linux-kernel - Re: [PATCH 0/8][RFC] IO latency/throughput fixes

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Date:	Mon, 6 Apr 2009 22:02:03 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"Trenton D. Adams" <trenton.d.adams@...il.com>
cc:	Chris Mason <chris.mason@...cle.com>, Theodore Tso <tytso@....edu>,
	Hua Zhong <hzhong@...il.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/8][RFC] IO latency/throughput fixes

On Mon, 6 Apr 2009, Trenton D. Adams wrote:
> 
> What about a procfs setting instead?  Is there a policy about why
> something should be in procfs or /sys, or as a kernel config option?
> That's basically as small as the patch you just made, right?

I'm never really against making things dynamically tunable, but this 
already was, and that wasn't really the issue.

Sure, you can just re-mount your filesystem with different options. That's 
what I did while testing - my /home is on a drive of its own, so I would 
just log out and as root unmount and re-mount with data=ordered/writeback, 
and log in and test again.

So dynamic tuning is good. But at the same time, having a tuning option is 
_never_ an excuse for not getting the default right in the first place. 
It's just a cop-out to say "hey, the default may be wrong for you, but you 
can always tune it locally with XYZ".

The thing is, almost nobody does that. Partly because it needs effort and 
knowledge, partly because after a few years the number of tuning knobs are 
in the hundreds for every little thing. 

So instead, leave the tuning for the _really_ odd cases (if you use your 
machine as an IP router, you hopefully know enough to tune it if you 
really care). Not for random general-purpose "use for whatever" kind of 
thing. 

> I'm just thinking that something like this, where people want one
> thing or the other, but may not know it when they install Linux, might
> like to change it realtime.  Especially if they are a Linux newbie,
> and don't know how to compile their own kernel.  Or don't have time to
> maintain their own kernel installs.

Oh absolutely. I'm not expecting people to compile their own kernels. I'm 
expecting that within a few months, most modern distributions will have 
(almost by mistake) gotten a new set of saner defaults, and anybody who 
keeps their machine up-to-date will see a smoother experience without ever 
even realizing _why_.

			Linus
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