linux-ext4 - Re: [PATCH 5/6] libext2fs/e2fsck: provide routines to read-ahead metadata

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Date:	Mon, 11 Aug 2014 11:05:09 -0700
From:	"Darrick J. Wong" <darrick.wong@...cle.com>
To:	"Theodore Ts'o" <tytso@....edu>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: [PATCH 5/6] libext2fs/e2fsck: provide routines to read-ahead
 metadata

On Mon, Aug 11, 2014 at 10:34:23AM -0400, Theodore Ts'o wrote:
> On Sun, Aug 10, 2014 at 11:31:20PM -0700, Darrick J. Wong wrote:
> > 
> > Hmm, I'll try that, though I'm almost tempted just to issue io_cache_readahead
> > calls directly from the for loop.  The only reason I'm using the list here at
> > all is to handle the case of reading multiple groups in a flexbg (pass5 RA).
> 
> In that case you could just use an accumulator for each of the block
> and inode bitmaps, and so long as the next group's block and inode
> bitmaps are contiguous with the previous one, you can bump a counter.
> Once you reach a discontiguous bitmap block, you can issue a single
> readahead request for N blocks starting at block B.  That way you only
> issue a single syscall, instead of one for every single block, and you
> don't have the overhead involved with storing the list of blocks and
> then sorting them.

Using the bitmap turns out to be pretty quick (~130us to start RA for 4 groups
vs. ~70us per group if I issue the RA directly).  Each fadvise call seems to
cost us ~1ms, so I'll keep using the bitmap to minimize the number of fadvise
calls, since it's also a lot less code.

--D

> 
> 					- Ted
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