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Date: Fri, 24 Jan 2003 09:56:26 -0600
From: Oliver Xymoron <oxymoron@waste.org>
To: Andrew Morton <akpm@digeo.com>
Cc: Alex Tomas <bzzz@tmi.comex.ru>, linux-kernel@alex.org.uk,
       linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: 2.5.59-mm5
Message-ID: <20030124155626.GC9417@waste.org>
References: <20030123195044.47c51d39.akpm@digeo.com> <946253340.1043406208@[192.168.100.5]> <20030124031632.7e28055f.akpm@digeo.com> <m3d6mmvlip.fsf@lexa.home.net> <20030124035017.6276002f.akpm@digeo.com>
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On Fri, Jan 24, 2003 at 03:50:17AM -0800, Andrew Morton wrote:
> Alex Tomas <bzzz@tmi.comex.ru> wrote:
> >
> > >>>>> Andrew Morton (AM) writes:
> > 
> >  AM> But writes are completely different.  There is no dependency
> >  AM> between them and at any point in time we know where on-disk a lot
> >  AM> of writes will be placed.  We don't know that for reads, which is
> >  AM> why we need to twiddle thumbs until the application or filesystem
> >  AM> makes up its mind.
> > 
> > 
> > it's significant that application doesn't want to wait read completion
> > long and doesn't wait for write completion in most cases.
> 
> That's correct.  Reads are usually synchronous and writes are rarely
> synchronous.
> 
> The most common place where the kernel forces a user process to wait on
> completion of a write is actually in unlink (truncate, really).  Because
> truncate must wait for in-progress I/O to complete before allowing the
> filesystem to free (and potentially reuse) the affected blocks.
> 
> If there's a lot of writeout happening then truncate can take _ages_.  Hence
> this patch:

An alternate approach might be to change the way the scheduler splits
things. That is, rather than marking I/O read vs write and scheduling
based on that, add a flag bit to mark them all sync vs async since
that's the distinction we actually care about. The normal paths can
all do read+sync and write+async, but you can now do things like
marking your truncate writes sync and readahead async.

And dependent/nondependent or stalling/nonstalling might be a clearer
terminology.

-- 
 "Love the dolphins," she advised him. "Write by W.A.S.T.E.." 
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