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Re: [Xen-devel] [PATCHv5 2/3] grant_table: convert grant table rwlock to percpu rwlock

>>> On 18.12.15 at 17:08, <malcolm.crossley@xxxxxxxxxx> wrote:
> The per domain grant table read lock suffers from significant contention when
> performance multi-queue block or network IO due to the parallel
> grant map/unmaps/copies occurring on the DomU's grant table.
> On multi-socket systems, the contention results in the locked compare swap
> operation failing frequently which results in a tight loop of retries of the
> compare swap operation. As the coherency fabric can only support a specific
> rate of compare swap operations for a particular data location then taking
> the read lock itself becomes a bottleneck for grant operations.
> Standard rwlock performance of a single VIF VM-VM transfer with 16 queues
> configured was limited to approximately 15 gbit/s on a 2 socket Haswell-EP
> host.
> Percpu rwlock performance with the same configuration is approximately
> 48 gbit/s.
> Oprofile was used to determine the initial overhead of the read-write locks
> and to confirm the overhead was dramatically reduced by the percpu rwlocks.
> Signed-off-by: Malcolm Crossley <malcolm.crossley@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

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