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Date: Wed, 29 Apr 2009 09:37:42 +0200
From: Eric Dumazet <dada1@cosmosbay.com>
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To: Andi Kleen <andi@firstfloor.org>
CC: Andrew Morton <akpm@linux-foundation.org>,
       linux kernel <linux-kernel@vger.kernel.org>,
       David Miller <davem@davemloft.net>, cl@linux.com,
       jesse.brandeburg@intel.com, netdev@vger.kernel.org, haoki@redhat.com,
       mchan@broadcom.com, davidel@xmailserver.org,
       Ingo Molnar <mingo@elte.hu>
Subject: Re: [PATCH] poll: Avoid extra wakeups in select/poll
References: <alpine.DEB.1.10.0904241554100.29995@qirst.com> <49F3308B.1030507@cosmosbay.com> <20090426.020411.157511269.davem@davemloft.net> <49F43B8F.2050907@cosmosbay.com> <87ab60rh8t.fsf@basil.nowhere.org> <49F71B63.8010503@cosmosbay.com> <20090429002049.4bbc8105.akpm@linux-foundation.org> <20090429073539.GR27382@one.firstfloor.org>
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Andi Kleen a ?crit :
>> 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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