Patent Application: US-9806198-A

Abstract:
a method and apparatus for performing efficient interprocess communication in a computer system . with this invention , a memory region called the ipc transfer region is shared among all processes of the system to enable more efficient ipc . the unique physical address of the region is mapped into a virtual address from each of the address spaces of the processes of the system . when one of the processes needs to transfer data to another of the processes , the first process stores arguments describing the data in the region using the virtual address in its address space that maps into the unique physical address . when the other or second process needs to receive the data , the second process reads the data from the second region using the virtual address in its memory space that maps into the unique physical address . with this invention , in most cases , control of the ipc transfer region occurs automatically without any kernel intervention .

Description:
though our invention is not limited to the following described implementation , this implementation demonstrates the potential advantages of using our mechanism over other mechanisms . the preferred embodiment of this inventions is illustrated in fig1 - 4 . fig1 is a high - level representation of the mechanism used during the common path of performing an ipc , while fig2 is a more detailed version of the same figure . fig3 is a flow chart representing the entire algorithm , and fig4 represents the actions that need to occur in the uncommon path of involuntary preemption while performing an ipc . when a process is created , the kernel sets up a region of virtual memory region marked as the ipc transfer region . see 51 in fig3 and 11 and 14 in fig1 and 2 . establishment of virtual memory regions occurs once at process creation . this transfer region is mapped via the operating system ( 15 a and 15 b in fig2 ) at a virtual address in each process . the only requirement is that every process knows the virtual address of the mapped ipc transfer region . therefore , without loss of generality , we always map the region at a well known and constant location in the virtual address of each process . the key idea is that all the virtual mappings of the region point to the same physical address ( 18 in fig1 and 2 ). a special variable is used to indicate whether the ipc transfer region is in use and if so how much data it is communicating . this variable is referred to as the byte_count variable , and it is set dynamically for each ipc request . through this variable the need to perform explicit synchronization to access the ipc transfer region is eliminated . furthermore , the processes do not need to check this variable . we have optimized the common path ( no ipc contention ) of the mechanism by pushing the cost of any contention for the ipc transfer region to the uncommon case of contention among the processors for the ipc region . described below is the mapping of the ipc transfer region virtual address in each process to the same physical one . such mapping eliminates the need for synchronization in the common path , and reduces the likelihood of contention for the ipc transfer region occurring . for correctness and to obtain high performance on multiprocessor systems , all processes on a specific processor should reference cache and memory local to that processor . to achieve this , a different physical address for the ipc transfer region is chosen for each processor . thus , when a process is migrated from one processor to another , the operating system must change the mapping of the ipc transfer region virtual address so that it points to the appropriate physical address for that processor . in contrast to memory mapping or copying data through the kernel , the ipc transfer region mechanism can be implemented very efficiently . assuming that most ipc requests are implemented using hand - off scheduling , the ipc transfer region is ( like the processor registers ) automatically transferred to the server address space . because all processes can access the ipc transfer region , the “ automatic transfer ” is implicitly occurring without any action . hence , there is no processing overhead in the kernel to transfer the ipc transfer region in the common case . the ipc mechanism is also more tlb ( translation lookaside buffer ) and cache friendly than other schemes . since it uses the same address for all inter - process communication , there will not be any tlb misses , and the cache line ( s ) holding the data is / are very likely to be valid . as detailed in fig3 there is a series of steps that are followed to perform the ipc . the common path that occurs is that steps 52 - 56 ( fig3 ) are followed in succession without interruption . in this case the kernel is not involved in copying arguments as part of the ipc . this path is represented in fig1 and 2 and described in more detail below . in the rare event that an involuntary preemption ( 62 ) occurs during step 52 through step 56 , the kernel ( 57 ) needs to become involved via an involuntary preemption ( transition “ a ” in fig3 ). the kernel &# 39 ; s task at this point is to ensure that the accessing and sharing assumptions of the ipc transfer region are not violated and are correctly preserved for the next process . when such an involuntary preemption occurs , steps 58 - 61 are followed . the purpose of these steps is to save and restore the ipc transfer region so that when control is returned to the interrupted process , it can continue performing the operation is was prior to being interrupted . more specifically , these steps check to see if the ipc transfer region is in use ( 58 ), and if so saves the registers and ipc transfer region ( 59 ), handles the reason for preemption ( 60 ), and restores the registers and ipc transfer region , so the process can continue its ipc . fig4 and more detail below describes what needs to occur on this path . after step 61 the kernel returns back to the ipc in progress via a “ b ” transition . in summary then , to perform an ipc , steps 52 - 56 occur . should an involuntary preemption ( 62 ) occur during this sequence , the kernel needs to perform steps 58 - 61 , after which point it returns control ( 63 ) back to the process that was in the middle of the ipc . the following describes in more detail what needs to occur at each of these steps . in the common path the ipc proceeds smoothly from step 52 through 56 without interruption . after the description of the common path , we describe the steps needed to be followed in the uncommon path when an involuntary preemption occurs . here we describe the series of events for the common case . throughout time , the operating system manages mappings ( e . g ., 15 a and 15 b ) between the virtual address space of the different processes ( e . g ., 19 a and 19 b ) and the physical memory ( 20 ) of the machine . specifically , these mappings map different virtual regions ( 16 a , 16 b , 16 c , 16 d , 11 , 12 , 13 , and 14 ) of the process to the actual physical memory of the machine . these mappings generally occur on a page ( region of memory defined by the machine architecture ) by page basis . of concern to this invention is a particular region called the ipc transfer region . as mentioned above , at process creation time , the operating system maps a common piece of physical memory into a process &# 39 ; s virtual address space and marks it as the ipc transfer region . in our system this is mapped at a constant and well - known