linux-kernel - Re: Performance regression in scsi sequential throughput (iozone) due to "e084b - page-allocator: preserve PFN ordering when __GFP

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Message-ID: <4B82A603.9030602@linux.vnet.ibm.com>
Date:	Mon, 22 Feb 2010 16:42:59 +0100
From:	Christian Ehrhardt <ehrhardt@...ux.vnet.ibm.com>
To:	Mel Gorman <mel@....ul.ie>
CC:	Nick Piggin <npiggin@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	epasch@...ibm.com, SCHILLIG@...ibm.com,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	christof.schmitt@...ibm.com, thoss@...ibm.com, hare@...e.de,
	gregkh@...ell.com
Subject: Re: Performance regression in scsi sequential throughput (iozone)
 due to "e084b - page-allocator: preserve PFN ordering when	__GFP_COLD is
 set"

Mel Gorman wrote:
[...]
> 
>> Unfortunately even now knowing the place of the issue so well I don't see
>> the connection to the commits e084b+5f8dcc21
> 
> Still a mystery.
> 
>>  - I couldn't find something but
>> did they change the accounting somewhere or e.g. changed the timing/order
>> of watermark updates and allocations?
>>
> 
> Not that I can think of.
> 
>> Eventually it might come down to a discussion of allocation priorities and
>> we might even keep them as is and accept this issue - I still would prefer
>> a good second chance implementation, other page cache allocation flags or
>> something else that explicitly solves this issue.
>>
> 
> In that line, the patch that replaced congestion_wait() with a waitqueue
> makes some sense.

[...]

> I'll need to do a number of tests before I can move that upstream but I
> don't think it's a merge candidate. Unfortunately, I'll be offline for a
> week starting tomorrow so I won't be able to do the testing.
> 
> When I get back, I'll revisit those patches with the view to pushing
> them upstream. I hate to treat symptoms here without knowing the
> underlying problem but this has been spinning in circles for ages with
> little forward progress :(

I'll continue with some debugging in search for the real reasons, but if 
I can't find a new way to look at it I think we have to drop it for now.

While I hate fixing symptoms too, I completely agree that  it is time to 
bring this fix upstream and in fact into the stable kernel too, to have 
at least a ~98% workaround out there.

I'm looking forward for your revised patch after you are back and I'm 
eager to test this one again.


-- 

Grüsse / regards, Christian Ehrhardt
IBM Linux Technology Center, System z Linux Performance
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