Abstract:
A method and system of relaxing the ordering of a read completion by setting an ordering attribute in the read completion. The relaxed ordering allows the read completion to bypass pending writes.

Description:
RELATED APPLICATION 
     None. 
     FIELD 
     Embodiments of the invention are generally related transaction ordering and in particular to systems and methods allowing modification of strict ordering. 
     BACKGROUND 
     Peripheral Component Interconnect (PCI) is a second generation parallel bus architecture developed in 1992 as a replacement for the Industry Standard Architecture (ISA) bus. In PCI, all the devices share the same bidirectional, 32-bit (or 64-bit), parallel signal path. The PCI bus brought a number of advantages over the ISA bus, including processor independence, buffered isolation, bus mastering, and true plug-and-play operation. PCI Express (PCIe) is a third generation general-purpose serial I/O interconnect designed to replace the PCI bus. Rather than being a bus, PCIe is structured around point-to-point serial links called lanes. 
     The point-to-point serial link architecture of PCI Express is well suited for distributed processing via a distributed multiprocessor architecture model. Distributed processors are generally optimized to implement data packet processing functions. Unlike general-purpose CPUs that rely heavily on caching for improving performance, distributed processors have a lack of locality in packet processing and need for high-performance I/O that has pushed designers to come up with innovative architectures to reduce processing latency while still processing packets at high data rates. 
     Currently, transaction ordering attributes in PCIe and similar interconnects must be set by the requester. Since the host CPUs are a general resource, they do not typically have the ability to set ordering attributes according to the specific requirements of the activity being performed, and must fall back to the lowest common denominator, which leads to low performance. CPU to IO reads are often the most performance critical system transactions because a CPU core may stall waiting for the result. Improving the performance of such reads therefore leads directly to an improvement of the overall system performance by freeing CPU resources for other, more useful, work. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may best be understood by referring to the following description and accompanying drawings that are used to illustrate embodiments of the invention. 
         FIG. 1  is a schematic illustration of a PCIe Request Header illustrating the Relaxed Ordering Attribute. 
         FIG. 2  is a schematic illustration of a PCIe Completion Header illustrating the Relaxed Ordering Attribute. 
         FIG. 3  is a schematic illustration of one embodiment of the invention. 
         FIG. 4  is a schematic illustration of another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     As used in the specification and claims, the singular forms “a”, “an” and “the” include plural references unless the context clearly dictates otherwise. A “root complex” (“RC”) is a PCI Express device that connects a central processing unit (“CPU” or “processor”) and memory to the PCI Express switch fabric. The root complex generates transaction requests on behalf of the processor. A “requester” is any device that makes a request. A “completer” is any entity that services a request. An “intermediate” is any device, such as a switch, between the requester and the completer. Typically, intermediates merely pass along the request. “Upstream” as used in the specification and claims indicates flow towards the root complex. “Downstream indicates flow away from the root complex. A “region” is a part of memory defined by an address or range of addresses. A device might, for example, might desire different behaviors for one region used to store instructions to the device and another region used for data produced or consumed by the device. 
     Embodiments of this invention describe how some of the performance lost to unnecessarily strict ordering can be recovered for CPU to IO device reads by having the IO device modify the ordering attributes of the completion based on the device&#39;s understanding of the transaction-specific requirements. As an example of the problem of strict ordering, consider that if a read that initially takes 2 μs to complete can be improved to 1 μs, the 1 μs difference can be “returned” to the core to do useful work—enough time to execute many thousands of instructions. Current systems frequently incur latencies on CPU to IO device reads of 0.6 μs to several μs. Large systems may incur latencies for conventionally ordered CPU to IO device reads of 4 μs. Much of this delay is caused by the conventional ordering requirement that forbids completions from bypassing writes. Since there is often a great deal of IO device to main memory write traffic, read completions to the CPU are often delayed simply by the queuing delays caused by these many writes. 
     PCIe and similar interconnects associate ordering attributes with transactions to allow optimized handling. For example, in PCI/PCIe the default ordering rules require read completions to wait for all previously issued writes flowing in the same direction, as a correctness requirement of the PCI Producer/Consumer Ordering model. However, this model is overly conservative for most cases. For example, it is usually necessary before reading a data structure describing an IO device&#39;s status to flush the IO device writes to main memory. Flushing the device writes is often done by having the CPU read from the device. However, once the flushing read is completed, additional device status reads typically do not need to be ordered against upstream writes. Further, it is not usually necessary to order the CPU&#39;s read against upstream writes from unrelated devices, even though the PCI/PCIe default ordering rules require this behavior. 
     Typically, there is no way for the host CPU to know how to mark read requests to indicate the ordering attributes required. However, the IO device typically knows which regions require conventional ordering and which do not. The ordering requirements can vary significantly from one device to another, from one register within a device to another register in the same device, and even for one register based on how the register is accessed. The knowledge of the ordering requirements could, theoretically, be communicated through device driver software running on the host, but most current Architecture CPUs provide no mechanisms for doing this. Typically, however, the IO device itself has the knowledge of the ordering requirements. The IO device, however, cannot tag the CPU to device request—this part of the transaction must be handled assuming the most conservative ordering requirements apply, unless some other mechanism is used to indicate otherwise. 
