Abstract:
A method and computing system for handling a page fault while executing a cross-platform system call with a shared page cache. A first kernel running in a first computer system receives a request for a faulted page associated with raw data from a second kernel running in a second computer system. In response to the request for the faulted page: (i) a first validity flag is updated to denote that the faulted page is unavailable to the first computer system in a first copy of the shared page cache and (ii) the faulted page is transmitted to the second kernel for insertion of the faulted page in a second copy of the shared page cache and for updating a second validity flag to denote that the faulted page is available to the second computer system in the second copy of the shared page cache.

Description:
[0001]    This application is a continuation application claiming priority to Ser. No. 12/342,704, filed Dec. 23, 2008. 
       CROSS REFERENCES TO RELATED INVENTIONS 
       [0002]    This invention is related to U.S. patent application Ser. No. 12/341,301 entitled “SYSTEM AND METHOD FOR HANDLING CROSS-PLATFORM SYSTEM CALL IN A HYBRID SYSTEM”, filed on Dec. 22, 2008, and U.S. patent application Ser. No. 12/341,342 (Attorney Docket No. DE920080095US1) entitled “SYSTEM AND METHOD FOR SHIFTING WORKLOADS ACROSS PLATFORM IN A HYBRID SYSTEM”, filed on Dec. 22, 2008. 
     
    
     FIELD OF THE INVENTION 
       [0003]    The present invention discloses a system and associated method for efficiently combining high-end mainframe systems and lower-end server systems to provide stable and cost-effective computing environments. 
       BACKGROUND OF THE INVENTION 
       [0004]    Mission-critical applications require stable and efficient computing environment. High-end mainframes are employed to stably host mission-critical applications. However, the high-end mainframes are prohibitively expensive to perform computation-intensive applications. Consequently, lower-end servers that primarily execute computation-intensive applications are often combined with high-end mainframes to provide both stability and cost-effective computation. However, issues regarding system integration, application migration, system management and administration must be solved to interconnect two types of platforms. 
         [0005]    Thus, there is a need for a system and associated method that efficiently interconnect high-end mainframes and lower-end servers. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention provides a method for handling a cross-platform system call with a shared page cache in a hybrid system, the method comprising: 
         [0007]    receiving a request to execute the cross-platform system call from a user application running on a first computer system, wherein the cross-platform system call provides a kernel service upon being remotely executed on a second computer system of the hybrid system, the first computer system comprising the user application and a first kernel, the second computer system comprising a second kernel, and wherein the first kernel is coupled to the second kernel; 
         [0008]    converting the cross-platform system call into a second configuration that is compatible with the second computer system such that the second kernel executes the converted cross-platform system call to provide the kernel service to the user application, wherein parameters of the cross-platform system call are converted upon determining that the parameters refer to raw data in the first computer system; 
         [0009]    transferring the converted cross-platform system call to the second kernel; 
         [0010]    handling a page fault that had occurred in the second kernel via transferring the corresponding page from the first kernel; 
         [0011]    obtaining a result of executing the converted cross-platform system call, wherein the result is produced by the second kernel; 
         [0012]    adjusting the obtained result to generate an adjusted result such that the adjusted result is in a first configuration that is compatible with the first computer system; 
         [0013]    handling a page fault that had occurred in the first kernel via transferring the corresponding page from the second kernel; and 
         [0014]    sending the adjusted result to the user application that had sent the request, 
         [0015]    wherein said receiving, said converting, said updating, said transferring, said handling, said obtaining, said adjusting, and said sending are performed by the first kernel. 
