Date: Thu, 26 Mar 2009 17:47:04 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Theodore Tso <tytso@mit.edu>, David Rees <drees76@gmail.com>,
       Jesper Krogh <jesper@krogh.cc>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: Linux 2.6.29
Message-Id: <20090326174704.cd36bf7b.akpm@linux-foundation.org>
In-Reply-To: <alpine.LFD.2.00.0903261723250.3994@localhost.localdomain>
References: <alpine.LFD.2.00.0903231617550.3032@localhost.localdomain>
	<49C87B87.4020108@krogh.cc>
	<72dbd3150903232346g5af126d7sb5ad4949a7b5041f@mail.gmail.com>
	<49C88C80.5010803@krogh.cc>
	<72dbd3150903241200v38720ca0x392c381f295bdea@mail.gmail.com>
	<20090325183011.GN32307@mit.edu>
	<alpine.LFD.2.00.0903251139260.3032@localhost.localdomain>
	<20090325220530.GR32307@mit.edu>
	<alpine.LFD.2.00.0903251622420.3032@localhost.localdomain>
	<20090326171148.9bf8f1ec.akpm@linux-foundation.org>
	<alpine.LFD.2.00.0903261723250.3994@localhost.localdomain>
Mime-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 2309
Lines: 64

On Thu, 26 Mar 2009 17:27:43 -0700 (PDT) Linus Torvalds <torvalds@linux-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.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/