linux-kernel - Re: [RFC][PATCH 0/3] Skip I/O merges when disabled

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Date:	Thu, 24 Apr 2008 17:04:26 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	"Alan D. Brunelle" <Alan.Brunelle@...com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH 0/3] Skip I/O merges when disabled

On Thu, Apr 24 2008, Andi Kleen wrote:
> 
> > Not a good idea IMHO, it's much better with an explicit setting. That
> > way you don't introduce indeterministic behavior.
> 
> So you would be deterministically slower.

Yes, absolutely. Think about the case for a second - the potential gain is in
fractions of a percent basically, the potential loss however is HUGE.
There's absolutely no way on earth I'd ever make this dynamic.

> Another way to avoid this problem would be to keep the statistics per
> IO context, then the same run of a program would always get the same
> behaviour. Drawback is that if your non mergeable workload consists of
> lots of short running processes (like a shell script) the optimization
> wouldn't work. Not sure if it's really practical, but it would be an option.

Complexity for basically zero gain, no thanks.

> I think in modern systems with caches etc. you typically have enough
> non quite deterministic and other surprising and hard to analyze
> behaviour anyways, so a little more doesn't make much difference.

Sorry Andi, but that is nonsense. Not merging IOs when you should can
cut your performance to a 5th or something of that order, it's an
entirely different ballgame.

-- 
Jens Axboe

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