netdev - Re: big picture UDP/IP performance question re 2.6.18 -> 2.6.32

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Date:	Sun, 02 Oct 2011 10:03:23 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	starlight@...nacle.cx
Cc:	linux-kernel@...r.kernel.org, netdev <netdev@...r.kernel.org>,
	Willy Tarreau <w@....eu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: big picture UDP/IP performance question re 2.6.18 -> 2.6.32

Le dimanche 02 octobre 2011 à 09:21 +0200, Eric Dumazet a écrit :

> You might try to disable any fancy power saving mode in your machine.
> Maybe on your machine, cost to enter/exit deep sleep state is too high.
> 
> I see nothing obvious in the profile but userland processing, futex
> calls. 

Just to clarify a bit :

Sometimes, optimizing one part of the kernel can have a negative impact
on some workloads because we end up doing more sleep/wakeup of
consumers : Several threads might try to acquire a lock at the same
time, while previously they got no contention.

In 2.6.35, commit c377411f2494a (net: sk_add_backlog() take rmem_alloc
into account) changed backlog limit, avoid taking socket lock on flood,
allowing to receive 200.000 pps on a test machine instead of 100pps.
But the receiver was doing a plain

while (1)
	recv(...);

And maximum throughput was reached because task never called
scheduler...

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