linux-kernel - Re: [PATCH] block: fix q->max_segment_size checking in blk_recalc_rq

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Date:	Thu, 24 Jul 2008 12:34:31 -0400 (EDT)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
cc:	David Miller <davem@...emloft.net>, fujita.tomonori@....ntt.co.jp,
	jens.axboe@...cle.com, linux-kernel@...r.kernel.org,
	linux-scsi@...r.kernel.org, linux-parisc@...r.kernel.org
Subject: Re: [PATCH] block: fix q->max_segment_size checking in blk_recalc_rq_segments
 about VMERGE

On Thu, 24 Jul 2008, James Bottomley wrote:

> On Thu, 2008-07-24 at 11:07 -0400, Mikulas Patocka wrote:
> > So try to #define BIO_VMERGE_BOUNDARY 0 for Pa-Risc and tell us what 
> > performance degradation do you see (and what driver do you use and what is 
> > the I/O pattern).
> > 
> > If you show something specific, we can consider that --- but you haven't 
> > yet told us anything, except generic talk.
> 
> You keep ignoring inconvenient facts.  For about the third time:
> 
> I run a test bed for sg_tables (large chaining of requests).  This runs
> on parisc using virtual merging (has to because the final physical table
> size can't go over the sg list of the SCSI card).  If I turn off virtual
> merging I can no longer test sg_tables in vanilla kernels.
> 
> James

What sg_tables test do you mean? What does the test do? Why couldn't you 
run the test if BIO_VMERGE_BOUNDARY is 0? Normal I/O obviously can work 
with BIO_VMERGE_BOUNDARY 0, the kernel will just send more smaller 
requests.

Mikulas
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