netdev - Re: [PATCH] poll: Avoid extra wakeups in select/poll

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Date:	Wed, 29 Apr 2009 09:37:42 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	linux kernel <linux-kernel@...r.kernel.org>,
	David Miller <davem@...emloft.net>, cl@...ux.com,
	jesse.brandeburg@...el.com, netdev@...r.kernel.org,
	haoki@...hat.com, mchan@...adcom.com, davidel@...ilserver.org,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] poll: Avoid extra wakeups in select/poll

Andi Kleen a écrit :
>> Seems that this is a virtuous patch even though Christoph is struggling
>> a bit to test it?
> 
> The main drawback is that the select/poll data structures will get
> larger. That could cause regression in theory. But I suspect
> the win in some situations is still worth it. Of course
> it would be nice if it handled more situations (like
> multiple reader etc.)

On poll()/select() interface, we must wakeup every pollers, because we dont know
if they really will consume the event

thread 1:
poll();
<insert an exit() or something bad here>
read();

thread 2:
poll(); /* no return because event was 'granted' to thread 1 */
read();


We could try to optimize read()/recvfrom() because we can know if event
is consumed, as its a whole syscall.


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