linux-ext4 - Re: [RFC, PATCH] Avoid hot statistics cache line in ext4 extent cache

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Date:	Fri, 13 Apr 2012 15:01:47 -0400
From:	Eric Whitney <eric.whitney@...com>
To:	Ted Ts'o <tytso@....edu>
CC:	Andi Kleen <andi@...stfloor.org>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Vivek Haldar <haldar@...gle.com>,
	Andreas Dilger <adilger@...ger.ca>, linux-ext4@...r.kernel.org
Subject: Re: [RFC, PATCH] Avoid hot statistics cache line in ext4 extent cache

On 04/13/2012 02:48 PM, Ted Ts'o wrote:
> On Fri, Apr 13, 2012 at 08:41:58PM +0200, Andi Kleen wrote:
>>> The reason why I ask this is we're seeing anything like this with Eric
>>> Whitney's 48 CPU scalability testing; we're not CPU bottlenecked, and
>>> I don't even see evidence of a larger than usual CPU utilization
>>> compared to other file systems.
>>
>> I bet Eric didn't test with this statistic counter.
>
> Huh?  You can't turn it off, and he's been doing regular scalability
> tests at least once per kernel release.

Yes, as recently as 3.4-rc1.  I saw Andi's patch, and tested it this 
week against that baseline with the ffsb profiles we've been using for 
ext4 (and other filesystem) scalability measurements.

I didn't get a noticeable delta for throughput or reported CPU 
utilization on my 48 core eight node NUMA test setup.  That said, I plan 
to look at this more closely to verify that my workloads should have 
seen a delta in the first place.  Ted knows them well, though.  It's 
worth noting that I've got plenty of free CPU capacity while running the 
workload, which differs from Andi's/Tim's description.

>
> Can you say a bit more about exactly how you are doing this test and
> what are the "other issues" where this is becoming a bottleneck?  If
> possible I'd like to ask Eric if he can add it to his regular
> scalability tests.

Yes, I'm certainly willing to do that if practical, and I'm curious to 
know more about what the workload looks like.

Eric

>
> Thanks,
>
> 						 - Ted
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