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Date: Wed, 10 Nov 2010 20:13:29 +0000
Message-ID: <AANLkTint6mbhCxFDni63ye5TJV_Rkj+CqL6ORazCgQAq@mail.gmail.com>
Subject: Re: [Qemu-devel] [PATCH 1/3] Make paio subsystem use threadlets
	infrastructure
From: Stefan Hajnoczi <stefanha@gmail.com>
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To: arun@linux.vnet.ibm.com
Cc: qemu-devel@nongnu.org

On Wed, Nov 10, 2010 at 5:54 PM, Arun R Bharadwaj
<arun@linux.vnet.ibm.com> wrote:
> * Stefan Hajnoczi <stefanha@gmail.com> [2010-11-10 13:45:29]:
>> On Wed, Nov 10, 2010 at 1:19 PM, Arun R Bharadwaj
>> <arun@linux.vnet.ibm.com> wrote:
>> I wonder if the condition variable has a measurable performance
>> overhead. =A0We unconditionally broadcast on paiocb completion. =A0One
>> idea would be to keep a counter of waiters (should only ever be 0 or
>> 1) protected by aiocb_mutex and broadcast only when there is a waiter.
>> =A0I just want to share this idea, I don't know if it's necessary to
>> implement it or if it could even work without a race condition.
>>
>
> I did not understand exactly why we are going to see a performane hit.
> We will be doing a broadcast only after the aio_thread has finished
> the work right? So how is this going to affect performance even if we
> do a useless broadcast?

If aio_thread() broadcasts before raising the signal then POSIX aio
request completion is delayed by the time it takes to broadcast.  I
don't know if it matters though.

Stefan