linux-kernel - Re: -mm merge plans for 2.6.23

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Date:	Wed, 25 Jul 2007 17:25:12 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	david@...g.hm
CC:	Eric St-Laurent <ericstl34@...patico.ca>,
	Rene Herman <rene.herman@...il.com>,
	Ray Lee <ray-lk@...rabbit.org>,
	Jesper Juhl <jesper.juhl@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	ck list <ck@....kolivas.org>, Ingo Molnar <mingo@...e.hu>,
	Paul Jackson <pj@....com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: -mm merge plans for 2.6.23

david@...g.hm wrote:
> On Wed, 25 Jul 2007, Nick Piggin wrote:

>> And constructed test cases of course are useful as well, I didn't say
>> they weren't. I don't know what you mean by "acceptable", but you should
>> read my last paragraph again.
> 
> 
> this problem has been around for many years, with many different people 
> working on solutions. it's hardly a case of getting a proposal and 
> trying to get it in without anyone looking at other options.

What is "this problem"? People have an updatedb problem that is solved
by swap prefetching which I want to fix in a different way.

There would be a different problem of "run something that uses heaps of
memory and swap everything else out, then quit it, wait for a while, and
swap prefetching helps". OK, definitely swap prefetching would help there.
How much? I don't know. I'd be slightly surprised if it was like an order
of magnitude, because not only swap but everything else has been thrown
out too.

> it seems that there are some people (not nessasarily including you) who 
> will oppose this feature until a test is created that shows that it's 
> better. the question is what sort of test will be accepted as valid? I'm 
> not useing this patch, but it sounds as if the people who are useing it 
> are interested in doing whatever testing is required, but so far the 
> situation seems to be a series of "here's a test", "that test isn't 
> valid, try again" loops. which don't seem to be doing anyone any good 

And yet despite my repeated pleas, none of those people has yet spent a
bit of time with me to help analyse what is happening.

> and are frustrating lots of people, so like several people over the last 
> few days O'm asking the question, "what sort of test would be acceptable 
> as proof that this patch does some good?"

I don't think any further proof is needed that the patch does "some"
good. Rig up a test case and you could see some seconds shaved off it.
Maybe you want to know "how to get this patch merged"? And I don't know
that one. I do know that it is fuzzy, and probably doesn't include
demanding things of Andrew or Linus.

BTW. If you find out the answer to that one, let me know because I have
this lockless pagecache patch that has also been around for years, is
also just a few hundred lines in the VM, and does do some good too. I'm
sure the buffered AIO people and many others would also like to know.

-- 
SUSE Labs, Novell Inc.
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