On 11/6/2015 12:22 PM, Bjorn Helgaas wrote:
I've never been thrilled about the way Linux ties MRRS and MPS
together. I don't think the spec envisioned MRRS being used to
control segment size on the link. My impression is that the purpose
of MRRS is to limit the amount of time one device can dominate a link.
I am sympathetic to the idea of having MRRS larger than MPS. The
question is how to accomplish that. I'm not really happy with the
current set of "pcie_bus_tune_*" parameters, so I'd hesitate to add
yet another one. They feel like they're basically workarounds for the
fact that Linux can't optimize MPS directly itself.
Can you give any more specifics of your MRRS/MPS situation? I guess
you hope to improve bandwidth to some device by reducing the number of
read requests? Do you have any quantitative estimate of what you can
gain?
Xilinx has a nice whitepaper about PCIe performance here. See the
section about Maximum Read Request Size.
http://www.xilinx.com/support/documentation/white_papers/wp350.pdf
The benefits of maximum read request is seen when moving large amounts
of data usually.
--
Sinan Kaya
Qualcomm Technologies, Inc. on behalf of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a
Linux Foundation Collaborative Project
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