         [0016]    The present invention provides a computer program product, comprising a computer usable storage medium having a computer readable program code embodied therein, said computer readable program code containing instructions that when executed by a processor of a computer system implement a method for handling a cross-platform system call with a shared page cache in a hybrid system, the method comprising: 
         [0017]    receiving a request to execute the cross-platform system call from a user application running on a first computer system, wherein the cross-platform system call provides a kernel service upon being remotely executed on a second computer system of the hybrid system, the first computer system comprising the user application and a first kernel, the second computer system comprising a second kernel, and wherein the first kernel is coupled to the second kernel; 
         [0018]    converting the cross-platform system call into a second configuration that is compatible with the second computer system such that the second kernel executes the converted cross-platform system call to provide the kernel service to the user application, wherein parameters of the cross-platform system call are converted upon determining that the parameters refer to raw data in the first computer system; 
         [0019]    transferring the converted cross-platform system call to the second kernel; 
         [0020]    handling a page fault that had occurred in the second kernel via transferring the corresponding page from the first kernel; 
         [0021]    obtaining a result of executing the converted cross-platform system call, wherein the result is produced by the second kernel; 
         [0022]    adjusting the obtained result to generate an adjusted result such that the adjusted result is in a first configuration that is compatible with the first computer system; 
         [0023]    handling a page fault that had occurred in the first kernel via transferring the corresponding page from the second kernel; and 
         [0024]    sending the adjusted result to the user application that had sent the request, 
         [0025]    wherein said receiving, said converting, said updating, said transferring, said handling, said obtaining, said adjusting, and said sending are performed by the first kernel. 
         [0026]    The present invention provides a system comprising a processor and a computer readable memory unit coupled to the processor, said memory unit containing instructions that when executed by the processor implement a method for handling a cross-platform system call with a shared page cache in a hybrid system, the method comprising: 
         [0027]    receiving a request to execute the cross-platform system call from a user application running on a first computer system, wherein the cross-platform system call provides a kernel service upon being remotely executed on a second computer system of the hybrid system, the first computer system comprising the user application and a first kernel, the second computer system comprising a second kernel, and wherein the first kernel is coupled to the second kernel; 
         [0028]    converting the cross-platform system call into a second configuration that is compatible with the second computer system such that the second kernel executes the converted cross-platform system call to provide the kernel service to the user application, wherein parameters of the cross-platform system call are converted upon determining that the parameters refer to raw data in the first computer system; 
         [0029]    transferring the converted cross-platform system call to the second kernel; 
         [0030]    handling a page fault that had occurred in the second kernel via transferring the corresponding page from the first kernel; 
         [0031]    obtaining a result of executing the converted cross-platform system call, wherein the result is produced by the second kernel; 
         [0032]    adjusting the obtained result to generate an adjusted result such that the adjusted result is in a first configuration that is compatible with the first computer system; 
         [0033]    handling a page fault that had occurred in the first kernel via transferring the corresponding page from the second kernel; and 
         [0034]    sending the adjusted result to the user application that had sent the request, 
         [0035]    wherein said receiving, said converting, said updating, said transferring, said handling, said obtaining, said adjusting, and said sending are performed by the first kernel. 
         [0036]    The present invention provides a method and system that overcomes at least one of the current disadvantages of conventional method and system for providing kernel services in a hybrid system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0037]      FIG. 1  illustrates a hybrid system that integrates two architecturally different systems respectively comprising kernels U and K at kernel levels, in accordance with embodiments of the present invention. 
           [0038]      FIG. 2A  is a flowchart depicting a method for handling a system call by the kernel U in the hybrid system of  FIG. 1 , in accordance with the embodiments of the present invention. 
           [0039]      FIG. 2B  is a flowchart depicting a method for handling a system call by the kernel K in the hybrid system of  FIG. 1 , in accordance with the embodiments of the present invention. 
           [0040]      FIG. 3A  is a flowchart depicting a method for handling a page fault by a page fault handler K utilizing a shared page cache in the hybrid system, in accordance with the embodiments of the present invention. 
           [0041]      FIG. 3B  is a flowchart depicting a method for handling a page fault by a page fault handler U utilizing a shared page cache in the hybrid system, in accordance with the embodiments of the present invention. 
           [0042]      FIG. 4  illustrates a computer system used for handling a cross-platform system call with a shared page cache in the hybrid system, in accordance with embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0043]      FIG. 1  illustrates a hybrid system  100  that integrates two architecturally different systems respectively comprising kernels U and K at kernel levels, in accordance with embodiments of the present invention. 
