linux-kernel - Re: IPC between application and xserver is causing extra context switches

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Date:	Fri, 3 Sep 2010 17:24:32 +0300
From:	Pauli Nieminen <ext-pauli.nieminen@...ia.com>
To:	ext Peter Zijlstra <peterz@...radead.org>
Cc:	ext Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Tony Lindgren <tony@...mide.com>,
	Jarkko Nikula <jhnikula@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Pauli <suokkos@...il.com>
Subject: Re: IPC between application and xserver is causing extra context
 switches

On 03/09/10 09:31 +0200, ext Peter Zijlstra wrote:
> On Fri, 2010-09-03 at 10:17 +0300, Pauli Nieminen wrote:
> > Scheduling at write is wrong because xserver doesn't know about client
> > priorities. 
> 
> Waking up the client at write is correct because you don't know if there
> is more to be written.
> 

IMO xserver is already signaling kernel when there is nothing more to write.
There is nothing more to write when xserver calls select after processing
request and writing responses.

O_NONBLOCK is set for file descriptors if it matters.

syscalls will be:

select(all_fds);

read(2, ...);
read(5, ...);

writev(3, ...);
writev(5, ...);
writev(6, ...);

select(all_fds);

Of course this is a lot simplified what really happens.

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