linux-kernel - Re: [PATCH] sysctl: Add a feature to drop caches selectively

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Message-ID: <1403690184.20275.11.camel@sauron.fi.intel.com>
Date:	Wed, 25 Jun 2014 12:56:24 +0300
From:	Artem Bityutskiy <dedekind1@...il.com>
To:	Thomas Knauth <thomas.knauth@....de>
Cc:	David Rientjes <rientjes@...gle.com>,
	Maksym Planeta <mcsim.planeta@...il.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sysctl: Add a feature to drop caches selectively

On Wed, 2014-06-25 at 10:25 +0200, Thomas Knauth wrote:
> On Wed, Jun 25, 2014 at 8:25 AM, Artem Bityutskiy <dedekind1@...il.com> wrote:
> > Plus some explanations WRT why proc-based interface and what would be
> > the alternatives, what if tomorrow we want to extend the functionality
> > and drop caches only for certain file range, is this only for regular
> > files or also for directories, why posix_fadvice(DONTNEED) is not
> > sufficient.
> 
> I suggested the idea originally. Let me address each of your questions in turn:

Thanks for the answer, although you forgot to comment on the question
about possibly extending the new interface to work with file ranges in
the future. For example, I have a 2 TiB file, and I am only interested
in dropping caches for the first couple of gigabytes. Would I extend
your interface, or would I come up with another one?

> Why a selective drop? To have a middle ground between echo 2 >
> drop_caches and echo 3 > drop_caches. When is this interesting? My
> particular use case was benchmarking. I wanted to repeatedly measure
> the timing when things were read from disk. Dropping everything from
> the cache, also drops useful things, not just the few files your
> benchmark intends to measure.

Sounds like a reasonable motivation for me.

> Why /proc? Because this is where the current drop_caches mechanism is
> located. If it should go somewhere else, please do suggest so.

I do not have particular suggestions, just pulling the information about
how much efforts were put into choosing the interface.

> Why not use posix_fadvice()? Because it is exactly this, an advice.
> The kernel is free to do whatever, i.e., also ignore the request. We
> want a mechanism that reliably drops select content from the cache.

OK, thanks.

-- 
Best Regards,
Artem Bityutskiy

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