Abstract:
A method, system and computer program product for handling write requests in a data processing system is disclosed. The method comprises receiving on an interconnect bus a first write request targeted to a first address and receiving on the interconnect bus a subsequent second write request targeted to a subsequent second address. The subsequent second write request is completed prior to completing the first write request, and, responsive to receiving a read request targeting the second address before the first write request has completed, data associated with the second address of the second write request is supplied only after the first write request completes.

Description:
BACKGROUND OF THE INVENTION 
   1. Technical Field 
   The present invention relates in general to data processing systems and in particular to managing memory access in data processing systems. Still more particularly, the present invention relates to a system, method and computer program product for preserving the ordering of read and write operations in a direct memory access system by delaying read access. 
   2. Description of the Related Art 
   A conventional symmetric multiprocessor (SMP) computer system, such as a server computer system, includes multiple processing units coupled to a system interconnect, which typically comprises one or more address, data and control buses. Coupled to the system interconnect is a system memory, which represents the lowest level of volatile memory in the multiprocessor computer system and which generally is accessible for read and write access by all processing units. In order to reduce access latency to instructions and data residing in the system memory, each processing unit is typically further supported by a respective multi-level cache hierarchy, the lower level(s) of which may be shared by one or more processor cores. 
   One aspect of design that affects cache performance and design complexity is the handling of writes initiated by the processor or by an alternate bus master. Because two copies of a particular piece of data or instruction code can exist, one in system memory and a duplicate copy in the cache, writes to either the system memory or the cache memory can result in an inconsistency between the contents of the two storage units. For example, consider the case in which the same data in both the cache memory and the system memory in association with a particular address. If the processor subsequently initiates a write cycle to store a new data item at the predetermined address, a cache write “hit” occurs and the processor proceeds to write the new data into the cache memory. Since the data is modified in the cache memory but not in the system memory, the cache memory and system memory become inconsistent. Similarly, in systems with an alternate bus master, direct memory access (DMA) write cycles to system memory by the alternate bus master modify data in system memory but not in the cache memory. Again, the data in the cache memory and system memory become inconsistent. 
   Inconsistency between data in the cache memory and data in system memory during processor writes can be prevented or handled by implementing one of several commonly employed techniques. In the first technique, a “write-through” cache guarantees consistency between the cache memory and system memory by writing the same data to both the cache memory and system memory. The contents of the cache memory and system memory are always identical, and so the two storage systems are always coherent. In a second technique, a “write back” cache handles processor writes by writing only to the cache memory and setting a “dirty” bit to indicate cache entries which have been altered by the processor. When “dirty” or altered cache entries are later replaced during a “cache replacement” cycle, the modified data is written back into system memory. 
   Inconsistency between data in the cache memory and corresponding data in system memory during a DMA write operation is handled somewhat differently. Depending upon the particular caching architecture employed, one of the variety of bus monitoring or “snooping” techniques may be used. One such technique involves the invalidation of cache entries which become “stale” or inconsistent with system memory after a DMA write to system memory occurs. Another technique involves the “write-back” to system memory of all dirty memory blocks within the cache memory prior to the actual writing of data by the alternate bus master. After the dirty memory blocks that are targeted by the DMA write is written back to the system memory, the memory blocks are invalidated in the cache, and the write by the alternate bus master may be performed. 
   As systems become larger and the latency required to resolve cache coherence increases, this latency can limit the bandwidth that a DMA device is able to achieve in the system. To sustain full DMA write throughput, the system must balance the amount of time to resolve cache coherence with the amount of data transferred per request. The traditional method of balancing time required to resolve cache coherence and the amount of data transferred per request is to design the system with a larger cache line size. Thus, with a larger cache line size, more data can be invalidated per cache line invalidation request. However, the major drawbacks of increasing the cache line size include trailing edge effects and the increased likelihood of false sharing of data within the larger cache lines. 
   Therefore, there is a need for an improved system and method of increasing the throughput capacity of DMA devices without increasing the size of the cache line within the cache memory. 
