Patent Application
20060143402
The present embodiments of the invention relate generally to processors and, more specifically, relate to processors using an uncacheable memory type for reads and writes to memory.
Media adapters connected to the input/output space in a computer system generate isochronous traffic that results in high-bandwidth direct memory access (DMA) writes to main memory. Because the snoop response in modern processors can be unbounded, and because of the requirements for isochronous traffic, systems are forced to use an uncacheable memory type for these transactions to avoid snoops to the processor. Such snoops to the processor can slow down a processor and interfere with its processing capabilities.
Uncacheable memory types include memory types such as Uncacheable Speculative Write Combining (USWC) memory and Uncacheable (UC) memory. These memory types are defined and allocated by the processor. Any access to the data of these memory types may not be cached in the processor. Use of uncacheable memory types avoids snoops to the processor by other processors and devices, which can interfere with the processor's own functions and throughput.
Since media data is usually non-temporal in nature, it is not desirable to use cacheable memory for such operations, as this will create unnecessary cache pollution. But, processing the media data by the processor, using the UC memory type results in low processing bandwidth and high latency. The effective throughput of the media data is limited by the processor, and is likely to become a limiting factor in the ability of future systems to deal with high-bandwidth isochronous media processing, such as processing of video data. In some processors, the latency can be slightly improved by using the USWC memory type.
Increasing the bandwidth and lowering the latency of the uncacheable memory types, while still preserving their uncacheable behavior, would greatly benefit the throughput of high-bandwidth, isochronous media data in a processor.
The present invention will be understood more fully from the detailed description given below and from the accompanying drawings of various embodiments of the invention. The drawings, however, should not be taken to limit the invention to the specific embodiments, but are for explanation and understanding only.
A method and apparatus for processing uncacheable streaming data is described. Reference in the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment.
Some embodiments of the present invention are implemented in a machine-accessible medium. A machine-accessible medium includes any mechanism that provides (i.e., stores and/or transmits) information in a form accessible by a machine (e.g., a computer, network device, personal digital assistant, manufacturing tool, any device with a set of one or more processors, etc.). For example, a machine-accessible medium includes recordable/non-recordable media (e.g., read only memory (ROM); random access memory (RAM); magnetic disk storage media; optical storage media; flash memory devices; etc.), as well as electrical, optical, acoustical or other form of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.); etc.
In the following description, numerous details are set forth. It will be apparent, however, to one skilled in the art, that the embodiments of the invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form, rather than in detail, in order to avoid obscuring the present invention.
The computer 100 further comprises a random access memory (RAM) or other dynamic storage device as a main memory 115 for storing information and instructions to be executed by the processors 110. Main memory 115 also may be used for storing temporary variables or other intermediate information during execution of instructions by the processors 110. The computer 100 also may comprise a read only memory (ROM) 120 and/or other static storage device for storing static information and instructions for the processor 110.
A data storage device 125 may also be coupled to the bus 105 of the computer 100 for storing information and instructions. The data storage device 125 may include a magnetic disk or optical disc and its corresponding drive, flash memory or other nonvolatile memory, or other memory device. Such elements may be combined together or may be separate components, and utilize parts of other elements of the computer 100.
The computer 100 may also be coupled via the bus 105 to a display device 130, such as a liquid crystal display (LCD) or other display technology, for displaying information to an end user. In some environments, the display device may be a touch-screen that is also utilized as at least a part of an input device. In some environments, display device 130 may be or may include an auditory device, such as a speaker for providing auditory information.
An input device 140 may be coupled to the bus 105 for communicating information and/or command selections to the processor 110. In various implementations, input device 140 may be a keyboard, a keypad, a touch-screen and stylus, a voice-activated system, or other input device, or combinations of such devices.
Another type of device that may be included is a media device 145, such as a device utilizing video, or other high-bandwidth requirements. The media device 145 communicates with the processor 110, and may further generate its results on the display device 130.
A communication device 150 may also be coupled to the bus 105. Depending upon the particular implementation, the communication device 150 may include a transceiver, a wireless modem, a network interface card, or other interface device. The computer 100 may be linked to a network or to other devices using the communication device 150, which may include links to the Internet, a local area network, or another environment. In an embodiment of the invention, the communication device 150 may provide a link to a service provider over a network.
