US 7213082 B2
A memory module includes a memory hub coupled to several memory devices. The memory hub is also coupled to receive a memory packet from a system controller containing a memory hint indicative of the subsequent operation of the memory devices. The memory module uses the hint to adjust the operation of the memory module, such as the number of pages to remain open or cache lines to be fetched.
1. A computer system, comprising:
a central processing unit (“CPU”);
a system controller coupled to the CPU, the system controller having an input port and an output port;
an input device coupled to the CPU through the system controller;
an output device coupled to the CPU through the system controller;
a storage device coupled to the CPU through the system controller;
a plurality of memory modules, each of the memory modules comprising:
a plurality of memory devices; and
a memory hub coupled to the system controller and the memory devices, the memory hub comprising:
a link interface receiving memory requests from the system controller for access to memory cells in at least one of the memory devices, at least some of the memory requests including respective memory hints providing information about the subsequent operation of the memory devices;
a memory device interface coupled to the memory devices and to the link interface, the memory device interface being operable to couple memory requests to the memory devices for access to memory cells in at least one of the memory devices and to receive read data responsive to at least some of the memory requests; and
a memory sequencer coupled to the link interface and the memory device interface, the memory sequencer being operable to couple memory requests to the memory device interface responsive to memory requests received from the link interface, the memory sequencer further being operable to dynamically adjust operability of the memory devices responsive to the memory hint.
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This invention relates to computer systems, and, more particularly, to a computer system having a memory hub coupling several memory devices to a processor or other memory access device.
Computer systems use memory devices, such as dynamic random access memory (“DRAM”) devices, to store data that are accessed by a processor. These memory devices are normally used as system memory in a computer system. In a typical computer system, the processor communicates with the system memory through a processor bus and a memory controller. The processor issues a memory request, which includes a memory command, such as a read command, and an address designating the location from which data or instructions are to be read. The memory controller uses the command and address to generate appropriate command signals as well as row and column addresses, which are applied to the system memory. In response to the commands and addresses, data are transferred between the system memory and the processor. The memory controller is often part of a system controller, which also includes bus bridge circuitry for coupling the processor bus to an expansion bus, such as a PCI bus.
Although the operating speed of memory devices has continuously increased, this increase in operating speed has not kept pace with increases in the operating speed of processors. Even slower has been the increase in operating speed of memory controllers coupling processors to memory devices. The relatively slow speed of memory controllers and memory devices limits the data bandwidth between the processor and the memory devices.
In addition to the limited bandwidth between processors and memory devices, the performance of computer systems is also limited by latency problems that increase the time required to read data from system memory devices. More specifically, when a memory device read command is coupled to a system memory device, such as a synchronous DRAM (“SDRAM”) device, the read data are output from the SDRAM device only after a delay of several clock periods. Therefore, although SDRAM devices can synchronously output burst data at a high data rate; the delay in initially providing the data can significantly slow the operating speed of a computer system using such SDRAM devices.
One approach to alleviating the memory latency problem is to use multiple memory devices coupled to the processor through a memory hub. In a memory hub architecture, a system controller or memory controller is coupled to several memory modules, each of which includes a memory hub coupled to several memory devices. The memory hub efficiently routes memory requests and responses between the controller and the memory devices. Computer systems employing this architecture can have a higher bandwidth because a processor can access one memory device while another memory device is responding to a prior memory access. For example, the processor can output write data to one of the memory devices in the system while another memory device in the system is preparing to provide read data to the processor.
Although computer systems using memory hubs may provide superior performance, they nevertheless often fail to operate at optimum speed for several reasons. For example, even though memory hubs can provide computer systems with a greater memory bandwidth, they still suffer from latency problems of the type described above. More specifically, although the processor may communicate with one memory device while another memory device is preparing to transfer data, it is sometimes necessary to receive data from one memory device before the data from another memory device can be used. In the event data must be received from one memory device before data received from another memory device can be used, the latency problem continues to slow the operating speed of such computer systems.