     In many cases the ordering requirements placed on the completion causes significant stalls. With systems and methods of the invention, however, the order requirement can be tagged by the TO device for ordering requirement relaxation where appropriate. The ordering requirement depends on the implementing architecture. In one embodiment of the invention, the architecture is PCIe. PCIe devices already have access to an ordering attribute called Relaxed Ordering (RO). 
       FIG. 1  shows the location of the RO bit in the PCIe Request Header. Typically, the RO bit is set to 0 in the request unless there is some mechanism that provides information for the CPU to comprehend which requests could allow the RO bit to be set. Per existing PCIe rules, the RO bit (and associated ordering implications) are simply copied by the completer from the Request into the Completion.  FIG. 2  shows the corresponding Relaxed Ordering (RO) bit in the PCIe Completion Header. 
     In PCI-x/PCIe today, the RO bit is only set in the Completion Header if it was set in the corresponding Request. This policy makes sense for device reads to main memory, because it is assumed that the device “knows” which requests can be marked RO and which cannot. Embodiments of the invention recognize that CPU reads from a device can be marked RO for the Completion returned by the device. This allows the completion to bypass unrelated device writes to memory. 
       FIG. 3  shows an example of how a relaxed ordering attribute (in this case, the RO bit of a PCI/PCIe completion) can be set by an TO device. In this example, the system  100  includes three PCI/PCIe Endpoints  1 ,  2 ,  3 . The system  100 , however, may have any number of PCI/PCIe endpoints. The first two PCI/PCIe endpoints  1 ,  2  connect to a switch  108  via PCIe interconnects  122 . The switch  108  connects to the root complex  110  through another PCIe interconnect  122 . The third PCI/PCIe endpoint  3  connects directly to the root complex  110  via a PCIe interconnect  122 . The root complex  110  includes an input/output controller hub controller (ICH)  112 , a memory controller hub (MCH)  114 , main memory  116 , and a CPU  118 . The input/output controller hub controller (ICH)  112  connects to the memory controller hub (MCH)  114  via a desktop management interface (DMI)  120 . 
     In this embodiment, Endpoints  2  and  3  are writing data to main memory  116 . These writes are unrelated to the activities of Endpoint  1 . A CPU core  118  is reading from Endpoint  1 . Conventional PCI ordering rules forbid the completion from bypassing the writes to memory under the (overly conservative) assumption that those writes could be related to the read data. This behavior is required by the PCI producer/consumer model. Typically, Endpoint  1  “knows” if the data being read by the CPU is related to the outstanding writes to memory, and will not indicate RO for the read completion if there is a relationship to the outstanding writes. In most cases, however, Endpoint  1  “knows” that the read is not related to any outstanding writes, and in this case Endpoint  1  can safely mark the completion for RO. 
       FIG. 4  illustrates another embodiment of the invention in which the addressing devices are integrated in a Root Complex  110 . The system  200  includes two integrated endpoints  124 ,  126  connected to an input/output controller hub controller (ICH)  112 . The input/output controller hub controller (ICH)  11  connects to a memory controller hub (MCH)  114  via a desktop management interface (DMI)  120 . The memory controller hub (MCH)  114  connects to a CPU  118  and main memory  116 . This embodiment includes a single PCI/PCIe endpoint  1 . The system  100 , however, may have any number of integrated addressing devices  124 ,  126  or PCI/PCIe endpoints  1 . 
     In many cases, the integrated addressing devices  124 ,  126  are used in very well defined ways, and it is possible to simplify the mechanisms used for determining when completion ordering relaxation is acceptable. Note that the specific handling of completion ordering need not follow the PCIe RO requirements. That is, other embodiments of the invention can be made simpler or more sophisticated, according to the cost/benefit desired. 
       FIG. 4  shows a system with Root Complex integrated devices participating in a completion ordering relaxation scheme. In the system illustrated in  FIG. 4  of the following policies, or other similar policies can be implemented:
         Recognizing that CPU reads from an integrated device may never have ordering requirements against writes to main memory from other integrated devices   Recognizing that CPU reads from an integrated device may never have ordering requirements against writes to main memory from non-integrated devices   Recognizing that CPU reads from a non-integrated device may never have ordering requirements against writes to main memory from other integrated devices   Recognizing that CPU reads from a non-integrated device may never have ordering requirements against writes to main memory from other non-integrated devices       

     The examples above were illustrated with the PCI/PCIe RO attribute. However, the invention is not limited to PCI/PCIe RO. In the more general case, the ordering attribute may be different from RO. Further, the way the ordering attribute is expressed may vary. It is sufficient that the Completer has the knowledge to determine if completion ordering can safely be modified from the default behavior. Additionally, the examples above were illustrated in terms of ordering against upstream writes. The system, however, is symmetric. That is, the ordering attribute can be relaxed to change to ordering against downstream writes as well. 
     The embodiments of the invention may provide performance improvement that is desirable for a broad array of computer systems. PC compatible architecture systems are particularly constrained because requirements associated with support for legacy hardware and software make it very difficult to implement ordering relaxations. The mechanism described above, however, works well in PC environments. 
     While the invention has been described in terms of several embodiments of the invention, those of ordinary skill in the art will recognize that the invention is not limited to the embodiments of the invention described, but can be practiced with modification and alteration within the spirit and scope of the appended claims. The description is thus to be regarded as illustrative instead of limiting.