         [0044]    The hybrid system  100  comprises at least one system U and a system K  150  interconnected through a data path  180 . A system U  110  of said at least one system U is a lower-end server that is configured to primarily execute user applications that have intensive computation workloads in the hybrid system  100 . The system K  150  is a high-end mainframe that is configured to primarily execute kernel services for the hybrid system  100 . The system U  150  and the system K  150  operate on respective hardware platforms and have respective software environments that have convertible system calls for respective kernels. The system U  150  and the system K  150  communicate with each other through the data path  180  that interconnects their respective kernels. The hybrid system  100  is configured to be perceived by users as a single system that provides functionalities and characteristics of both the system U  110  and the system K  150 . 
         [0045]    The system U  110  is the lower-end server that is configured to primarily execute user applications for both the system U  110  and the system K  150  in the hybrid system  100 . The system U  110  comprises at least one user application U and a kernel U  130 . 
         [0046]    A user application  120  of said at least one user application U is a user mode executable code of the system U  110 . When the user application  120  needs a kernel service, the user application  120  invokes a system call U  119  to interfaces with the kernel U  110 . The system call U  119  requests a service that is provided by the kernel U  130  and/or by the kernel K  170 . Memory pages relevant to a cross-platform execution of the system call U  119  by the kernel K  170  are shared by both the system U  110  and the system K  150  as described in this specification. 
         [0047]    The kernel U  130  is a main part of an operating system that controls the system U  110 . The operating system provides services such as, inter alia, time/clock related services, memory management services, process management services including scheduling, termination, and signalling, data services such as file/file system operations, network operations, and I/O calls, debugging and logging functionalities, inter-process communication (IPC) services without a shared memory, memory sharing services, I/O synchronizations, system administration services, and hardware services, etc. Examples of the operating system may be, inter alia, Linux, UNIX®, etc. (UNIX is a registered trademark of X/Open Company Ltd., in the United States and other countries.) 
         [0048]    The kernel U  130 , among other operating system functionalities, specifically refers to a system call handling facility of the system U  110  throughout this specification. The kernel U  130  comprises a receiver  131 , a splitter  132 , a converter  133 , a transporter  134 , a local system call handler  135 , and a page fault handler U  136 , which are functional components rather than physically separated units. 
         [0049]    The receiver  131  is an interface between the system call  119  invoked from the user application  120  and the kernel U  130 . The system call  119  is serviced by the system U  110  and/or the system K  150 . The receiver  131  sends the received system call  119  to the splitter  132 . 
         [0050]    The splitter  132  determines whether the received system call  119  is either a local system call of the system U  110  or a remote system call of the system K  150 . The splitter  132  forwards the local system call to the local system call handler  135  and the remote system call to the converter  133 . 
         [0051]    In one embodiment of the present invention, examples of local system calls are, inter alia, a clock request system call of the user application  120  that does not need to be synchronized with the system K  150 , a memory management system call such as mprotect( ) that modifies swapping properties of the user application, etc. 
         [0052]    In the same embodiment of the present invention, examples of remote system calls are, inter alia, time related services, process/signal management services, file/file system operations, network operations, I/O devices accesses, debugging and logging functionalities, inter-process communication (IPC) services without a shared memory, a shared memory, etc. 
         [0053]    The converter  133  converts the received remote system call to a corresponding system K system call that is executable on the system K  150 . The converter  133  also converts a result of executing the corresponding system K system call returned from the system K  150  to a corresponding result for the system U  110  to provide the corresponding result to the user application  120 . System calls of the system U  110  and system calls of the system K  150  are previously mapped according to their respective functionalities. Examples of issues to be addressed by the converter  133  may be, inter alia, a possible difference in endianness between the system U  110  and the system K  150 , memory mappings, etc. 
         [0054]    The transporter  134  sends the converted corresponding system K system call from the system U  110  to the system K  150 . After the system call is executed on the system K  150 , the transporter  134  receives a result of the converted corresponding system K system call from the system K  150 . 
