Abstract:
In a SCI based multi-node system, the write purge command joins the new node that is requesting to write to the memory of the sharing list, while maintaining the connection between the memory and the sharing list. The new node then issues the purging command to each node in the sharing list, while still maintaining the connection of the sharing list to the memory. Next, the new node issues the collapsing command to separate the sharing list from the memory after the purging command has been issued to each node. A send request data packet is used to distribute the write purge command to the memory node.

Description:
TECHNICAL FIELD OF THE INVENTION  
         [0001]    This invention relates in general to memory accesses in multi-node, multi-processor, cache coherent non-uniform access system and relates in particular to a system and method for performing a write purge operation in such a system.  
         BACKGROUND OF THE INVENTION  
         [0002]    The Scalable Coherent Interface (SCI) Direct Memory Access (DMA) write operations in the standard SCI specification completely over-writes the cache lines. If the DMA devices are only updating some of the bytes in the line, the information contained in the memory for the other bytes are lost in the complete overwrite.  
           [0003]    The DMA write operation also relies on software to maintain cache coherency. If two devices are writing the same line, the second device can believe that it has finished purging all caches that contain the old data, even though the lines are being purged because of the first device. This second device can then allow other software to read stale data. The use of stale data by the software will cause program errors.  
           [0004]    Other prior art methods for writing partial lines rely on reading the line into a local cache before the specified bytes are updated. This results in other desirable data being swapped out of the cache because of conflicts with the stored read data. In the end, this results in poor performance of other processes that are currently running because of the extra memory operations and the latency associated with refetching affected data.  
           [0005]    Therefore, there is a need in the art for a method and system that has a write command that does not allow the use of stale data by the software.  
           [0006]    In addition, there is a need in the art for a method and system that does not require the reading memory lines into a local cache before the updating memory bytes.  
         SUMMARY OF THE INVENTION  
         [0007]    These and other objects and features are achieved in a system in which follows the same general flow as the DMA Write as described in the SCI specification, however, the inventive system and method does not detach the cache sharing list from memory. Instead, it joins the sharing list. This prevents another write purge from another node from believing it has finished its operation while memory lines are still encached. If no sharing list exists, a mask supplied by the command is used to merge the new data into memory. The system and method tracks down stale data in remote caches and merges it into the memory line using a mask instead of discarding it.  
           [0008]    One technical advantage of the present invention is to issue a write purge command that joins the new node to the sharing list, while maintaining the connection between the memory and the sharing list.  
           [0009]    Another technical advantage of the present invention is to have the new node issue the purging command to each node in the sharing list, while maintaining the connection between the memory and the sharing list.  
           [0010]    A further technical advantage of the present invention is to have the new node issue the collapsing command to separate the sharing list from the memory after the purging command has been issued to each node. The collapsing command completes the destruction of the sharing list.  
           [0011]    A further technical advantage of the present invention is to use a write mask with the write purge command.  
           [0012]    The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:  
         [0014]    [0014]FIGS. 1A to  1 C show an example of the SCI specification write purge operation;  
         [0015]    [0015]FIGS. 2A to  2 C show an example of the inventive write purge partial operation;  
         [0016]    [0016]FIG. 3 describes the operations performed by an initialization state machine to execute the write purge operation;  
         [0017]    [0017]FIG. 4 describes the operations performed by a request state machine to execute the write purge operation;  
         [0018]    [0018]FIGS. 5A to  5 F describe the operations performed by a response state machine to execute the write purge operation;  
         [0019]    [0019]FIGS. 6A to  6 C describe the operations performed by a conflicting state machine to execute the write purge operation;  
         [0020]    [0020]FIG. 7 depicts the send request data packet for carrying the write purge command in the bytemask; and  
         [0021]    [0021]FIG. 8 shows the write mask.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]    [0022]FIGS. 1A to  1 C show how the prior art SCI specification performs a DMA write to a line that is fresh with a sharing list. The list starts out with memory  101  in the state of fresh with pointer to node  0   102 . Node  0   102  is head_fresh and has a forward pointer to node  1   103 , which is mid_valid. Node  1   103  has a back pointer back to the head, node  0   102 , and a forward pointer to node  2   104 . Node  2   104  is tail_valid and has a back pointer to the mid or node  1   103 .  
         [0023]    The first step that is performed by the standard SCI flow is to determine if another node wants to write this memory line, if so, then it will issue an MWRITE_FRESH_TO_HOME. As shown in FIG. 1A, node  3   105  wants to write to the memory line and issues the MWRITE command to begin the write operation. FIG. 1B shows that the list fresh_to_home is actually detached from the memory  101  so that the sharing list of nodes  0 ,  1 , and  2   102 ,  103 ,  104  is no longer connected to the memory  101 . So the memory is now marked home and any new accesses can retrieve the memory line directly from memory  101 .  