location . thus , when a process wishes to access this location , it knows exactly where in its virtual address space the ipc transfer region is . the type of inter - process communication provided by the invention is that of a procedure call interface . when process a ( a client ) wishes to communicate with process b ( a server ), process a makes a ( cross address space ) procedure call to process b . the data that is communicated is stored as arguments of the function call . the first step ( 52 ) process a must do in performing the ipc is to set the byte_count variable at the beginning of the ipc transfer region . this count should be the sum total of the arguments and critical information ( e . g ., what function is being called ) related to the procedure call . depending on the algorithm used , the byte_count can be modified multiple times until all the parameters have been filled in . it could be computed up front by the compiler or modified as the parameters are copied to the ipc transfer region . the important thing that needs to remain invariant is that the byte_count be larger than or equal to the amount of valid data in the ipc transfer region . after the byte_count is set , process a copies the data ( 53 in fig1 , and 3 ) represented by that count from a place in its memory to the ipc transfer region . the setting of the count and copying of data can be all at once or in passes . after all the data has been copied , process a makes a call to process b ( 54 ). as part of this call , the processor is now given to process b , which begins to execute . process b &# 39 ; s first job is to copy out the arguments that were placed in the ipc transfer region by process a ( 55 fig1 , and 3 ). the reason it is important for process b to immediately copy out the arguments is that there is only one ipc transfer region per processor ; thus each process needs to minimize the amount of time it uses this resource to avoid contention . for security reasons ( to prevent data from leaking to other processes ), the process may want to zero the data that was in the ipc transfer region . the final step in the ipc is for process b to zero the byte count variable ( 56 ). this indicates completion of the inter - process communication . at some point , process b may wish to provide information back to process a , or to call another entirely different process . in either event , the above described mechanism is repeated . in the common case this mechanism is very efficient , since the memory employed by the mechanism is frequently used . on most processor architectures , tlb entries may be shared across processes thus reducing the number of tlb misses . also , since the memory in the ipc transfer region is frequently used by all processes , the cache lines representing it have a high probably of being in the cache . it is possible that an involuntary preemption occurs while steps 52 through 56 are being performed . this preemption could be because an external interrupt comes in with higher priority and needs to be processed by another process , because the current process &# 39 ; s time slice is up , or for a variety of other reasons . if this preemption occurs when the system is in the middle of an ipc , then the ipc transfer region will be in use . since there is only one ipc transfer region on each processor , the kernel needs to preserve the data in the region so that once the interrupted process resumes execution , it may continue with the ipc . the data structures and representations for handling this uncommon case appears in fig4 . the operating system maintains a set of data structures keeping various information about the processes . in our system , the pertinent ( to ipc transfer region ) ones are a list of the running processes ( 41 ), a list of the blocked processes ( 40 , 42 a , and 42 b ), a set of free process states ( 49 ), a set of saved process states ( 48 a and 48 b ), and pointers from the blocked processes to the saved process states ( 43 a and 43 b ). if process a is currently running and performing an ipc , it will be in the set of running processes ( 41 ). if at that point process a is involuntarily preempted , the operating system creates a blocked process data structure to represent it ( 40 ). the operating system also takes a process state structure out of the free pool ( 49 ) of processor states , saves the appropriate state ( 44 , 45 , 46 , 47 a , and 47 b ), moves it to the current set of saved process states ( 48 a and 48 b ), and creates a pointer to it from the blocked process state data structure ( 43 a and 43 b ). the operating system must copy information about the state of the machine to the saved process state . the machine state normally consists of , among other things , register contents ( 45 ), but the introduction of the ipc transfer region introduces one more piece of information that must be saved . during the involuntary preemption , the operating system must preserve the data in the ipc transfer region by copying it to the saved state associated with the process that is being preempted . if the security level of the system requires that two processes can not share information unless explicitly requested , then the mechanism should also corrupt ( zero ) the data in the ipc transfer region before providing the processor to the next process , otherwise data could leak between unrelated processes . while zeroing the portion of the region where the data resided is not absolutely required , it is necessary to provide the same degree of security commonly offered today by operating systems . however , this zeroing is not sufficient for “ trusted ” systems requiring a high degree of security , e . g ., military installations . a different technique is needed , such as always zeroing the entire region rather than just where the data was stored . while the uncommon case introduces potential overhead to the possible ipc path , it occurs very infrequently and thus does not impact the performance of ipc . the ipc transfer region provides an efficient mechanism for processes wishing to communicate medium amounts of data . for small amounts of data , registers are sufficient , and for very large amounts of data a different technique may provide better performance . however , for moderate amounts of data ( between what fits in registers , and a couple of megabytes ) the ipc transfer regions offers the best performing method for communicating between processes in different address spaces . the process of copying arguments into the ipc transfer region before the ipc and out of the region afterwards can be tedious and is performance sensitive . while not required by this invention , as the user could manually copy the arguments into and out of the ipc transfer region when making ipcs , we recommend an automated method for copying the arguments into and out of the ipc transfer region . a stub compiler is run over decorated code in order to automatically produce the actual code for copying arguments to and from the ipc transfer region . the stub compiler automatically turns a normal - looking procedure call ( decorated with symbols to indicate it is an ipc ) to the full code needed to copy the arguments in or out and then to make the actual call . this automation leads to an easier programming model for the application writer , reduces potential errors , and provides better performance since we will make sure that the automated method performs this operation in the optimal way on the targeted architecture . 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