PCIe latency
Joseph H Allen
Hello,
Suppose I have a PCIe core in an FPGA on a PCIe card plugged into a modern
PC. What read latency should I expect? How much time is it from when I
submit a request to the PCIe core to read from the PC's main memory to when
I get back the first datum from the PCIe core? How does this compare to
PCI-X?
We're architecting a new system and trying to get typical FPGA-based PCIe
latency values.
Also, we're looking for recommendations for an 8-lane PCIe development/eval
card. Which cards are people using?
Thanks
--
/* jha...@world.std.com (192.74.137.5) */ /* Joseph H. Allen */
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Kolja Sulimma
> Hello,
>
> Suppose I have a PCIe core in an FPGA on a PCIe card plugged into a modern
> PC. What read latency should I expect? How much time is it from when I
> submit a request to the PCIe core to read from the PC's main memory to when
> I get back the first datum from the PCIe core? How does this compare to
> PCI-X?
DON'T READ.
Not only the PCI standards but also the chipsets are designed around the
concept that data is written back by a bus master device.
On some modern mainboards we see something like 10 PCI33 clock cycles
between consecutive reads by the processor. Those reads are adjacent
assembler instructions in the source code. This means that the chipset
one the mainboard thinks really hard about these read accesses and maybe
takes a nap inbetween.
What we also see is that one 128-bit read instruction is split into two
64-bit read accesses on PCI and PCIe.
Kolja Sulimma