   SUMMARY OF THE INVENTION 
   A method, system and computer program product for handling write requests in a data processing system is disclosed. The method comprises receiving on an interconnect bus a first write request targeted to a first address and receiving on the interconnect bus a subsequent second write request targeted to a subsequent second address. The subsequent second write request is completed prior to completing the first write request, and, responsive to receiving a read request targeting the second address before the first write request has completed, data associated with the second address of the second write request is supplied only after the first write request completes. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed descriptions of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
       FIG. 1  illustrates a high level block diagram of a processing unit in accordance with the present invention; 
       FIG. 2  depicts a high level block diagram of a memory controller in accordance with the present invention; 
       FIG. 3  is a high level logical flowchart of a process for assigning instructions to an appropriate queue in accordance with the present invention; 
       FIG. 4  is a high-level logical flowchart of a process for queuing read requests and performing read operations in accordance with a preferred embodiment of the present invention; and 
       FIG. 5  is a high-level logical flowchart of a process for queuing write requests and performing write operations in accordance with a preferred embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   In the present invention, DMA write requests are sent to interconnect logic upon receipt from the I/O controller or interconnect logic. If an older DMA write request receives retry response while a newer DMA write is in flight, the newer DMA write is allowed to complete, but the I/O controller issues a retry response to any subsequent read of the newer DMA write data until all older DMA writes have completed. 
   With reference now to the figures and, in particular, with reference to  FIG. 1 , there is illustrated a high level block diagram of an exemplary embodiment of a data processing system  10  containing a plurality of processing units  100  in accordance with the present invention. In the depicted embodiment, processing unit  100  is a single integrated circuit including two processor cores  102   a ,  102   b  for independently processing instructions and data. Each processor core  102  includes at least an instruction sequencing unit (ISU)  104  for fetching and ordering instructions for execution and one or more execution units  106  for executing instructions. The instructions executed by execution units  106  may include, for example, fixed and floating point arithmetic instructions, logical instructions, and instructions that request read and write access to a memory block. 
   The operation of each processor core  102   a ,  102   b  is supported by a multi-level volatile memory hierarchy having at its lowest level one or more shared system memories  132  (only one of which is shown in  FIG. 1 ) and, at its upper levels, one or more levels of cache memory. As depicted, processing unit  100  includes an integrated memory controller (IMC)  124  that controls read and write access to a system memory  132  in response to requests received from processor cores  102   a ,  102   b  and operations snooped on an interconnect fabric. 
   In the illustrative embodiment, the cache memory hierarchy of processing unit  100  includes a store-through level one (L1) cache  108  within each processor core  102   a ,  102   b  and a level two (L2) cache  110  shared by all processor cores  102   a ,  102   b  of the processing unit  100 . L2 cache  110  includes an L2 array and directory  114 , masters  112  and snoopers  116 . Masters  112  initiate transactions on the interconnect fabric and access L2 array and directory  114  in response to memory access (and other) requests received from the associated processor cores  102   a ,  102   b . Snoopers  116  detect operations on the interconnect fabric, provide appropriate responses, and perform any accesses to L2 array and directory  114  required by the operations. Although the illustrated cache hierarchy includes only two levels of cache, those skilled in the art will appreciate that alternative embodiments may include additional levels (L3, L4, etc.) of on-chip or off-chip in-line or lookaside cache, which may be fully inclusive, partially inclusive, or non-inclusive of the contents the upper levels of cache. 
   As further shown in  FIG. 1 , processing unit  100  includes integrated interconnect logic  120  by which processing unit  100  may be coupled to the interconnect fabric as part of a larger data processing system. In the depicted embodiment, interconnect logic  120  supports an arbitrary number N of interconnect links  121 , which include in-bound and out-bound links. With these interconnect links  121 , each processing unit  100  may be coupled for bi-directional communication to up to N/2+1 other processing units  100 . 
   Each processing unit  100  further includes an instance of response logic  122 , which implements a portion of a distributed coherency signaling mechanism that maintains cache coherency between the cache hierarchy of processing unit  100  and those of other processing units  100 . Finally, each processing unit  100  includes an integrated I/O (input/output) controller  128  supporting the attachment of one or more I/O devices, such as I/O device  130 . I/O controller  128  may issue I/O read and I/O write operations and transmit data to and receive data from the local IMC  124  and interconnect links  121  in response to requests by I/O device  130 . 