         [0055]    The local system call handler  135  executes the local system call received from the splitter  132  and returns a result to the user application  120  that had invoked the system call. 
         [0056]    The page fault handler U  136  manages a shared page cache of the system U  110  in concert with a page fault handler K  173  by loading a faulted page and updating a validity flag of an entry associated with a newly loaded page in the shared page cache of the system U  110 . The term “page cache” defines a multiplicity of memory pages which is provided by an Operating System for kernel and application execution purposes. The shared page cache of the system U  110  is a page cache of the kernel U  130  for pages of the system U  110  that are shared with the kernel K  170 . An entry representing a memory page in the shared page cache of the system U  110  has a respective validity flag indicating whether the memory page associated with the entry is available in the system U  110 . See the description of  FIG. 3B , infra, for details. 
         [0057]    The system K  150  is, as noted, the high-end mainframe that is configured to primarily execute kernel services for both the system U  110  and the system K  150  in the hybrid system  100 . The system K  150  comprises at least one user application K and a kernel K  170 . 
         [0058]    The kernel K  170  is a core of an operating system that controls the system K  150 .  110 . See the description of the kernel U  130 , supra, for services provided by the operating system. The kernel K  170  comprises a converter K  171 , a system call handler K  172 , and a page fault handler K  173 . 
         [0059]    The system call handler K  172  receives a system call request from the transporter  134  of the system U  110  through the data path  180 . The received system call request is serviced by the system call handler K  172 . After executing the system call request, the system call handler K  172  returns a result to the transporter of the system U  110  through the data path  180 . The system call handler K  172  controls an I/O device driver that handles an input/output (I/O) request that is directed to an I/O device that is controlled by the system K  150 . In the hybrid system  100 , the kernel K  170  services all I/O requests from both the system U  110  and the system K  150 . Examples of the I/O device may be, inter alia, HiperSockets®, a direct access storage device (DASD), enterprise tape systems, etc. (HiperSockets is a registered trademark of International Business Machines Corporation in the United States and other countries.) 
         [0060]    The page fault handler K  173  manages a shared page cache of the system K  150  in concert with a page fault handler U  136  by loading a faulted page and updating a validity flag of an entry associated with a newly loaded page in the shared page cache of the system K  150 . See the description of the page fault handler U  136  for the definition of the term “page cache.” The shared page cache of the system K  150  is a page cache of the kernel K  170  for pages of the system K  150  that are shared with the kernel U  130 . An entry representing a memory page in the shared page cache of the system K  150  has a respective validity flag indicating whether the memory page associated with the entry is available in the system K  150 . See the description of  FIG. 3A , infra, for details. 
         [0061]    The data path  180  interconnects the transporter  134  of the kernel U  130  and the system call handler  172  of the kernel K  170 . The data path  180  directly connects the system U  110  and the system K  150  in a kernel level with a high-speed communication I/O device such as 10 Gigabit Ethernet. 
         [0062]    In one embodiment of the present invention, the system U  110  is a blade server employing Intel® or AMD® processors and the system K  150  is an IBM® System z® mainframe. (Intel is a registered trademark of Intel Corporation in the United States and other countries; AMD is a registered trademark of Advanced Micro Devices, Inc in the United States and other countries; IBM and System z are registered trademarks of International Business Machines Corporation in the United States and other countries.) Both the blade server and the mainframe employ the Linux operating system or other UNIX family OS. The hybrid system  100  maintains input/output capability of the mainframe to HiperSockets, a direct access storage device (DASD), enterprise tape systems, etc. The hybrid system  100  retains superior hardware stability and management features of the mainframe. The hybrid system  100  also executes computation intensive applications on the blade server to achieve cost effectiveness. The hybrid system  100  provides a unified view of all resources for all applications on both the mainframe and the blade server. 
         [0063]    The hybrid system  100  may be employed in areas of, inter alia, dynamic web servicing, data mining, SAP® R/3 application serving, Domino® groupware processing, application servicing and Java® processing. (SAP is a registered trademark of SAP AG in the United States and other countries; Domino is a registered trademark of International Business Machines Corporation in the United States and other countries; Java is a registered trademark of Sun Microsystems, Inc., in the United States and other countries.) 
