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

From: Rick Jones <[email-masked]>
Date: 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