   Turning now to  FIG. 2 , a high-level block diagram of a memory controller in accordance with the present invention is depicted. Integrated memory controller  124  contains dispatch logic  200  for routing incoming read and writes requests to a read queue  202  and a write queue  204 , respectively. Read queue  202  holds read requests before servicing by reference to them to system memory  132 . Read queue  202  contains several entries  206   a – 206   n , each of which has a Ttype  208  and an address  210 , regulated by a read queue control  212 . 
   Similarly, write queue  204  holds write requests before servicing by reference to them to system memory  132 . Write queue  204  contains several entries  220   a – 220   n , each of which has a reorder bit  222 , a Ttype  224  and an address  226 , regulated by a write queue control  230 . As will be explained below with respect to  FIGS. 3–5 , IMC  124  allows multiple DMA writes from a single I/O device  130  to remain ordered as observed by any potential consumer of data within data processing system  10  by reordering writes  220   a – 220   n  through adjustment of reorder bit  222  and control of read queue  202 . 
   Referring now to  FIG. 3 , a high-level logical flowchart of a process by which IMC  124  assigns read and write requests to an appropriate queue in accordance with the present invention is illustrated. The process starts at step  300  and then moves to step  302 , which depicts dispatch logic  200  of integrated memory controller  124  determining whether or not a read-type request has been received. If not, then the process iterates at step  302 . If a request is received at step  302 , then the process next proceeds to step  304 . At step  304 , dispatch logic  200  of integrated memory controller  124  determines the Ttype (transaction type) of the request received in step  302 . If the request is a read-type request, the process next moves to step  306 , which depicts dispatch logic  200  of integrated memory controller  124  allocating an entry in read queue  202  to the read-type request received in step  302  and placing the read-type request in the allocated entry in read queue  202 . The process then ends at step  308 . 
   Returning to step  304 , if dispatch logic  200  of integrated memory controller  124  determines that the Ttype of the request received in step  302  is a write-type request, then the process next moves to step  310 . At step  310 , dispatch logic  200  of integrated memory controller  124  allocates an entry in write queue  204  to the request received in step  302  and places the write-type request in the allocated entry in write queue  204 . The process then ends at step  308 . 
   Turning now to  FIG. 4 , a high-level logical flowchart of a process by which read queue  202  services a read-type request in accordance with the preferred embodiment of the present invention is depicted. The process starts at step  400  and then moves to step  404 , which depicts read queue controller  212  determining whether a read-type request has been received from dispatch logic  200 . If no read-type request has been received, then the process iterates to step  404 . 
   If read queue controller  212  determines that a read-type request has been received in one of the entries  206  of read queue  202 , then the process next moves to step  406 , which depicts read queue controller  212  determining whether any pending re-ordered write request exists within write queue  204  having a matching request address. In one preferred embodiment, read controller  212  queue makes this determination by reference to comparing address field  210  of the read request with the address fields  226  of the pending write requests and by checking the reorder flag  222  of any matching entry. In a preferred embodiment, if no address match is found for a re-ordered write request, then the process proceeds to step  408 . At step  408 , read queue controller  212  performs the requested read-type operation and routes the requested data to the appropriate destination. Thereafter, at block  410 , read queue controller  212  de-allocates the entry in read queue  202  allocated to the read-type request. The process then ends at step  412 . 
   Returning to step  406 , if read queue controller  212  determines that any pending re-ordered write request exists within write queue  204  having a matching request address, the process will next proceed to step  414 . At step  414 , integrated memory controller  124  will provide a retry partial response to the sender of the read request, which can be any consumer of data on data processing system  10 . 
   In an alternative embodiment, at step  406 , if read queue controller  212  determines that any pending re-ordered write request exists within write queue  204  having a matching request address, then the process will proceed to step  416 . At step  416 , will allow read queue control  212  on integrated memory controller  124  will queue and hold the read-type request until any pending re-ordered write request that exists within write queue  204  having a matching request address completes. The process then moves to step  408 , which is described above. As will be apparent to those skilled in the art, source queuing is generally preferred in a memory system. However, those skilled in the art will realize that some specialized applications may require destination queuing, such as is indicated with respect to step  416 . Destination queuing, such as is indicated with respect to step  416 , lies within the scope and spirit of the present invention. 
