Re: [Xen-devel] [PATCH] Persistent grant maps for xen blk drivers

[Oliver Chick <oliver.chick@xxxxxxxxxx>]

I have attached a graph that shows the results of my benchmarking.

The setup is:
-Xeon X5650
-32GB of Ram
-Xen 4.2
-Linux 3.5.0 for dom0 and domus
-Dom0 has 24 CPUs.
-Each guest has a (separate) xvdb backed by a 1GB ramdisk (/dev/ramX) in
dom0. No LVM on these.
-The setup is that initially 1 guest does an fio sequential read of the
ramdisk, then 2, then 3 etc.
-The y axis is the sum of the iops, as reported by the guests' fio
output.

On Wed, 2012-09-19 at 14:16 +0100, X5650Pasi KÃrkkÃinen wrote:
> On Wed, Sep 19, 2012 at 11:51:27AM +0100, Oliver Chick wrote:
> > This patch implements persistent grants for the xen-blk{front,back}
> > mechanism. The effect of this change is to reduce the number of unmap
> > operations performed, since they cause a (costly) TLB shootdown. This
> > allows the I/O performance to scale better when a large number of VMs
> > are performing I/O.
> > 
> > Previously, the blkfront driver was supplied a bvec[] from the request
> > queue. This was granted to dom0; dom0 performed the I/O and wrote
> > directly into the grant-mapped memory and unmapped it; blkfront then
> > removed foreign access for that grant. The cost of unmapping scales
> > badly with the number of CPUs in Dom0. An experiment showed that when
> > Dom0 has 24 VCPUs, and guests are performing parallel I/O to a
> > ramdisk, the IPIs from performing unmap's is a bottleneck at 5 guests
> > (at which point 650,000 IOPS are being performed in total). If more
> > than 5 guests are used, the performance declines. By 10 guests, only
> > 400,000 IOPS are being performed.
> > 
> > This patch improves performance by only unmapping when the connection
> > between blkfront and back is broken.
> >
> 
> So how many IOPS can you get with this patch / persistent grants ? 
> 
> -- Pasi
>

Attachment: pers-vs-nonpers.pdf
Description: Adobe PDF document

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