netdev - Re: [RFC] tcp: use order-3 pages in tcp

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Message-ID: <CAAM7YAkC+xMwJCjsi0ojkj+JYgXQgqzM+6NF9L-=j5eh7xh2mQ@mail.gmail.com>
Date:	Wed, 21 Nov 2012 16:05:27 +0800
From:	"Yan, Zheng" <zheng.z.yan@...ux.intel.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev <netdev@...r.kernel.org>
Subject: Re: [RFC] tcp: use order-3 pages in tcp_sendmsg()

On Thu, Nov 15, 2012 at 9:47 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Thu, 2012-11-15 at 15:52 +0800, Yan, Zheng wrote:
>> LLC misses happen on the receiver size. It means most pages allocated by the
>> senders are cache hot. But when using order-3 pages, 2048 * 32k = 64M, 64M
>> is much larger than LLC size.
>
> By the way, this 2048*32k is wrong, as the receiver only uses fragments
> in pages, and its not related to the "tcp: use order-3 pages in
> tcp_sendmsg()" commit
>
> Many drivers use order-0 pages to hold ethernet frames, regardless of
> what was used by the sender ;)
>
>
Hi,

I think we found root cause of this regression. The test case runs
2048 instance of
netperf TCP loopback stream test on a two sockets core2 machine. There is more
LLC misses when using order-3 pages. core2 is not NUMA architecture, there is
only one memory node. Order-3 pages used by one socket may be later re-used by
another socket, which causes lots of LLC invalidation.  Using order-0
page doesn't
have this issue is because the kernel page allocator uses per-cpu list
to optimize
order-0 page allocation.

Regards
Yan, Zheng
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