         [0064]      FIG. 2A  is a flowchart depicting a method for handling a system call by the kernel U in the hybrid system of  FIG. 1 , supra, in accordance with the embodiments of the present invention. 
         [0065]    Initially, the user application runs in a user mode on the system U. To perform a kernel operation, the user application invokes a system call to interface with the kernel U of the system U. Upon invoking the system call, the user application passes control over to the kernel U, and the system U starts running in a kernel mode to handle the system call. 
         [0066]    In step  210 U, the kernel U receives the system call request and parameters of the system call. The receiver of  FIG. 1 , supra, represents step  210 U, which may be a separate sub-procedure. The kernel U proceeds with step  220 U. 
         [0067]    In step  220 U, the kernel U determines whether the system call should be executed remotely on the system K or whether the system call can be executed locally on the system U. If the kernel U determines that the system call can be executed remotely on the system K, then the system call handler of the kernel U proceeds with step  230 U. If the kernel U determines that the system call can be executed locally on the system U, then the kernel U proceeds with step  270 U. 
         [0068]    In one embodiment of the present invention, the kernel U uses a predefined number scheme for each system call in determining whether the system call is remote or local in step  220 U. Each system call and required parameters for each system call are analyzed and assigned a respective number describing a function of the system call. Examples of system call numbering based on the function of each system call may be, inter alia, {1=write, 2=read, 3=fork, 4=get-time-of-day, . . . }, etc. Based on the assigned number and necessary parameters of the system call, the kernel U determines whether to execute the system call either locally or remotely. 
         [0069]    Whether a system call is either locally or remotely executed is determined based on, inter alia, concerns as to the overall effectiveness of the hybrid system, the seamless integration of the hybrid system, the utilization of each platform in the hybrid system, the workload distribution among platforms in the hybrid system, locality of resources that the system call is to operate on, etc. 
         [0070]    There are certain system calls that require special/combined actions rather than simple remote/local execution. These actions are implementation specific and usually simulate the system call in a manner that provides services of the system call and satisfies system-specific requirements of both the kernel U and the kernel K. Examples of system calls require special actions may be, inter alia, I/O synchronizing system calls, exit( ) system call, administrative system calls, and hardware-specific system calls, etc. Examples of administrative system calls are, inter alia, reboot( ), kernel module handling calls, virtualization system calls, etc. 
         [0071]    In cases of I/O synchronizing system calls such as sync( ), close( ), shared page cache handling through the page fault handler U and the page fault handler K will synchronize all dirty pages on both systems correctly. 
         [0072]    In step  230 U, the kernel U converts the system call and parameters associated with the system call into a corresponding system K system call and associated parameters for the converted corresponding system K system call, respectively. The kernel U proceeds with step  240 U. 
         [0073]    System calls and parameters are inherent to a platform and a kernel on which system calls are executed. Thus, to provide a consistent result in cross-platform system call execution, the kernel U system call and the parameters must be converted to the kernel K system call and corresponding system K data before an execution. System calls in respective systems are mapped by number and/or function. The parameters are converted with respect to differences between the systems, inter alia, endianness for numeric parameters, address space usage, pointers, namespace of object identifiers, memory object synchronization, etc. 
         [0074]    When a parameter of the system call refers to a memory page consisting of raw data, conventional method converting the parameter from a system U configuration to a system K configuration is copying the raw data to the system K and converting the parameter to point the copied raw data. Example of the raw data may be, inter alia, content of I/O operations, etc. The conventional method copies all raw data back and forth between the system U and the system K, regardless of which pages are actually accessed by the system K kernel operations or applications on the system U which request kernel services. To optimize memory operations for a parameter referring to raw data areas, the system U and the system K shares a page cache comprising at least one entry that represents a respective memory page in both the system U and the system K. If an entry of the shared page cache in one system is valid, another entry of the shared page cache in the other system that represents the same page is invalid. An access to an invalid page results in a call to the page fault handler of a local system. The page fault handler of the local system and a page fault handler of a remote system that has a valid entry for the faulted page coordinate handling of the faulted page as described in  FIGS. 3A and 3B , infra. 