Using common terminology for cache memories, the illustration shown in
Embodiments of the present invention allow the processor 205 to read uncacheable streaming data at a high throughput (the same throughput as reading cacheable data) without violating the uncacheability requirements. Uncacheable streaming data includes the Uncacheable Speculative Write Combining (USWC) memory type. Uncacheable memory types are not cached in the processor, and thus the data is only used once when accessed from memory. Embodiments of the invention also allow the processor 205 to read non-temporal streaming data without polluting the cache.
Embodiments of the present invention utilize the USWC memory type, but other embodiments are not precluded from the possibility of utilizing any other memory type to accomplish a particular objective. For example, although the Uncacheable (UC) memory type is non-speculatable, some embodiments of the present invention may employ this memory type.
Embodiments of the invention consist of two tightly coupled components:
(1) Streaming Read Buffer: A hardware mechanism that allows the processor to generate a cache-line-wide read request to uncacheable streaming memory (such as USWC), place the data in a buffer, and supply the data to the program while maintaining conventional uncacheability behavior.
(2) An instruction or other software visible mean to activate the streaming read buffer mechanism.
The Streaming Read Buffer:
The contents 330 of the LFB entry are illustrated in
In embodiments of the present invention, the new Streaming Read Buffer (SRB) may be implemented in the already existing LFB structure 320 of the L1 cache 310. The conventional LFB structure 320 in the L1 cache 310 is enhanced by a new SRB type designator 321. This type designator 321 is added to the structure to indicate that an entry is allocated to a request originated by a special SRB instruction (discussed infra) to a particular memory type, such as USWC. Furthermore, the LFB structure 320 is enhanced by a new status bit (AR bit) 323 indicating if certain data within the SRB was already read (AR).
In other embodiments, the SRB may be implemented as a separate, individual structure in the L1 cache 310. In one embodiment, the SRB is not required to be implemented in the already-existing LFB structure 320. One skilled in the art will appreciate that there may be various implementations of the SRB structure
In one embodiment, the SRB maintains coherency and uncacheability of the memory type it is storing. In another embodiment, a SRB is invalidated, flushed, and, if necessary, the proper request is reissued to refetch the data from external memory, if any of the following conditions occur:
If not, the process continues at decision block 450, where the processor determines whether a snoop hit the SRB. If not, the process continues at decision block 460, where the processor determines whether all of the AR bits in the SRB are set to one. If not the process continues at decision block 470, where the processor determines whether a fencing operation instruction has been executed. If not, then at processing block 480 the processor determines the SRB to be valid. If at any of decision blocks 420-470 the answer had been yes, then the process would continue to processing block 490, where the processor determines the SRB to be invalid.
One embodiment of the invention will mark the SRB entry invalid if any one of the above conditions occurs. Other embodiments may only mark an SRB entry invalid if one of only a subset of the above conditions occurred. One skilled in the art will appreciate that the above conditions may be altered to achieve a particular desired objective. One skilled in the art will also appreciate that the above conditions can be evaluated in parallel and not sequentially as described above.
When a SRB is flushed it is marked as invalid, but the LFB entry on which it resides is deallocated only after all data has arrived. If a SRB is invalidated for any reason, a new SRB instruction to that line will reissue a new line read to external memory. No pre-defined addressing order is required between multiple SRB instructions to the same line.
The SRB Instruction:
In one embodiment, a SRB instruction forces a cache-line-wide read of the line containing the desired memory location to be accessed. In one embodiment, the SRB instruction is a regular load instruction with a SRB hint. The SRB instruction is implemented with uncacheable memory types, such as USWC. In one embodiment, if the memory type being accessed is not of the uncacheable type, then the SRB hint has no effect and the instruction is treated as a regular load instruction of the same category.
