Re: [Xen-devel] [PATCHv2 3/3] p2m: convert grant table rwlock to percpu rwlock

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 20/11/15 16:03, Malcolm Crossley wrote:
> The per domain p2m read lock suffers from significant contention when
> performance multi-queue block or network IO due to the parallel
> grant map/unmaps/copies occuring on the DomU's p2m.
> 
> 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 p2m operations.
> 
> Percpu rwlock p2m performance with the same configuration is approximately
> 64 gbit/s vs the 48 gbit/s with grant table percpu rwlocks only.
> 
> 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.
> 
> Note: altp2m users will not achieve a gain if they take an altp2m read lock
> simultaneously with the main p2m lock.
> 
> Signed-off-by: Malcolm Crossley <malcolm.crossley@xxxxxxxxxx>

Looks good to me.  I'll save the reviewed-by until the question about
the calling convention has been discussed.

 -George


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