linux-kernel - Re: readahead on directories

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Message-ID: <4BCF5C87.8060509@cfl.rr.com>
Date:	Wed, 21 Apr 2010 16:13:59 -0400
From:	Phillip Susi <psusi@....rr.com>
To:	Jamie Lokier <jamie@...reable.org>
CC:	Evgeniy Polyakov <zbr@...emap.net>, linux-fsdevel@...r.kernel.org,
	Linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: readahead on directories

On 4/21/2010 4:01 PM, Jamie Lokier wrote:
> Ok, this discussion has got a bit confused.  Text above refers to
> needing to asynchronously read next block in a directory, but if they
> are small then that's not important.

It is very much important since if you ready each small directory one
block at a time, it is very slow.  You want to queue up reads to all of
them at once so they can be batched.

> FIEMAP suggestion is only if you think you need to issue reads for
> multiple blocks in the _same_ directory in parallel.  From what you say,
> I doubt that's important.

That may be why you suggested it, but it is also exactly what
readahead() does.  It also queues the read asynchronously which is what
I really want so that I can queue more reads on other directories in one
big batch.

> That was my first suggestion: threads with readdir(); I thought it had
> been rejected hence the further discussion.

Yes, it was sort of rejected, which is why I said it's just a workaround
for now until readahead() works on directories.  It will produce the
desired IO pattern but at the expense of ram and cpu cycles creating a
bunch of short lived threads that go to sleep almost immediately after
being created, and exit when they wake up.  readahead() would be much
more efficient.

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