Re: [PATCH] mlx4_en: map entire pages to increase throughput

Rick Jones <rick.jones2@xxxxxx> · Mon, 16 Jul 2012 10:27:57 -0700

On 07/16/2012 10:01 AM, Thadeu Lima de Souza Cascardo wrote:

In its receive path, mlx4_en driver maps each page chunk that it pushes
to the hardware and unmaps it when pushing it up the stack. This limits
throughput to about 3Gbps on a Power7 8-core machine.

That seems rather extraordinarily low - Power7 is supposed to be a 

rather high performance CPU.  The last time I noticed O(3Gbit/s) on 10G 

for bulk transfer was before the advent of LRO/GRO - that was in the x86 

space though.  Is mapping really that expensive with Power7?

One solution is to map the entire allocated page at once. However, this
requires that we keep track of every page fragment we give to a
descriptor. We also need to work with the discipline that all fragments will
be released (in the sense that it will not be reused by the driver
anymore) in the order they are allocated to the driver.

This requires that we don't reuse any fragments, every single one of
them must be reallocated. We do that by releasing all the fragments that
are processed and only after finished processing the descriptors, we
start the refill.

We also must somehow guarantee that we either refill all fragments in a
descriptor or none at all, without resorting to giving up a page
fragment that we would have already given. Otherwise, we would break the
discipline of only releasing the fragments in the order they were
allocated.

This has passed page allocation fault injections (restricted to the
driver by using required-start and required-end) and device hotplug
while 16 TCP streams were able to deliver more than 9Gbps.

What is the effect on packet-per-second performance?  (eg aggregate, 

burst-mode netperf TCP_RR with TCP_NODELAY set or perhaps UDP_RR)

rick jones
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