One technique that has been used to reduce latency in memory devices is to prefetch data, i.e., read data from system memory before a program being executed requests the data. Generally the data that are to be prefetched are selected based on a pattern of previously fetched data. The pattern may be as simple as a sequence of addresses from which data are fetched so that data can be fetched from subsequent addresses in the sequence before the data are needed by the program being executed. The pattern, which is known as a “stride,” may, of course, be more complex.
Further, even though memory hubs can provide computer systems with a greater memory bandwidth, they still suffer from throughput problems. For example, before data can be read from a particular row of memory cells, digit lines in the array are typically precharged by equilibrating the digit lines in the array. The particular row is then opened by coupling the memory cells in the row to a digit line in respective columns. A respective sense amplifier coupled between the digit lines in each column then responds to a change in voltage corresponding to the data stored in respective memory cell. Once the row has been opened, data can be coupled from each column of the open row by coupling the digit lines to a data read path. Opening a row, also referred to as a page, therefore consumes a finite amount of time and places a limit on the memory throughput.
Finally, the optimal decision of whether or not to prefetch data (and which data to prefetch), as well as whether or not to precharge or open a row, and whether or not to cache accessed data, may change over time and vary as a function of an application being executed by a processor that is coupled to the memory hub.
There is therefore a need for a computer architecture that provides the advantages of a memory hub architecture and also minimizes the latency and/or throughput problems common in such systems, thereby providing memory devices with high bandwidth, high throughput, and low latency. Such a system would also desirably allow the operation of the memory hub to change over time.
According to one aspect of the invention, a memory module and method is provided including a plurality of memory devices and a memory hub. The memory hub contains a link interface, such as an optical input/output port, that receives memory requests for access to memory cells in at least one of the memory devices. The memory hub further contains a memory device interface coupled to the memory devices, the memory device interface being operable to couple memory requests to the memory devices for access to memory cells in at least one of the memory devices and to receive read data responsive to at least some of the memory requests. The memory hub further is coupled to a system controller, the system controller operable to generate a memory hint. The memory hub further contains a memory sequencer coupled to the link interface and the memory device interface. The memory sequencer is operable to couple memory requests to the memory device interface responsive to memory requests received from the link interface. The memory sequencer is further operable to dynamically adjust operability responsive to the memory hint.
A computer system 100 according to one example of the invention is shown in
The system controller 110 is coupled to several memory modules 130 a,b . . . n, which serve as system memory for the computer system 100. The memory modules 130 are preferably coupled to the system controller 110 through a high-speed link 134, which may be an optical or electrical communication path or some other type of communications path. The high-speed link 134 may be either a bi-directional link, or it may include two separate bi-directional links, one of which couples signals from the system controller 110 to the memory modules 130 and the other of which couples signals from the memory modules 130 to the system controller 110. In the event the high-speed link 134 is implemented as an optical communication path, the optical communication path may be in the form of one or more optical fibers, for example. In such case, the system controller 110 and the memory modules will include an optical input/output port or separate input and output ports coupled to the optical communication path. The memory modules 130 are shown coupled to the system controller 110 in a point-to-point coupling arrangement in which a separate segment of the high-speed link 134 is used to couple each of the memory modules 130 to either each other or to the system controller 110. However, it will be understood that other topologies may also be used, such as a multi-drop arrangement in which the single high-speed link (not shown) is coupled to all of the memory modules 130. A switching topology may also be used in which the system controller 110 is selectively coupled to each of the memory modules 130 through a switch (not shown). Other topologies that may be used will be apparent to one skilled in the art.
