical host  160 B. 
   Once the virtual server information transfer is complete, the old physical host  160 A has one fewer virtual server, and the new physical host  160 B has one additional virtual server. The quality of service tables for both the old and new physical hosts are modified  260  to reflect this change. The quality of service table entries for virtual server  162 B will also reflect the virtual server&#39;s resource increase. 
   The virtual server user is transferred  270  from the old physical host ( 160 A) to the new physical host ( 160 B) by transferring the virtual server address. The transfer process may use either “break, then make” timing, or “make, then break” timing. The timing of the transfer process determines whether all processes and configuration information associated with the virtual server to be transferred are first shut down in the old physical host, or first started up in the new physical host, before the virtual server address is transferred. Transferring the virtual server address transfers the virtual server user from one virtual server location to another. For example, using “break, then make” timing, the virtual server  162 B is first shut down in the old physical host  160 A, a new virtual server is created in new physical host  160 B and started up, and the virtual server  162 B address is then transferred over to the new physical host  160 B. In another embodiment using “make, then break” timing, a new virtual server is created in new physical host  160 B and started up, the virtual server  162 B address is transferred over to the new physical host  160 B, and the virtual server  162 B is then shut down in old physical host  160 A. 
   As used herein, the terms “customer,” “user,” and “virtual server user” refer to individuals or groups of individuals accessing the same virtual server. Typically, a virtual server “user” is a group of individuals with a shared association. For example, “user” may collectively refer to the employees of a company, or to certain employees within a division of a company. One company (a “customer”) may have several different users, each corresponding to a different group within the company, and each having many different individuals. Additionally, a “user” may also refer to a single individual. 
   The process for virtual server resource configuration is dynamic and ongoing during the operation of the virtual servers. After the virtual server user transfer  270  is completed, the process continues to monitor  210  virtual server resource denials. 
     FIG. 2B  is another embodiment of a flowchart of the process for dynamically modifying the resources of a virtual server. The method shown in  FIG. 2B  is similar to the method shown in  FIG. 2A . However, the method of  FIG. 2B  includes three additional steps, steps  242 ,  244  and  246 , which together provide a method for reclaiming unused virtual server resources. 
   As before, virtual server resource denials are monitored  210 . If a determination  220  is made that a particular virtual server resource is overloaded, and a determination  230  is made that the corresponding physical host resources are not overloaded, the virtual server resource allocation is increased  240 . 
   Next, a timer is set  242  for a pre-specified interval. Upon timer expiry, the method determines  244  whether the newly increased virtual server resource is currently operating at its resource limit. If one or more resource denial signals corresponding to the newly increased virtual server resource are received during the timer period, the virtual server is assumed to be operating at its resource limit. 
   If the virtual server is determined  244  to be operating at its limit for a particular resource, the method continues  210  to monitor resource denials. However, if the virtual server is not operating at its limit for a particular resource, the method decreases  246  the virtual server resource allocation by a pre-specified amount. Steps  242 ,  244 , and  246  allow the dynamic resource configuration module  100  to reclaim unused resources within the virtual server system, by temporarily increasing resources allocated to a virtual server as needed. 
   In another embodiment, a recently transferred virtual server  162  may also allow unused resources to be reclaimed by the virtual server  162 &#39;s new physical host. In this embodiment, step  270  would be followed by steps  242 ,  244  and  246 . 
     FIG. 3  shows an embodiment of one process for determining whether an individual resource in a virtual server has reached its resource limit. A virtual server resource monitor  110  receives a set of input signals  312  from a virtual server  162 B. The virtual server resource monitor  110  processes these signals to determine if any resources from virtual server  162 B are overloaded. If an overloaded resource is found, the virtual server resource monitor  110  sends a “resource overloaded” signal  350  to the virtual server resource modifier  120 . 
   Many different types of input signals  312  may be processed to determine if a resource is overloaded. The virtual server resource monitor  110  monitors different types of resource denials, which are instances wherein a request for additional resources is either implicitly or explicitly denied.  FIG. 3  shows four examples of resource denial signals: a create file denial signal  312 A generated, for example, by a lack of disk space, a memory allocation denial signal  312 B, a network packet delay signal  312 C generated by a lack of network bandwidth, and a central processing unit (CPU) process scheduling delay signal  312 D generated by exceeding CPU usage limits. It is to be understood that there may be many other types of signals indicating an implicit or explicit denial of resources. The examples shown herein are used purely for illustrative purposes. 