         [0075]    In one embodiment of the present invention, the kernel U converts only parameters that do not refer to memory areas but the kernel K converts such memory referring parameters for efficiency in the performance of the hybrid system as a whole. 
         [0076]    In step  240 U, the kernel U transfers the system call request including all parameters and data to the system K through the data path, without copying the raw data referred to by parameters. The system call request sent in step  240 U is received by the kernel K in step  241 K of  FIG. 2B , infra. The kernel U proceeds with step  250 U. 
         [0077]    In step  244 U, the kernel U receives a request to synchronize a faulted memory page with the kernel K in step  243 K of  FIG. 2B , infra. The kernel U transfers the faulted memory page to the kernel K and invalidates an entry in the shared page cache that is associated with the transferred memory page because the memory page in the kernel K is the only valid version of the memory page after the transfer. 
         [0078]    In step  250 U, the kernel U receives results from the kernel K performing step  246 K of  FIG. 2B , infra. The kernel U proceeds with step  260 U. 
         [0079]    In step  260 U, the kernel U adjusts the result received in step  250 U from the system K for the system U kernel and application. The adjustment in step  260 U is a reverse process of the conversion in step  230 U. Thus the adjustment in step  260 U also comprises reverse conversions of data due to little/big endian differences, reverse conversions of data, pointers, raw data, and/or structured data to be configured for the system U, reverse translations of namespaces from the system K to the system U, etc. Because the result received in step  250 U is executed on the system K, the result should be configured for the system U before the result is returned to the user application. If the system call has an impact on structures of the kernel U, the kernel U structure is updated in step  260 U as a part of the adjustment. For example, if the system call is for signal or process management, a result of executing the system call changes processing environments of the system U in the kernel space and/or the user space. The kernel U proceeds with step  280 U. 
         [0080]    In step  270 , the kernel U executes the system call locally on the system U as in a conventional single platform system consisting of the system U. The kernel U proceeds with step  280 U. 
         [0081]    In step  280 , the kernel U returns the adjusted result from either step  260 U or step  270 U to the user application that had invoked the system call U. If any status updates resulting from executing the system call remotely on the system K need to be made available to the system U, the kernel U transmits such information regarding status updates to the system U prior to returning the control to the user application such that the user application on the system U obtains the result of the system service produced by executing the system call. The user application takes control of the thread of execution back from the kernel U and resumes the execution. 
         [0082]      FIG. 2B  is a flowchart depicting a method for handling a system call by the kernel K in the hybrid system of  FIG. 1 , supra, in accordance with the embodiments of the present invention. 
         [0083]    In step  241 K, the kernel K receives a system call request and parameters from the kernel U performing step  240 U of  FIG. 2A , supra. 
         [0084]    In step  242 K, the kernel K determines whether the requested system call needs memory reference to execute the requested system call. If the requested system call has parameters referring to a memory space, the kernel U marks the system call for a memory reference during the conversion in step  230 U of  FIG. 2A , supra. Thus the kernel K determines that the requested system call needs a memory reference pursuant to the information received from the kernel U. If the kernel K determines that the requested system call needs a memory reference, the kernel K proceeds with step  243 K. If the kernel K determines that the requested system call does not need a memory reference, the kernel K proceeds with step  245 K. 
         [0085]    In step  243 K, the kernel K finds a mapping associated with each memory reference parameter of the requested system call. When a page fault for each memory reference parameter occurs, the kernel K handles the page fault in concert with the kernel U as illustrated in descriptions of  FIGS. 3A and 3B , infra, to copy raw data pointed to by each memory reference parameter that has triggered the page fault. Parameters that do not refer to memory spaces are already converted in the system U as illustrated in the description of step  230 U of  FIG. 2A , supra. The mappings associated with each memory pointer parameter point to respectively corresponding pages of the shared page cache of the system K, which is not valid until a page fault occurs in the system K for a mapped page in the system U. The kernel K uses the mapping associated with each memory reference parameter to locate parameters of the requested system call on the system K, even though the kernel K has not copied data pointed by each memory reference parameter. 
