Re: poor domU VBD performance.

[Jens Axboe <axboe@suse.de>]

On Thu, Mar 31 2005, Jens Axboe wrote:
> On Thu, Mar 31 2005, Kurt Garloff wrote:
> > Hi,
> > 
> > On Thu, Mar 31, 2005 at 09:33:12AM -0500, Philip R Auld wrote:
> > > This effects merging though, right? I don't think the the front
> > > end has done any merging. 
> > 
> > The noop elevator does front and back merging.
> > My understanding is that it's used in the frontend driver.
> > 
> > Otherwise, unplugging on every block would indeed be quite bad ...
> 
> Not necessarily - either your io rate is not fast enough to sustain a
> substantial queue depth, in that case you get plugging on basically
> every io anyways. If on the other hand the io rate is high enough to
> maintain a queue depth of > 1, then the plugging will never take place
> because the queue never empties.
> 
> So all in all, I don't think the temporary work-around will be such a
> bad idea. I would still rather implement the queue tracking though, it
> should not be more than a few lines of code.

There are still cases where it will be suboptimal of course, I didn't
intend to claim it will always be as fast as queue tracking! If you are
unlucky enough that the first request will reach the target device and
get started before the next one, you will have a small and a large part
of any given request executed. This isn't good for performance,
naturally. But queueing is so fast, I would be surprised if this
happened much in the real world.

-- 
Jens Axboe