netdev - Re: Performance regression on kernels 3.10 and newer

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Date:	Fri, 15 Aug 2014 11:49:02 -0700
From:	Tom Herbert <therbert@...gle.com>
To:	Alexander Duyck <alexander.h.duyck@...el.com>
Cc:	David Miller <davem@...emloft.net>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: Performance regression on kernels 3.10 and newer

On Fri, Aug 15, 2014 at 10:15 AM, Alexander Duyck
<alexander.h.duyck@...el.com> wrote:
> On 08/14/2014 04:20 PM, David Miller wrote:
>> From: Alexander Duyck <alexander.h.duyck@...el.com>
>> Date: Thu, 14 Aug 2014 16:16:36 -0700
>>
>>> Are you sure about each socket having it's own DST?  Everything I see
>>> seems to indicate it is somehow associated with IP.
>>
>> Right it should be, unless you have exception entries created by path
>> MTU or redirects.
>>
>> WRT prequeue, it does the right thing for dumb apps that block in
>> receive.  But because it causes the packet to cross domains as it
>> does, we can't do a lot of tricks which we normally can do, and that's
>> why the refcounting on the dst is there now.
>>
>> Perhaps we can find a clever way to elide that refcount, who knows.
>
> Actually I would consider the refcount issue just the coffin nail in all
> of this.  It seems like there are multiple issues that have been there
> for some time and they are just getting worse with the refcount change
> in 3.10.
>
> With the prequeue disabled what happens is that the frames are making it
> up and hitting tcp_rcv_established before being pushed into the backlog
> queues and coalesced there.  I believe the lack of coalescing on the
> prequeue path is one of the reasons why it is twice as expensive as the
> non-prequeue path CPU wise even if you eliminate the refcount issue.
>
> I realize most of my data is anecdotal as I only have the ixgbe/igb
> adapters and netperf to work with.  This is one of the reasons why I
> keep asking if someone can tell me what the use case is for this where
> it performs well.  From what I can tell it might have had some value
> back in the day before the introduction of things such as RPS/RFS where
> some of the socket processing would be offloaded to other CPUs for a
> single queue device, but even that use case is now deprecated since
> RPS/RFS are there and function better than this.  What I am basically
> looking for is a way to weight the gain versus the penalties to
> determine if this code is even viable anymore.
>
Alex, I tried to repro your problem running your script (on bnx2x).
Didn't see see the issue and in fact ip_dest_check did not appear in
top perf functions on perf. I assume this is more related to the
steering configuration rather than the device (although flow director
might be a fundamental difference).

> In the meantime I think I will put together a patch to default
> tcp_low_latency to 1 for net and stable, and if we cannot find a good
> reason for keeping it then I can submit a patch to net-next that will
> strip it out since I don't see any benefit to having this code.
>
> Thanks,
>
> Alex
>
>
>
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