   In order to associate resource request denials with a particular virtual server executing in a physical host computer, certain selected system calls are intercepted. For example, not all CPU scheduling within the physical host computer is associated with a virtual server. The monitor  110  must be able to distinguish between resource requests made from virtual servers, and other resource requests. The monitor  110  must also be able to distinguish between resource requests made by different virtual servers within the same physical server. 
   A system call performs some system operation, such as the access of a system hardware or software resource, when the system call is executed. In order to make a system call, arguments are programmatically loaded into specific registers of the central processing unit on which the operating system is executing. One of these arguments identifies the specific system call that is being made. This argument is typically in the form of a number that is an offset into the operating system interrupt vector table, which contains pointers to the actual executable code of the system calls. The other loaded arguments include parameters to be passed to the system call. 
   Once the arguments have been loaded, a software interrupt is generated, signaling to the operating system that a process is requesting execution of a system call. The operating system reads the registers, and executes the requested system call with the specified parameters. The system call executes and performs the desired functionality. If the system call generates a return value, it places the generated return value (or a pointer thereto) in a pre-designated register where it can be accessed by the calling process. 
   In order to intercept a system call, a pointer in an interrupt vector table to a system call is replaced with a pointer to alternative object code to be executed instead of the system call. Then, when the system call is made, the alternative object code will execute instead. The alternative object code is known as a system call wrapper. 
   The method of the related application may be used to selectively intercept system calls such that a system call wrapper only executes when a system call is made by a select process associated with one of the virtual servers being monitored. When a system call is made by a non-select process, the default system call is executed. Furthermore, only certain types of system calls relating to resource allocation, as described above, are selectively intercepted. 
   The system call wrapper for the intercepted system call allows the resource request by a particular virtual server and the resulting response to be monitored. Request denial responses are monitored by the virtual server resource monitor  110 . As will be evident to one of skill in the art, the specific system calls to be monitored will be system-dependent, and may vary based upon the type of operating system and physical server machine being used. 
   Each resource denial signal  312  is input into an individual resource denial table  320  for tracking purposes. Create file denial signals  312 A are recorded in a disk denial table  320 A; memory allocation denial signals  312 B are recorded in a memory denial table  320 B; network packet delay signals  312 C are recorded in a network denial table  320 C; and CPU process scheduling delay signals  312 D are recorded in a CPU denial table  320 D. A calculation  330  is performed on the signals stored in each table to determine the mean number of times a particular resource denial occurs in a pre-specified time window. Different time windows may be specified for each type of resource denial. The calculation of mean resource denials is performed individually for each different type of resource denial being monitored ( 330 A,  330 B,  330 C and  330 D). 
   The mean number of resource denials may be calculated using one of several well-known techniques for averaging a signal rate over a period of time. Each technique determines whether the number of received resource denial signals a received in a particular time window t exceeds a certain threshold T:
 
 a ( t )&gt; T? 
 
   In one embodiment, a “jumping-window” technique is used. The jumping-window technique measures the number of resource denials a received in consecutive windows of time length t. A new time interval t starts immediately after the end of the last time interval t. In another embodiment, a “moving-window” technique is used. The moving-window technique measures the number of resource denials a received in a continuously moving window of time length t. In the moving-windows technique, all windows of time length t are measured. 
   The virtual server resource monitor  110  checks  340  if the metric a(t) calculated is beyond the pre-specified threshold T. This determination is made individually for each type of resource denial signal ( 340 A,  340 B,  340 C and  340 D), and need not be made simultaneously. Each different type of resource denial signal  312  may have a different pre-specified threshold T. 
   If the metric a(t) representing the average resource denial rate does not exceed the threshold T, the method continues to calculate a(t)  330  so that resource denials are continuously monitored. Using the jumping-window technique, after the next consecutive time interval t passes, the method will again check  340  if a(t)&gt;T. Using the moving-windows technique, a continuous loop of steps  330  and  340  is used to measure each continuously-moving window of time t. In another embodiment, a pre-specified schedule for repeating calculating mean resource denials  330  and checking  340  if the threshold T has been exceeded can be established to limit the amount of processing required by the virtual server resource monitor  110 . 