         [0024]    Now node  3   105  goes in and cleans up or purges the sharing list. Node  3   105  sends CREAD 00 _FRESH_TO_INVALID from node  3  to node  0 . When node  3   105  receives a response, it uses the response to get the forward pointer. Node  3   105  then uses the forward pointer to send CREAD 00 _VALID_TO_INVALID from node  3   105  to node  1   103 . When node  3   105  receives a response, it then uses the forward pointer from the response to send the CREAD 00 _VALID_TO_INVALID from node  3   105  to node  2   104 . When this operation is complete or done, only memory in the home state contains a copy of the data.  
         [0025]    The SCI specification relies on software interlocking in such a way that it does not access the fresh memory until the list is totally purged. This can be very difficult if multiple write purges occur to the same memory line almost simultaneously, as the second one will immediately see that the line is marked HOME and allow software to access this line, which could still have stale copies.  
         [0026]    [0026]FIGS. 2A to  2 C shows that the operation of the invention commences similarly to that shown in FIG. 1A, beginning with the initial state with memory  101  being marked fresh and its pointer to node  0   102 . Node  0   102  is marked head_fresh with a forward pointer to node  1   103  which is mid_valid. Mid_valid has a back pointer to the head and a forward pointer to node  2   104 , which is tail_valid. Again, node  3   105  wants to do a write_purge or an update_memory with the new data. So node  3   105  sends an MWRITE_purge command, MWRITE_PU_ATTACH_TO_LIST, from node  3   105  to memory  101 . As shown in FIG. 2B, memory  101  and node  3   105  uses the mask  500  from the write_purge_partial command to merge in the new data and does not detach the list as in FIG. 1B.  
         [0027]    So now the list is still connected with the memory  101  marked fresh. The forward pointer of the memory  101  points to node  3   105 . Node  3   105  points to node  0   102 . Node  0   102  remains in the head_fresh state since it has not received any communication from node  3   105 . Node  0   102  has a forward pointer pointing to node  1   103  mid_valid, and the mid_valid has a forward pointer to tail_valid node  2   104 .  
         [0028]    After node  3   105  receives its response from the mwrite_purge command, it then does a CPURGE not a CREAD, because in this case the line is FRESH and does not need new data. So after the mwrite_purge response, node  3   105  starts purging the list with a CPURGE_FRESH_TO_INVALID from node  3   105  to node  0   102 . Node  3   105  then receives a response from node  0  containing the next forward pointer, and issues a CPURGE_VALID_TO_INVALID from node  3   105  to node  1   103 .  
         [0029]    When node  3   105  receives a response from node  1   103  with the next forward pointer, it then performs a CPURGE_VALID_TO_INVALID from node  3   105  to node  2   104 . When node  3   105  sees that the tail_valid node, here node  2   104 , has been reached, it then does an MUPDATE_LIST_TO_HOME from node  3   105  to the memory  101  and the memory is left in the home state as shown in FIG. 2C. Thus, this method achieves the same results as shown in FIG. 1C.  
         [0030]    FIGS.  3  to  6  describe the operations handled by the various state machines inside the SCI controller to do the write_purge_partial flow.  
         [0031]    [0031]FIG. 3 describes the operations for the memory access controller Request State Machine Logic. This state machine will take requests from the memory access controller, and given the state passed by the memory access controller and the flow, will decide on the first state that is inside the SCI controller. For example, in the write_purge case, where the memory access controller requests with head_fresh, the SCI controller would initialize this request to a CS_HF_MODS_HD state, or for in a different case, head_fresh_MODS_only_head_dirty state.  
         [0032]    [0032]FIG. 4 describes how that state is turned into a request on the ring. In the previous case where the state is CS_HF_MODS_HD, this state machine would then make a ring request of mupdate_list_to_gone.  
         [0033]    FIGS.  5 A-F describes the operations for the Response State Machine. This state machine describes what is done when a response is received from the remote memory or cache. The machine decides whether to make another request to the ring, make a response to the memory access controller, or in some cases, perform both. Again following the previous case where CS_HF_MODS_HD, if the response is not nullified and is FRESH, in that case, then the machine will transition to the CS_HD_INVAL_OD state and then the FIG. 3B state machine will take this state and generate a new request.  
         [0034]    The state machine of FIGS.  6 A-C describes what happens if a request for the same line is received from another node while actively in the write purge flow. Again following the previous case where CS_HF_MODS_HD state, the state machine will allow a cupdate_prevmid or cupdate_prevtail to complete. All other requests will be nullified. The prev-TAIL will update the forward pointer and change the state.  
         [0035]    [0035]FIG. 7 depicts the symbols in a send request packet  400  on the SCI rings. The significant field in this particular packet is the second symbol in which we have a field labeled bytemask  401 . The bytemask field  401  is used to carry the byte mask information for the write_purge_partial.  
         [0036]    [0036]FIG. 8 describes the mask  500  in more detail. The mask consists of a start  501  and an end  502 , each being 5 bits. If the start is all 0&#39;s and the end is all 1&#39;s that means we had started at 0 and ended at 31, meaning we write the whole line per write_purge. Other values may be used, for example, start at 5 and end at 20, however the sub-field must always be of continuous bytes.  
         [0037]    Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.