   Turning now to  FIG. 5 , a high-level logical flowchart of a process by which write queue  204  services a write-type request in accordance with the preferred embodiment of the present invention is depicted. The process starts at step  500  and moves to step  504 . At step  504 , write queue controller  230  determines whether a write-type request has been received from dispatch logic  200 . If no write request is received at dispatch logic  200 , then the process iterates to step  504 . 
   If write queue controller  230  determines that a write-type request has been received from dispatch logic  200 , then the process next moves to step  508 , which depicts write queue controller  230  determining whether any pending re-ordered write-type request exists within write queue  204  having a matching request address. If write queue controller  230  determines that any pending re-ordered write-type request exists within write queue  204  having a matching request address, then the process next proceeds to step  510 . 
   At step  510 , write queue controller  230  on integrated memory controller  124  determines whether re-ordering is enabled by inspecting reorder bit  222 . If write queue controller  230  on integrated memory controller  124  determines that reorder bit  222  indicates re-ordering is enabled, then the process next moves to step  512 , which depicts write queue controller  230  on integrated memory controller  124  performing a second subsequent received write request before a first received write request. The process then ends at step  514 . 
   Returning to step  508 , if write queue controller  230  determines that no pending re-ordered write-type request exists within write queue  204  having a matching request address, then the process next proceeds to step  516 , which depicts write queue controller  230  determining whether the received write request is the next write-type request to be serviced. Those skilled in the art will realize that while a first-in first-out buffering and queuing system will be common in the art, alternative queuing mechanisms can be used to determine priority of fulfillment of write requests without departing from the spirit and scope of the present invention. If the write request received at step  504  is determined by write queue controller  230  to be the next to be serviced, then the process moves to step  518 , which depicts integrated memory controller  124  performing the write-type request. The process then ends at step  514 . 
   Returning to step  516 , if write queue controller  230  determines that the write request received at step  504  is not the next to be serviced, then the process next moves to step  520 , which depicts write queue controller  230  determining by inspecting reorder bit  222  of each entry  220   a – 220   n  whether there is a later-received write-type request that is to be re-ordered. If write queue controller  230  determines that there is no later write request to be re-ordered, then the process returns to step  516 . If write queue controller  230  determines that there is a later write request to be re-ordered, then the process proceeds to step  522 , which depicts write queue controller  230  on integrated memory controller  124  performing a second subsequent received write request before a first received write request. The process then returns to step  516 , which is described above. 
   An example is provided below. While the example below is explained with respect to an environment with two write requests and one read request, those skilled in the art will quickly anticipate that the present invention applies equally to any set of multiple writes and multiple reads, and that the present invention is substantially scalable. The following example of system behavior illustrates the performance of a preferred embodiment:
     1. DMA address A is broadcast by interconnect logic  120 .   2. DMA address B is broadcast by interconnect logic  120 .   3. DMA address A receives a response indicating that the operation must be retried.   4. DMA address B receives a response indicating that the operation is successful.   5. DMA address A is broadcast on interconnect logic  120 .   6. DMA address A receives a response indicating that the operation is successful.   

   During the time required to complete step 5 and step 6, if any processor or other consumer of data attempts to read the data from DMA write to address B, I/O controller  128  issues a retry response to prevent the read from completing, thereby restricting read access. By allowing DMA writes to deliver data independently and enforcing coherency by restricting subsequent read access when required, the DMA write ordering rules are met without substantial negative impact to bandwidth and throughput. 
   While the present invention is explained with respect to an environment with two write requests and one read request, those skilled in the art will quickly anticipate that the invention applies equally to any set of multiple writes and multiple reads, and that the present invention is substantially scalable. Further, as used with respect to the present invention, the terms second and second subsequent refer to any subsequent write request without regard to how many intervening write requests have accumulated. 
   While the invention has been particularly shown as described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention. It is also important to note that although the present invention has been described in the context of a fully functional computer system, those skilled in the art will appreciate that the mechanisms of the present invention are capable of being distributed as a program product in a variety of forms, and that the present invention applies equally regardless of the particular type of signal bearing media utilized to actually carry out the distribution. Examples of signal bearing media include, without limitation, recordable type media such as floppy disks or CD ROMs and transmission type media such as analog or digital communication links.