   However, if the metric a(t) does exceed the threshold T, a “resource overloaded” signal is sent  350  to the virtual server resource modifier  120 . Each type of resource denial signal  312  has an associated resource overloaded signal.  FIG. 3  shows four examples of resource overloaded signals: disk resource overloaded signal  350 A, memory resource overloaded signal  350 B, network resource overloaded signal  350 C, and CPU resource overloaded signal  350 D. It is to be understood that there may be many other types of signals indicating an overloaded resource. The examples shown herein are used purely for illustrative purposes. 
     FIG. 4  shows a flowchart of an embodiment of a method for determining when to increase or decrease a particular resource allocation within a virtual server. The virtual server resource modifier  120  performs the method shown in  FIG. 4 . A separate analysis using the method of  FIG. 4  is performed for each type of resource being monitored. 
   The modifier  120  waits  410  to receive a resource overloaded signal  350  from the virtual server resource monitor  110 . When a resource overloaded signal  350  is received, the modifier  120  checks  420  to determine whether the signal  350  falls within a pre-specified “hysteresis time window” H. The hysteresis time window H check  420  damps the modifier  120  system to avoid rapid changes in the system state. For example, in a situation in which a virtual server has overloaded its existing memory resource allocation, the virtual server may attempt to access memory repeatedly before the memory resource allocation is increased. Each memory access attempt may generate a memory resource overloaded signal  350 B. The modifier  120  only needs to respond to one of these signals. The hysteresis time window H check  420  avoids repetitive responses to resource overloaded messages. Thus, the modifier  120  checks  420  whether the most recently received resource overloaded signal  350  (received at T 1 ) is close in time (within the hysteresis time window H) to a previously received resource overloaded signal  350  (received at T 0 ) for a particular resource:
 
 T   1   −T   0   &lt;H? 
 
   If the recent and previous resource overloaded signals have occurred close enough in time to fall within the pre-specified hysteresis time window H, no further action will be taken and the modifier  120  returns and waits  410  to receive another resource overloaded signal  350 . If the current resource overloaded message is not received within the hysteresis time window H, the modifier  120  proceeds to increase  430  the virtual server resource allocation. 
   The resource allocation for a particular overloaded resource is increased  430  by a pre-specified amount i. Amount i may be specified as a certain percentage of the resources of a physical host, or alternatively amount i may be specified as a certain number of resource units. Amount i may also be specified as a certain percentage of each particular virtual server&#39;s current resource allocation, e.g. increase a resource by 5% of its current value. After a particular resource has been increased the modifier  120  sets  440  a timer for a pre-specified time period. 
   When the timer expires, the modifier  120  determines  450  if the recently increased resource is being fully utilized. In one embodiment, a resource is fully utilized if a corresponding resource denial signal has been received within the timer period  440  after the resource was increased. 
   If the resource is determined  450  to be fully utilized, the modifier  120  returns and waits  410  for an overloaded signal. However, if it is determined that the resource is not being fully utilized, the modifier  120  decreases  460  the resource by a pre-specified amount d. Amount d may be specified as a certain percentage of the resources of a physical host, or amount d may be specified as a certain number of resource units. Amount d may also be specified as a certain percentage of each particular virtual server&#39;s current resource allocation, e.g. decrease a resource by 10% of its current value. 
   In one embodiment, d (the resource decreases amount) is larger than i (the resource increase amount). This allows unused resources to be decreased aggressively, but overloaded resources to be increased cautiously. In another embodiment, d and i are set such that the resource allocation is increased and decreased by equal amounts. For example, assume that the increase in virtual server resources i is specified as a percentage of each virtual server&#39;s current resource allocation. The decrease in virtual server resources d is specified as d=1−(1/1+i), which returns the resource allocation to its previous level. Once the resource reaches a fully utilized state, the modifier  120  then returns to waiting  410 . 
     FIG. 5  shows a block diagram of an embodiment of a process for performing resource load balancing among physical hosts, in the context of a working example of overloaded physical host  160 A. The physical host load balancer  130  periodically monitors the resource usage of a group of physical hosts  160  ( 160 A,  160 B and  160 C) and transfers virtual servers to different ones of these physical hosts  160  in order to balance the resource loads between the physical hosts  160 . Requests to increase virtual server resource allocations are also sent to the physical host load balancer  130  in order to assist in the balancing of physical host  160  resource loads. This process is next explained by example. 
   In this example, physical host load balancing module  130  receives a signal  510  from the virtual server resource modifier  120  indicating that virtual server  162 B requires an increased resource allocation. This signal is used as an input  520  into the load-balancing calculator  530 . The load-balancing calculator  530  also requests and receives as input the current physical host resource loads  535  from the physical host resource monitor  540 . 
