PCI Express bridging: Optimizing PCI read performance

November 1, 2008 OpenSystems Media

PCI Express (PCIe) is now a ubiquitous interconnect standard on PC chipsets and embedded processors. Although using a bridge can provide existing PCI devices with a cost-effective upgrade path to PCIe, the resultant PCI reads produce additional latency that can significantly hinder system performance. Craig explores how implementing PCIe bridges can resolve performance issues caused by PCI reads.

While PCIe is replacing the original PCI bus standard, many peripherals and devices such as FPGAs and I/O modules still use PCI. Components without an integrated PCIe interface need a bridge from the PCI bus to PCIe. Two application examples that require a bridge include a PCIe add-in processor card that uses a PCI-based DSP for communications applications and an embedded video recorder that uses I/O devices with PCI to connect to an embedded processor with PCIe ports.

In these and other systems, adding a bus bridge presents design challenges. Performance often depends on the bridge's PCI read performance. Because of limitations in the older PCI protocol, performance problems can arise once a bridge is introduced. These problems can be eliminated using commercially available bus bridges to maximize system performance.

Heavy-duty reading burdens PCI devices

Devices on the PCI bus often depend heavily on reading large amounts of data from the host processor. PCI I/O devices typically control data movement in the system and initiate both read and write cycles to the host's memory. For example, the host processor might organize and orchestrate data movement by writing to registers on the peripherals to set up DMA transfers in the peripherals, but it will remove itself from data movement to focus on other tasks. The peripheral device will then read or write data to service the DMA request. At other times, the processor might read status information and write to registers for control. This traffic typically does not involve high bandwidth or contribute significantly to overall system performance.

In the case of a processor add-in card, its DSP must read data from the host PC memory for data processing or compression tasks. Likewise, an embedded processing system such as a security DVR will capture and compress video that will be written to disk storage via the disk controllers. The disk controllers achieve this by reading data from the host's memory via the PCI bus.

Writes from PCI peripherals to a bridge are usually posted in an internal write buffer to overcome the inherent performance penalty a bridge imposes. However, PCI reads introduce problems as the PCIe bridge must retry the peripheral device until it obtains the requested data from the host's memory. This usually involves attempting to read many small PCIe packets, thus adding delay.

While the PCI-X protocol skirts this problem through split transactions, the conventional PCI protocol does not implement this feature. Additionally, some PCI devices were designed to automatically release the bus after receiving one or two cache lines of data, compounding the performance challenge with PCI reads.

Take the DSP processor card application, for example. This particular DSP uses a 32-bit PCI interface. As with many PCI devices, it will read one or two cache lines of data before releasing the PCI bus. A cache line in this case consists of 16 to 128 bytes depending on the system design and device capabilities.

The card will read large blocks of raw data for processing, such as audio bit streams for processing within telecom applications. In legacy systems where the DSP communicates with the host processor directly over a PCI bus, read performance will be better than after a bridge is added because of the additional latency for each transaction.

Bridge-induced performance deterioration

Introducing a PCIe bridge can cause a significant hit in performance. This read performance degradation can occur through the following process (Figure 1):

  1. The DSP will initiate a read from PC main memory. The bridge will latch the transaction and keep retrying until the bridge receives the data.
  2. The bridge will pre-fetch data from memory and store it in the internal buffer.
  3. The DSP will read a portion of the data (one or two cache lines) and then disconnect, releasing the PCI bus.
  4. Once disconnected, the bridge will discard any remaining data in its buffer. With the next read initiated by the DSP, the bridge will need to fetch the data again, retrying until the data is available for the DSP.

Figure 1

In this example, step 4 introduces significant delay between read transactions, which dramatically affects read performance. The impact on read performance that results from using a PCIe bridge can thus diminish system performance to a much greater degree than what can be achieved using the PCI bus directly.

Consider another common situation using an embedded DVR. In this case, the system must write continuous streams of compressed video data to disk for storage and later retrieval or analysis. In this system, one or more SATA disk controllers will read the video data from the system's main memory to store in the attached disk array. These types of systems may contain additional peripherals such as an Ethernet controller sharing the PCI bus.

Like the previous example, bus efficiency in the embedded DVR is seriously affected by continuous data reads followed by a bus disconnect and many retries and pre-fetches, as illustrated in Figure 2.

Figure 2: PCI bus utilization in a DVR system

As evidenced by the PCI_TRDYn (PCI Target Ready) trace that indicates where data is being read, there is a large gap between the first read (step 3) and the next read (step 6). This translates into a significant reduction in the maximum speed at which video data can be written to disk, therefore limiting system performance. In this case, maximizing the speed at which video data can be written is critical to the end product, providing the ability to store as many high-quality video channels as necessary for surveillance.

Solving the problem: Short-Term Caching

Tundra's PCIe bridges (Tsi381, Tsi382, and Tsi384) incorporate a feature known as Short-Term Caching (STC) to help overcome this performance challenge. STC allows data to be pre-fetched from the attached PCIe device during an initial PCI read cycle and temporarily stored in the bridge for quick turnaround during subsequent read cycles. Data that would be read in subsequent reads is not immediately discarded when the requested device stops the transaction.

STC's effect on performance can be dramatic compared to the initial bridging situation. Take the previous example of the embedded DVR but with a Tsi381 bridge added to the system. By enabling STC, subsequent reads are not delayed because they remain in cache. Furthermore, the bridge does not have to reread the data from memory after the first read, ensuring that the bus is used efficiently (see Figure 3).

Figure 3: Embedded DVR system using Tsi381 with Short-Term Caching

To demonstrate the difference in performance, compare the timing for the PCI bus before and after enabling STC (see Figure 4). In this example where the system performs 32-bit reads per device, the two devices can perform seven reads during the same period that only two reads were performed without STC. Hence, overall system throughput can be improved by a factor of more than three with STC. For a system with a single device performing such reads, the improvement will be even greater.

Figure 4: PCI bus timing before and after enabling Short-Term Caching

PCIe bridges allow developers to further modify the system by adjusting the following parameters:

  • The time that data is held in the cache. This allows the designer to ensure that stale data is discarded and pre-fetched once stale.
  • PCI read pre-fetch length. Ideally, the bridge should not pre-fetch more data than what the PCI peripheral typically requires, so designers can set this parameter based on the typical transfer lengths expected.

Depending on the system design and device behaviour, designers can improve overall performance or performance for critical functions by adjusting the pre-fetch length and short-term cache discard timer.

Bridge options help eliminate bottlenecks

Designers who use PCIe bridges to migrate designs from PCI to PCIe face considerable design challenges. Reads initiated by PCI peripheral devices introduce additional latency, which contributes significantly to overall system performance. PCIe bridges such as Tundra's Tsi381 give designers options to tune bridges, providing optimum system performance. Using STC, these bridges can easily remove performance bottlenecks associated with PCI reads.

Craig Downing is product marketing manager for PCIe products at Tundra Semiconductor, based in Ottawa, Ontario, Canada. During the past 20 years, he has held several engineering, sales, marketing, and product management positions at various high-tech companies with experience in applications including videographics, industrial machine vision, image processing systems, and media processors for consumer electronics. Craig has a Bachelor of Engineering (Electrical) degree from McGill University, Montreal.

Tundra Semiconductor

Craig Downing (Tundra Semiconductor)
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