linux-kernel - Re: AIO, FIO and Threads ...

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Date:	Thu, 22 Mar 2007 07:47:47 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Davide Libenzi <davidel@...ilserver.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: AIO, FIO and Threads ...

On Wed, Mar 21 2007, Davide Libenzi wrote:
> On Wed, 21 Mar 2007, Jens Axboe wrote:
> 
> > On Tue, Mar 20 2007, Davide Libenzi wrote:
> > > 
> > > I was looking at Jens FIO stuff, and I decided to cook a quick patch for 
> > > FIO to support GUASI (Generic Userspace Asyncronous Syscall Interface):
> > > 
> > > http://www.xmailserver.org/guasi-lib.html
> > > 
> > > I then ran a few tests on my Dual Opteron 252 with SATA drives (sata_nv) 
> > > and 8GB of RAM.
> > > Mind that I'm not FIO expert, like at all, but I got some interesting 
> > > results when comparing GUASI with libaio at 8/1000/10000 depths.
> > > If I read those result correctly (Jens may help), GUASI output is more 
> > > then double the libaio one.
> > > Lots of context switches, yes. But the throughput looks like 2+ times.
> > > Can someone try to repeat the measures and/or spot the error?
> > > Or tell me which other tests to run?
> > > This is kinda a suprise for me ...
> > 
> > I don't know guasi at all, but libaio requires O_DIRECT to be async. I'm
> > sure you know this, but you may not know that fio default to buffered IO
> > so you have to tell it to use O_DIRECT :-)
> > 
> > So try adding a --direct=1 (or --buffered=0, same thing) as an extra
> > option when comparing depths > 1.
> 
> I knew about AIO and O_DIRECT, but I thought FIO was using it by default :)

It actually used to, but I changed the default a few months ago as I
think that is more appropriate.

> I used it for the first time yesterday night, and there are a pretty wide 
> set of options. Will re-run today with --direct.

Yep, I try to add good explanations to all of them though, also
available through --cmdhelp or --cmdhelp=option so you don't have to
lookup the documentation all the time.

-- 
Jens Axboe

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