         [0086]    For a system call having memory reference parameters, a memory area referred to by a parameter of the system call may be a structured object rather than raw data. For example, the ioctl( ) system call may have an object supporting a buffer interface. The structured object referred to by the parameter should be evaluated and converted element by element of the structured object pursuant to the usage of the parameter in the system call to execute the system call remotely on the system K. Each element of the structured object may be, inter alia, a numeric parameter, a memory reference with raw data, or a memory reference with a structured object. Any element of the structured object that is not a memory reference parameter is converted by the kernel U in step  230 U prior to the system call is transferred. An element of the structured object that is a memory reference parameter is converted by the kernel K and pages referred to by the memory reference parameter are copied to the system K from the system U through page fault handling described in  FIGS. 3A and 3B , infra. After performing step  243 K, memory reference parameters point to respective valid pages in system K that correspond to respective pages in system U, which have been referenced by the system call on system U and have been copied from the system U. The kernel K proceeds to step  245 K. 
         [0087]    In step  245 K, the kernel K executes the requested system call with parameters that are configured for the kernel K. 
         [0088]    In step  246 K, the kernel K sends a result from executing the requested system call to the kernel U. 
         [0089]      FIGS. 3A and 3B  describe how a page fault is handled in the hybrid system. 
         [0090]    The system K and the system U both have a respective page fault handler, the page fault handler K and the page fault handler U. The system K and the system U also have a respective copy of a shared page cache comprising entries associated with memory pages in each system. Memory pages available in one system are marked as valid entries in the shared page cache, and if a memory page is marked as valid in the shared page cache of one system, the same memory page in the other system is invalid, because only one system keeps the valid copy at a time. Also the page fault handler K and the page fault handler U coordinate validity flags of entries in the respective shared page cache such that the hybrid system can access valid memory pages regardless of the location of faulted pages. 
         [0091]    The page fault handler K and the page fault handler U lock respective page entry in the respective shared page cache while handling the page fault, to assure one page entry in either the shared page cache of the system K or the shared page cache of the system U must be valid. 
         [0092]    In this specification, the page fault occurs in the system K in step  243 K of  FIG. 2B , supra. Because memory pages are not necessarily copied to the system K while converting parameters of the system call in step  230 U of  FIG. 2A , supra, the kernel U keeps memory pages that are referred to by the parameters. The page fault handler U handles the page fault in step  244 U of  FIG. 2A , supra, in concert with the page fault handler K. Steps in  FIGS. 3A and 3B  are performed in an order of  300 K,  310 K,  320 U,  330 U,  340 U,  350 K, and  360 K. 
         [0093]      FIG. 3A  is a flowchart depicting a method for handling a page fault by a page fault handler K utilizing a shared page cache in the hybrid system, in accordance with the embodiments of the present invention. 
         [0094]    In step  300 K, the kernel K detects a page fault upon accessing a page that is marked as invalid in a shared page cache in the system K. The kernel K proceeds with step  310 K. 
         [0095]    In step  310 K, the kernel K requests the faulted page to the page fault handler U in the system U. The kernel K proceeds with step  350 K. 
         [0096]    In step  350 K, the kernel K copies the faulted page to the system K as the faulted page is transferred from the system U in step  340 U of  FIG. 3B , infra. 
         [0097]    In step  360 K, the kernel K validates an entry in the shared page cache that corresponds to the copied page in step  350 K supra such that the page is available for executing the system call in the system K. 
         [0098]      FIG. 3B  is a flowchart depicting a method for handling a page fault by a page fault handler U utilizing a shared page cache in the hybrid system, in accordance with the embodiments of the present invention. 
         [0099]    In step  320 U, the kernel U receives a request for the faulted page from the kernel K performing step  310 K of  FIG. 3A , supra. The kernel U proceeds with step  330 U. 