The high-speed link 134 serves as the path for communicating command, address and data signals between the system controller 110 and the memory modules. The command, address and data signals can assume a variety of formats. However, in the embodiment shown in
Each of the memory modules 130 includes a memory hub 140 for controlling access to 32 memory devices 148, which, in the example illustrated in
A memory hub 200 according to an embodiment of the present invention is shown in
Further included in the memory hub 200 are link interfaces 210 a–d and 212 a–d for coupling the memory module on which the memory hub 200 is located to a first high speed data link 220 and a second high speed data link 222, respectively. As previously discussed with respect to
The link interfaces 210 a–d, 212 a–d are coupled to a switch 260 through a plurality of bus and signal lines, represented by busses 214. The busses 214 are conventional, and include a write data bus and a read data bus, although a single bi-directional data bus may alternatively be provided to couple data in both directions through the link interfaces 210 a–d, 212 a–d. It will be appreciated by those ordinarily skilled in the art that the busses 214 are provided by way of example, and that the busses 214 may include fewer or greater signal lines, such as further including a request line and a snoop line, which can be used for maintaining cache coherency.
The link interfaces 210 a–d, 212 a–d include circuitry that allow the memory hub 200 to be connected in the system memory in a variety of configurations. For example, the point-to-point arrangement, as shown in
The switch 260 is further coupled to four memory interfaces 270 a–d which are, in turn, coupled to the system memory devices 240 a–d, respectively. By providing a separate and independent memory interface 270 a–d for each system memory device 240 a–d, respectively, the memory hub 200 avoids bus or memory bank conflicts that typically occur with single channel memory architectures. The switch 260 is coupled to each memory interface through a plurality of bus and signal lines, represented by busses 274. The busses 274 include a write data bus, a read data bus, and a request line. However, it will be understood that a single bi-directional data bus may alternatively be used instead of a separate write data bus and read data bus. Moreover, the busses 274 can include a greater or lesser number of signal lines than those previously described.
In an embodiment of the present invention, each memory interface 270 a–d is specially adapted to the system memory devices 240 a–d to which it is coupled. More specifically, each memory interface 270 a–d is specially adapted to provide and receive the specific signals received and generated, respectively, by the system memory device 240 a–d to which it is coupled. Also, the memory interfaces 270 a–d are capable of operating with system memory devices 240 a–d operating at different clock frequencies. As a result, the memory interfaces 270 a–d isolate the processor 104 from changes that may occur at the interface between the memory hub 230 and memory devices 240 a–d coupled to the memory hub 200, and it provides a more controlled environment to which the memory devices 240 a–d may interface.
The switch 260 coupling the link interfaces 210 a–d, 212 a–d and the memory interfaces 270 a–d can be any of a variety of conventional or hereinafter developed switches. For example, the switch 260 may be a cross-bar switch that can simultaneously couple link interfaces 210 a–d, 212 a–d and the memory interfaces 270 a–d to each other in a variety of arrangements. The switch 260 can also be a set of multiplexers that do not provide the same level of connectivity as a cross-bar switch but nevertheless can couple the some or all of the link interfaces 210 a–d, 212 a–d to each of the memory interfaces 270 a–d. The switch 260 may also includes arbitration logic (not shown) to determine which memory accesses should receive priority over other memory accesses. Bus arbitration performing this function is well known to one skilled in the art.
With further reference to
The write buffer 282 in each memory interface 270 a–d is used to store write requests while a read request is being serviced. In such a system, the processor 104 can issue a write request to a system memory device 240 a–d even if the memory device to which the write request is directed is busy servicing a prior write or read request. Using this approach, memory requests can be serviced out of order since an earlier write request can be stored in the write buffer 282 while a subsequent read request is being serviced. The ability to buffer write requests to allow a read request to be serviced can greatly reduce memory read latency since read requests can be given first priority regardless of their chronological order. For example, a series of write requests interspersed with read requests can be stored in the write buffer 282 to allow the read requests to be serviced in a pipelined manner followed by servicing the stored write requests in a pipelined manner. As a result, lengthy settling times between coupling write request to the memory devices 270 a–d and subsequently coupling read request to the memory devices 270 a–d for alternating write and read requests can be avoided.
