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

[Pasi Kärkkäinen]

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

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