Furthermore, in some embodiments, the SRB instruction is implemented as a hint that does not have to be implemented every time. Instead, the processor may revert to the old behavior of a regular uncacheable load. In some embodiments, the implementation of the SRB hint is processor-dependent, and can be ignored by a particular processor implementation. The amount of data prefetched is also processor implementation-dependent, but limited, in one embodiment, to the size of a cache line.
The first time the SRB instruction is executed it allocates an SRB in the LFB structure 320 and issues a cache-line-wide read request to the bus. The read request includes the requested data, plus any other data included on the line containing the memory location. For example, in some processors, a cache-line-wide request for 16 bytes of data with the SRB instruction may return 64 bytes of data (including the 16 bytes desired).
In one embodiment, upon the SRB allocation, all of the AR bits in the SRB entry are cleared to indicate that the data designated by the particular AR bit was not read yet. In this embodiment, the SRB internally prevents caching of the returning data in any cache level, or activation of any hardware prefetcher. The execution of the SRB instruction forces the uncacheable data into the SRB while keeping the uncacheable semantics of the memory type. The uncacheable semantics include not allowing the line to be cached anywhere, and not allowing each datum in the line to be used more than once.
When the data specified in the SRB instruction is available, the data value is stored in a specified register, and its corresponding AR bit in the SRB is set to indicate this particular datum was already used. The rest of the data coming from the bus is placed in the SRB. When a SRB instruction hits a SRB already allocated and the data is available with its AR bit cleared, the datum is extracted from the SRB and written back to the register and the corresponding AR bit is set.
The SRB instruction is intended for processing data generated by a device or processor that produces sequential writes to uncacheable memory types, such as USWC. Software provides proper synchronization prior to use of this instruction to ensure that all the data residing in the cache line was already written by the generating agent. In one embodiment, a fencing operation is used after a series of SRB instructions to ensure that future reads will observe subsequent writes by other processors or devices.
In one embodiment of the present invention, the streaming read buffer and SRB instruction may be implemented in a processor with an IA-32 instruction set and Pentium-M™ like micro-architecture. The SRB instruction may be implemented as a MOVDQASR xmm1, m128 (Move Aligned Double Quadword using Streaming Read hint) instruction. The MOVDQASR instruction moves the double quadword in the source operand (second operand m128) to the destination operand (first operand xmm1). The destination operand is an XMM register. The source operand is an aligned 128-bit memory location.
When the MOVDQASR instruction is executed, a SRB entry is allocated in the LFB structure with the AR bits cleared. A 64-byte read request is issued to the bus, with the read request including an attribute that internally prevents caching of the returning data to any cache level or activation of any hardware prefetcher. Each AR bit in the allocated SRB entry is associated with a particular double quadword of the 64-byte read request to memory, so that there are 4 AR bits in total.
When the double quadword specified in the instruction is available on the bus, the value is stored in the XMM register and the corresponding AR bit in the SRB is set to indicate this particular datum was used. The rest of the data (48 bytes) and the data already placed in the XMM register is placed in the allocated SRB. The SRB entry should follow the coherency and uncacheability rules mentioned above with respect to the SRB and SRB instruction description.
Many media adapters and processors use uncacheable memory types, such as USWC, for certain media data transactions. Media devices issue line-wide DMA writes to uncacheable memory type to fill a data buffer, and invoke a software routine via an interrupt or other proper synchronization method. The software routine is invoked to either copy the data to write back (WB) memory or to process the data buffer directly. The software routine may make heavy usage of the new SRB instruction to improve throughput.
Software may also utilize this SRB instruction for high-performance reads of large amounts of non-temporal data without polluting the processor cache. For example, a video capture application may utilize this operation to read high-bandwidth video data from a TV tuner device such that the non-temporal video data does not unnecessarily pollute the processor cache.
In alternative embodiments, a new Uncacheable Speculatable Streaming Read memory type may be created to produce the same results as the SRB and SRB instruction.
Whereas many alterations and modifications of the present invention will no doubt become apparent to a person of ordinary skill in the art after having read the foregoing description, it is to be understood that any particular embodiment shown and described by way of illustration is in no way intended to be considered limiting. Therefore, references to details of various embodiments are not intended to limit the scope of the claims, which in themselves recite only those features regarded as the invention.
