linux-kernel - Re: [patch 9/9] mm: thrash detection-based file cache sizing v4

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Date:	Thu, 22 Aug 2013 14:15:20 +0300
From:	Metin Doslu <metin@...usdata.com>
To:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 9/9] mm: thrash detection-based file cache sizing v4

Hey everbody,

I run following tests, and it shows in what cases this patch is
beneficial for us.

Test Environment:

* Ubuntu Server 12.04.2 LTS Linux 3.2.0-40-virtual #64-Ubuntu on EC2.
* 15 GB memory (DMA32 4GB + Normal 11GB).

Test Settings:

We have two PostgreSQL tables with same size of 9.75GB (65% of total
memory), where these tables contain clickstream events for March and
April. We call these two tables "events_march" and "events_april"
respectively.

Problem (Before Patch is Applied):

I pass over events_march data twice with an example query, such as
"select count(*) from events_march". This activates all of
events_march's pages.

I then pass over events_april dozens of times with a similar query. No
matter how many times I query events_april, I can't get completely get
this table's pages into memory. This happens even when events_march
isn't touched at all, events_april easily fits into memory, and
events_april has been referenced dozens of times.

After Patch is Applied:

This time, after three passes over events_april, all the pages are
cached in memory. (4th access is completely served from memory.)

I also repeated this test with a bigger dataset of size 12GB (80% of
total memory) for both events_march and events_april, and observed the
same results. (after 3rd pass, all of pages in events_april are
cached.)

Thank you,
Metin

On Wed, Aug 21, 2013 at 12:04 AM, Andrew Morton
<akpm@...ux-foundation.org> wrote:
> On Sat, 17 Aug 2013 15:31:14 -0400 Johannes Weiner <hannes@...xchg.org> wrote:
>
>> This series solves the problem by maintaining a history of pages
>> evicted from the inactive list, enabling the VM to tell streaming IO
>> from thrashing and rebalance the page cache lists when appropriate.
>
> I can't say I'm loving the patchset.  It adds significant bloat to the
> inode (of all things!), seems to add some runtime overhead and
> certainly adds boatloads of complexity.
>
> In return for which we get...  well, I don't know what we get - no data
> was included.  It had better be good!
>
> To aid in this decision, please go through the patchset and calculate
> and itemize the overhead: increased inode size, increased radix-tree
> consumption, lengthier code paths, anything else I missed  Others can
> make their own judgements regarding complexity increase.
>
> Then please carefully describe the benefits, then see if you can
> convince us that one is worth the other!
>
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