linux-kernel - Re: [RFC Patch] fs: implement per-file drop caches

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Date:	Mon, 04 Jun 2012 11:28:24 +0800
From:	Cong Wang <amwang@...hat.com>
To:	John Stoffel <john@...ffel.org>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Pádraig Brady <P@...igBrady.com>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Cong Wang <xiyou.wangcong@...il.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Matthew Wilcox <matthew@....cx>, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [RFC Patch] fs: implement per-file drop caches

On Fri, 2012-06-01 at 09:08 -0400, John Stoffel wrote:
> >>>>> "Cong" == Cong Wang <amwang@...hat.com> writes:
> 
> Cong> Yeah, at least John Stoffel expressed his interests on this, as
> Cong> a sysadmin. So I believe there are some people need it.
> 
> I expressed an interest if there was a way to usefully *find* the
> processes that are hogging cache.  Without a reporting mechanism of
> cache usage on per-file or per-process manner, then I don't see a
> great use for this.  It's just simpler to drop all the caches when you
> hit a wall.  
> 
> Cong> Now the problem is that I don't find a proper existing utility
> Cong> to patch, maybe Pádraig has any hints on this? Could this
> Cong> feature be merged into some core utility? Or I have to write a
> Cong> new utility for this?
> 
> I'd write a new tutorial utility, maybe you could call it 'cache_top'
> and have it both show the biggest users of cache, as well as exposing
> your new ability to drop the cache on a per-fd basis.
> 
> It's really not much use unless we can measure it.

Fair enough.

We could do that with Keiichi's page cache tracepoint patches:
https://lkml.org/lkml/2011/7/18/326

with that patch, we can measure page caches with `perf`. I tried to
carry Keiichi's patches, but those patch depend on other patches too,
the main problem is still translating the inode number to file name for
user-space users to read, which is not trivial at all.

Also, will vmtouch work for you too? You can get it at
http://hoytech.com/vmtouch/

I can patch it too if you want.

Thanks!

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