Re: [Xen-devel] Odd blkdev throughput results

[Daniel Stodden <stodden@xxxxxxxxxx>]

On Sun, 2008-03-16 at 21:15 +0000, Mark Williamson wrote:
> >
> > > Block IO doesn't require a copy at all.
> >
> > Well, not in blkback by itself, but certainly from the in-memory disk
> > image. Unless I misunderstoode Keirs post recently, page flipping is
> > basically dead code, so I thought the number should at least point into
> > roughly the same directions.
> 
> Blkback has always DMA-ed directly into guest memory when reading data from 
> the disk drive (normal usecase), in which case there's no copy - I think that 
> was Ian's point.  In contrast the Netback driver has to do a copy in the 
> normal case.
> 
> If you're using a ramdisk then there must be a copy somewhere, although I'm 
> not sure exactly where it happens!

I checked it, this is comparatively easy to find. Since DMA-or-not is
ultimately up to the driver, it's that single memcpy() in rd.c. Looks
rather straightforward. 

In theory, such a pseudo-device could make use of the host DMA engine
embedded into newer Intel chipsets to save a few cycles. But looking at
the source that does not seem to be the case (and typical usage
scenarios for the ramdisk driver (4MB default size iirc) would hardly
justify the effort).

Blkdev peak throughput at 500MB/s is certainly not a usability issue :)
I just asked because I hoped someone who spent more time one the PV
drivers than I did might have experienced (or even profiled) similar
effects already, and could explain.

Thanks and greetings,
Daniel
 
-- 
Daniel Stodden
LRR     -      Lehrstuhl fÃr Rechnertechnik und Rechnerorganisation
Institut fÃr Informatik der TU MÃnchen             D-85748 Garching
http://www.lrr.in.tum.de/~stodden         mailto:stodden@xxxxxxxxxx
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