linux-kernel - Re: Why does __do_page_cache_readahead submit READ, not READA?

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Message-ID: <20090730060649.GC4148@kernel.dk>
Date:	Thu, 30 Jul 2009 08:06:49 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Chris Mason <chris.mason@...cle.com>
Cc:	Lars Ellenberg <lars.ellenberg@...bit.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org, dm-devel@...hat.com,
	Neil Brown <neilb@...e.de>
Subject: Re: Why does __do_page_cache_readahead submit READ, not READA?

On Wed, Jul 29 2009, Chris Mason wrote:
> On Wed, Jul 29, 2009 at 11:18:45PM +0200, Jens Axboe wrote:
> > On Wed, Jul 29 2009, Lars Ellenberg wrote:
> > > I naively assumed, from the "readahead" in the name, that readahead
> > > would be submitting READA bios. It does not.
> > > 
> > > I recently did some statistics on how many READ and READA requests
> > > we actually see on the block device level.
> > > I was suprised that READA is basically only used for file system
> > > internal meta data (and not even for all file systems),
> > > but _never_ for file data.
> > > 
> > > A simple
> > > 	dd if=bigfile of=/dev/null bs=4k count=1
> > > will absolutely cause readahead of the configured amount, no problem.
> > > But on the block device level, these are READ requests, where I'd
> > > expected them to be READA requests, based on the name.
> > > 
> > > This is because __do_page_cache_readahead() calls read_pages(),
> > > which in turn is mapping->a_ops->readpages(), or, as fallback,
> > > mapping->a_ops->readpage().
> > > 
> > > On that level, all variants end up submitting as READ.
> > > 
> > > This may even be intentional.
> > > But if so, I'd like to understand that.
> > 
> > I don't think it's intentional, and if memory serves, we used to use
> > READA when submitting read-ahead. Not sure how best to improve the
> > situation, since (as you describe), we lose the read-ahead vs normal
> > read at that level. I did some experimentation some time ago for
> > flagging this, see:
> > 
> > http://git.kernel.dk/?p=linux-2.6-block.git;a=commitdiff;h=16cfe64e3568cda412b3cf6b7b891331946b595e
> > 
> > which should pass down READA properly.
> 
> One of the problems in the past was that reada would fail if there
> wasn't a free request when we actually wanted it to go ahead and wait.
> Or something.  We've switched it around a few times I think.

Yes, we did used to do that, whether it was 2.2 or 2.4 I
don't recall :-)

It should be safe to enable know, whether there's a prettier way
than the above, I don't know. It works by detecting the read-ahead
marker, but it's a bit of a fragile design.

-- 
Jens Axboe

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