Re: [Xen-devel] poor domU VBD performance.

[Philip R Auld <pauld@xxxxxxxxxxx>]

Rumor has it that on Thu, Mar 31, 2005 at 10:19:01AM +0200 Jens Axboe said:
> On Thu, Mar 31 2005, Keir Fraser wrote:
> 
> > measurable benefit from reducing the number of unplugs?
> 
> Probably not, since the plugging happened at the front end as well. So
> you should get a nice stream of io in any way.

This effects merging though, right? I don't think the the front
end has done any merging. 

Also the BIO_RW_SYNC bit is sometimes ignored in __make_request
due to the bad queue locking interactions with scsi_request_fn.

The bio can be completed before the bio_sync() test in 
__make_request. Since there is no other reference to the bio it 
can be freed and reused by the time it is tested for BIO_RW_SYNC.

Cheers,

Phil


> 
> -- 
> Jens Axboe
> 
> 
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-- 
Philip R. Auld, Ph.D.                          Egenera, Inc.    
Software Architect                            165 Forest St.
(508) 858-2628                            Marlboro, MA 01752

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