The use of the cache memory unit 284 in each memory interface 270 a–d allows the processor 104 to receive data responsive to a read command directed to a respective system memory device 240 a–d without waiting for the memory device 240 a–d to provide such data in the event that the data was recently read from or written to that memory device 240 a–d. The cache memory unit 284 thus reduces the read latency of the system memory devices 240 a–d to maximize the memory bandwidth of the computer system. Similarly, the processor 104 can store write data in the cache memory unit 284 and then perform other functions while the memory controller 280 in the same memory interface 270 a–d transfers the write data from the cache memory unit 284 to the system memory device 240 a–d to which it is coupled.
Further included in the memory hub 200 is a built in self-test (BIST) and diagnostic engine 290 coupled to the switch 260 through a diagnostic bus 292. The diagnostic engine 290 is further coupled to a maintenance bus 296, such as a System Management Bus (SMBus) or a maintenance bus according to the Joint Test Action Group (JTAG) and IEEE 1149.1 standards. Both the SMBus and JTAG standards are well known by those ordinarily skilled in the art. Generally, the maintenance bus 296 provides a user access to the diagnostic engine 290 in order to perform memory channel and link diagnostics. For example, the user can couple a separate PC host via the maintenance bus 296 to conduct diagnostic testing or monitor memory system operation. By using the maintenance bus 296 to access diagnostic test results, issues related to the use of test probes, as previously discussed, can be avoided. It will be appreciated that the maintenance bus 296 can be modified from conventional bus standards without departing from the scope of the present invention. It will be further appreciated that the diagnostic engine 290 should accommodate the standards of the maintenance bus 296, where such a standard maintenance bus is employed. For example, the diagnostic engine should have a maintenance bus interface compliant with the JTAG bus standard where such a maintenance bus is used.
Further included in the memory hub 200 is a DMA engine 286 coupled to the switch 260 through a bus 288. The DMA engine 286 enables the memory hub 200 to move blocks of data from one location in the system memory to another location in the system memory without intervention from the processor 104. The bus 288 includes a plurality of conventional bus lines and signal lines, such as address, control, data busses, and the like, for handling data transfers in the system memory. The DMA engine 286 can implement conventional DMA operations well known by those ordinarily skilled in the art. The DMA engine 286 is able to read a link list in the system memory to execute the DMA memory operations without processor intervention, thus, freeing the processor 104 and the bandwidth limited system bus from executing the memory operations. The DMA engine 286 can also include circuitry to accommodate DMA operations on multiple channels, for example, for each of the system memory devices 240 a–d. Such multiple channel DMA engines are well known in the art and can be implemented using conventional technologies.
The diagnostic engine 290 and the DMA engine 286 are preferably embedded circuits in the memory hub 200. However, including separate a diagnostic engine and a separate DMA device coupled to the memory hub 200 is also within the scope of the present invention.
As mentioned above, the command, address and data signals are preferably coupled between the memory hub controller 128 and the memory modules 130 in the form of memory packets. In accordance with one embodiment of the present invention, a “hint,” which are bits indicative of the expected future performance of the memory modules 130, is embedded in the memory packets and coupled to one or more of the memory hubs 140 in the memory modules 130. The hint, or hints, modifies the behavior of one or more memory hubs 140, as explained in greater detail below. In particular, the hint modifies the memory sequencing based on information known to or estimated by the controller 128. For example, the controller 128 may have access to addressing information such as the memory requester or address stride.
In one example of an addressing hint, the controller 128 communicates a command placing the hub 140 in page mode and identifying a number of pages to keep open. In another example, the controller 128 provides a hint related to prefetching—such as 1, 2, or 4 cache lines that will follow. In another example, the controller 128 communicates a stride to the hub 140—such as skip the next 1, 2, or 4 cache lines. In another example of a hint, the controller 128 may indicate whether or not to place a particular cache line in a hub cache. Of course, other hints may be used, or other specific information provided with the hints described.
Read and write command packets, such as the packets 300 and 350, are sent to the hub 140. One implementation of the hub 140 for receiving the packets 300 and/or 350 is shown in
From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.
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