linux-ext4 - Re: [GIT PULL] Ext3 latency fixes

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Date:	Sat, 4 Apr 2009 08:57:28 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Jens Axboe <jens.axboe@...cle.com>
cc:	Theodore Tso <tytso@....edu>,
	Linux Kernel Developers List <linux-kernel@...r.kernel.org>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: [GIT PULL] Ext3 latency fixes

On Sat, 4 Apr 2009, Jens Axboe wrote:
> 
> Big nack on this patch. Ted, this is EXACTLY where I told you we saw big
> write regressions (sqlite performance drops by a factor of 4-5). Do a
> git log on fs/buffer.c and see the original patch (which does what your
> patch does) and the later revert. No idea why you are now suggestion
> making that exact change?!

Jens, if I can re-create the 'fsync' times (I haven't yet), then the 
default scheduler _will_ be switched to AS.

> Low latency is nice, but not at the cost of 4-5x throughput for real
> world cases.

I'm sorry, but that fsync thing _is_ a real-world case, and it's the one 
that a hell of a lot more people care about than some idiotic sqlite 
throughput issue.

You have a test-case now. Consider it a priority, or consider CFQ to be a 
"for crazy servers that only care about throughput".

Quite frankly, the fact that I can see _seconds_ of latencies with a 
really good SSD is not acceptable. The fact that it is by design is even 
less so.

Latency is more important than throughput. It's that simple.

			Linus
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