linux-kernel - Re: Linux 2.6.29

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Date:	Thu, 26 Mar 2009 17:47:04 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Theodore Tso <tytso@....edu>, David Rees <drees76@...il.com>,
	Jesper Krogh <jesper@...gh.cc>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Linux 2.6.29

On Thu, 26 Mar 2009 17:27:43 -0700 (PDT) Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> 
> 
> On Thu, 26 Mar 2009, Andrew Morton wrote:
> > 
> > userspace can do it quite easily.  Run a self-tuning script after
> > installation and when the disk hardware changes significantly.
> 
> Uhhuh.
> 
> "user space can do it".
> 
> That's the global cop-out.

userspace can get closer than the kernel can.

> The fact is, user-space isn't doing it, and never has done anything even 
> _remotely_ like it. 
> 
> In fact, I claim that it's impossible to do. If you give me a number for 
> the throughput of your harddisk, I will laugh in your face and call you a 
> moron.
>
> Why? Because no such number exists. It depends on the access patterns.

Those access patterns are observable!

> If 
> you write one large file, the number will be very different (and not just 
> by a few percent) from the numbers of you writing thousands of small 
> files, or re-writing a large database in random order.
> 
> So no. User space CAN NOT DO IT, and the fact that you even claim 
> something like that shows a distinct lack of thought.

userspace can get closer.  Even if it's asking the user "what sort of
applications will this machine be running" and then use a set of canned
tunables based on that.

Better would be to observe system behaviour, perhaps in real time and
make adjustments.

> > Maybe we should set the tunables to 99.9% to make it suck enough to
> > motivate someone.
> 
> The only times tunables have worked for us is when they auto-tune. 
> 
> IOW, we don't have "use 35% of memory for buffer cache" tunables, we just 
> dynamically auto-tune memory use. And no, we don't expect user space to 
> run some "tuning program for their load" either.
> 

This particular case is exceptional - it's just too hard for the kernel
to be able to predict the future for this one.

It wouldn't be terribly hard for a userspace daemon to produce better
results than we can achieve in-kernel.  That might of course require
additional kernel work to support it well.
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