Re: 2.6.20-rc2+: CFQ halving disk throughput.

[Andrew Morton]

On Mon, 01 Jan 2007 23:46:58 +0100
Rene Herman <rene.herman@xxxxxxxxx> wrote:

> > Everything seems fine in the dmesg.  Performance degradation is
> > probably some other issue in -rc kernel.  I'm suspecting recently
> > fixed block layer bug.  If it's still the same in the next -rc,
> > please report.
> 
> In fact, it's CFQ. The PATA thing was a red herring. 2.6.20-rc2 and 3 
> give me ~ 24 MB/s from "hdparm t /dev/hda" while 2.6.20-rc1 and below 
> give me ~ 50 MB/s.
> 
> Jens: this is due to "[PATCH] cfq-iosched: tighten allow merge 
> criteria", 719d34027e1a186e46a3952e8a24bf91ecc33837:
> 
> http://www2.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=719d34027e1a186e46a3952e8a24bf91ecc33837
> 
> If I revert that one, I have my 50 M/s back. config and dmesg attached 
> in case they're useful.

The patch would appear to need this fix:

--- a/block/cfq-iosched.c~a
+++ a/block/cfq-iosched.c
@@ -592,7 +592,7 @@ static int cfq_allow_merge(request_queue
 	if (cfqq == RQ_CFQQ(rq))
 		return 1;
 
-	return 1;
+	return 0;
 }
 
 static inline void
_

But that might not fix things...
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