         [0100]    In step  330 U, the kernel U invalidates an entry in the shared page cache that corresponds to the page that will be transferred in step  340 U infra such that the kernel U does not access the transferred page any more because the page is used to execute the system call in the system K. The kernel U proceeds with step  340 U. 
         [0101]    In step  340 U, the kernel U transfers the faulted page that had been requested in step  320 U to the system K. 
         [0102]    An example of a system call with memory reference may be, inter alia, write ( ). A write( ) system call has three parameters representing a file descriptor that is an integer, a buffer area that is a pointer to a starting address of a memory area to write, and a length of the buffer area that is an integer representing the size of the buffer area to write. The system call number of the write( ) system call is converted to match the write( ) system call number on the system K. With the shared page cache of the present invention, the data in the buffer area is not copied initially but only the parameter pointing to the buffer area is adjusted to point a corresponding data area in the system K. Later on, if the buffer area is accessed while executing the write( ) system call in the system K, a page fault occurs for the data in the buffer area of the system U and the data is copied to the system K and the shared page caches in both systems are updated to reflect a status of the page holding the data in the buffer area. 
         [0103]    In another embodiment of the present invention, the shared page caches of the system K and the system U may use a different cache/memory coherency protocol that utilizes four statuses of Modified, Exclusive, Shared or Invalid, which is referred to as the MESI protocol, instead of utilizing two statuses indicating only the validity as described in  FIGS. 3A and 3B . 
         [0104]      FIG. 4  illustrates a computer system  90  used for handling a cross-platform system call with a shared page cache in the hybrid system, in accordance with embodiments of the present invention. 
         [0105]    The computer system  90  comprises a processor  91 , an input device  92  coupled to the processor  91 , an output device  93  coupled to the processor  91 , and memory devices  94  and  95  each coupled to the processor  91 . The input device  92  may be, inter alia, a keyboard, a mouse, a keypad, a touchscreen, a voice recognition device, a sensor, a network interface card (NIC), a Voice/video over Internet Protocol (VOIP) adapter, a wireless adapter, a telephone adapter, a dedicated circuit adapter, etc. The output device  93  may be, inter alia, a printer, a plotter, a computer screen, a magnetic tape, a removable hard disk, a floppy disk, a NIC, a VOIP adapter, a wireless adapter, a telephone adapter, a dedicated circuit adapter, an audio and/or visual signal generator, a light emitting diode (LED), etc. The memory devices  94  and  95  may be, inter alia, a cache, a dynamic random access memory (DRAM), a read-only memory (ROM), a hard disk, a floppy disk, a magnetic tape, an optical storage such as a compact disk (CD) or a digital video disk (DVD), etc. The memory device  95  includes a computer code  97  which is a computer program that comprises computer-executable instructions. The computer code  97  includes, inter alia, an algorithm used for handling a cross-platform system call with a shared page cache in the hybrid system according to the present invention. The processor  91  executes the computer code  97 . The memory device  94  includes input data  96 . The input data  96  includes input required by the computer code  97 . The output device  93  displays output from the computer code  97 . Either or both memory devices  94  and  95  (or one or more additional memory devices not shown in  FIG. 4 ) may be used as a computer usable storage medium (or a computer readable storage medium or a program storage device) having a computer readable program embodied therein and/or having other data stored therein, wherein the computer readable program comprises the computer code  97 . Generally, a computer program product (or, alternatively, an article of manufacture) of the computer system  90  may comprise said computer usable storage medium (or said program storage device). 
         [0106]    While  FIG. 4  shows the computer system  90  as a particular configuration of hardware and software, any configuration of hardware and software, as would be known to a person of ordinary skill in the art, may be utilized for the purposes stated supra in conjunction with the particular computer system  90  of  FIG. 4 . For example, the memory devices  94  and  95  may be portions of a single memory device rather than separate memory devices. 
         [0107]    While particular embodiments of the present invention have been described herein for purposes of illustration, many modifications and changes will become apparent to those skilled in the art. Accordingly, the appended claims are intended to encompass all such modifications and changes as fall within the true spirit and scope of this invention.