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Date: Wed, 28 Nov 2001 13:26:55 -0800
From: Andrew Morton <akpm@zip.com.au>
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To: Marcelo Tosatti <marcelo@conectiva.com.br>
CC: Andreas Dilger <adilger@turbolabs.com>,
        "'lkml'" <linux-kernel@vger.kernel.org>
Subject: Re: Unresponiveness of 2.4.16
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Marcelo Tosatti wrote:
> 
> > ...
> > But so little code is actually using READA at this stage that I didn't
> > bother - I first need to go through those paths and make sure that they
> > are in fact complete, working and useful...
> 
> I've done some experiments in the past which have shown that doing this
> will cause us to almost _never_ do readahead on IO intensive workloads,
> which ended up decreasing performance instead increasing it.

Interesting.  Thanks.

One _could_ make the first readahead page non-READA, and then
make the rest READA.  That way, all block-contiguous requests
will be merged, and any non-contiguous requests will be dropped on
the floor if the request queue is full.  Which is probably what
we want to happen anyway.

Of course the alternative is to slot a little bmap() call into
the readhead logic :)

> Please make sure to extensively test the propagation of READA through the
> pagecache when you do so...

Extensivelytest is my middle name.

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