   The physical host resource monitor  540  performs periodic physical host resource checks  545  upon the group of physical hosts  160  ( 160 A,  160 B and  160 C). Resource checks  545  monitor the current virtual server resource guarantees in each quality of service table for each physical host  160 . 
   The load-balancing calculator  530  determines whether a virtual server&#39;s request for additional resources  510  will overload the particular physical host currently hosting the virtual server. Using the example shown in  FIG. 5 , the load-balancing calculator  530  determines whether physical host  160 A is capable of supporting the request for additional virtual server  162 B resources  510 . If the resource request  510  exceeds the available resources of physical host  160 A, the load-balancing calculator  530  determines that physical host  160 A is overloaded. 
   In one embodiment, the load-balancing calculator  530  uses an easiest fit heuristic to find the physical host that has the most available resources. Each different type of resource is associated with an ordinal and a weight. The i th  resource R i  has ordinal i and weight w i . For example, resource R 1  represents disk resources, R 2  represents memory resources, R 3  represents network resources and R 4  represents CPU resources. The weights for each respective resource are determined by the system operator. 
   Let R i (V) denote the resource requirement of the virtual server under consideration, e.g. virtual server  162 B, including the requested resource increase from signal  510 . Let R i (S j ) denote the resource availability at the j th  physical host. The load-balancing calculator  530  computes the weighted resource availability of physical host j as the sum over i: 
         ∑   i     ⁢       w   i     *     (         R   i     ⁡     (     S   j     )       -       R   i     ⁡     (   V   )         )           
 
   Using the easiest fit heuristic, the load-balancing calculator  530  will select the physical host with the largest weighted resource availability to receive the virtual server  162 B (in the example of  FIG. 5 , physical host  160 B). The choice of physical host  160 B is subject to the constraint that the selected physical host  160 B has sufficient resources to meet the resource demands of virtual server  162 B. The load-balancing calculator  530  sends  550  a signal  560  to the dynamic virtual server mover  140  indicating that virtual server  162 B is to be transferred to physical host  160 B. 
   It will be understood by one of skill in the art that load-balancing calculator  530  may use other criteria for selecting which virtual server to transfer out of an overloaded physical host. In the embodiment given above, the load balancing calculator  530  transfers the virtual server that has most recently requested additional resources. However, in another embodiment, the load balancing calculator could select, for example, the smallest virtual server within an overloaded physical host for transfer, regardless of which virtual server has recently made a request for increased resources. 
     FIG. 6  is a flowchart of an embodiment of the process for transferring a virtual server from one physical host to another physical host. The dynamic virtual server mover  140  directs the process of  FIG. 6 . This process is next explained by example. 
   In this example, virtual server  162 B is transferred from old physical host  160 A to new physical host  160 B. The mover  140  waits  610  to receive a transfer virtual server signal  560 . The mover  140  receives a signal  560  directing the transfer of virtual server  162 B from physical host  160 A to physical host  160 B. The mover  140  directs physical host  160 A to store  620  local state information associated with virtual server  162 B in the file system  150 . As shown in  FIG. 1 , file system  150  is commonly accessible from physical hosts  160 A,  160 B and  160 C. 
   Mover  140  next directs physical host  160 A to stop  630  local processes associated with the virtual server being moved, e.g. virtual server  162 B. Mover  140  directs physical host  160 B to access  640  the virtual server  162 B state information stored in file system  150 . Mover  140  directs physical host  160 B to start  650  processes associated with virtual server  162 B locally. This enables virtual server  162 B to begin running locally in physical host  160 B. The user of virtual server  162 B is then transferred  660  from physical host  160 A to physical host  160 B by transferring the virtual server  162 B address to the new physical host  160 B. As explained previously, the mover  140  may use either “make, then break” timing or “break, then make” timing for the transfer process. Although the invention has been described in considerable detail with reference to certain embodiments, other embodiments are possible. As will be understood by those of skill in the art, the invention may be embodied in other specific forms without departing from the essential characteristics thereof. For example, the dynamic resource configuration module may support different numbers of physical hosts. Additionally, different fitting heuristic methods may be used to select physical hosts for receiving transferred virtual servers during load balancing among the physical hosts. Accordingly, the present invention is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and scope of the appended claims and equivalents.