linux-kernel - Re: [PATCH 00/23] per device dirty throttling -v8

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Message-ID: <Pine.LNX.4.64.0708051158260.6905@asgard.lang.hm>
Date:	Sun, 5 Aug 2007 12:03:21 -0700 (PDT)
From:	david@...g.hm
To:	Diego Calleja <diegocg@...il.com>
cc:	Ingo Molnar <mingo@...e.hu>, Alan Cox <alan@...rguk.ukuu.org.uk>,
	J??rn Engel <joern@...fs.org>, Jeff Garzik <jeff@...zik.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>, linux-mm@...ck.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	miklos@...redi.hu, akpm@...ux-foundation.org, neilb@...e.de,
	dgc@....com, tomoki.sekiyama.qu@...achi.com, nikita@...sterfs.com,
	trond.myklebust@....uio.no, yingchao.zhou@...il.com,
	richard@....demon.co.uk
Subject: Re: [PATCH 00/23] per device dirty throttling -v8

On Sun, 5 Aug 2007, Diego Calleja wrote:

> El Sun, 5 Aug 2007 09:13:20 +0200, Ingo Molnar <mingo@...e.hu> escribió:
>
>> Measurements show that noatime helps 20-30% on regular desktop
>> workloads, easily 50% for kernel builds and much more than that (in
>> excess of 100%) for file-read-intense workloads. We cannot just walk
>
>
> And as everybody knows in servers is a popular practice to disable it.
> According to an interview to the kernel.org admins....
>
> "Beyond that, Peter noted, "very little fancy is going on, and that is good
> because fancy is hard to maintain." He explained that the only fancy thing
> being done is that all filesystems are mounted noatime meaning that the
> system doesn't have to make writes to the filesystem for files which are
> simply being read, "that cut the load average in half."
>
> I bet that some people would consider such performance hit a bug...
>

actually, it's popular practice to disable it by people who know how big a 
hit it is and know how few programs use it.

i've been a linux sysadmin for 10 years, and have known about noatime for 
at least 7 years, but I always thought of it in the catagory of 'use it 
only on your performance critical machines where you are trying to extract 
every ounce of performance, and keep an eye out for things misbehaving'

I never imagined that itwas the 20%+ hit that is being described, and with 
so little impact, or I would have switched to it across the board years 
ago.

I'll bet there are a lot of admins out there in the same boat.

adding an option in the kernel to change the default sounds like a very 
good first step, even if the default isn't changed today.

David Lang