Re: [Xen-devel] [PATCH RFC v2 0/5] Multi-queue support for xen-blkfront and xen-blkback

[Jens Axboe <axboe@xxxxxx>]

On 08/10/2015 05:03 AM, Rafal Mielniczuk wrote:
> On 01/07/15 04:03, Jens Axboe wrote:
> > On 06/30/2015 08:21 AM, Marcus Granado wrote:
> > > Hi,
> > >
> > > Our measurements for the multiqueue patch indicate a clear improvement
> > > in iops when more queues are used.
> > >
> > > The measurements were obtained under the following conditions:
> > >
> > > - using blkback as the dom0 backend with the multiqueue patch applied to
> > > a dom0 kernel 4.0 on 8 vcpus.
> > >
> > > - using a recent Ubuntu 15.04 kernel 3.19 with multiqueue frontend
> > > applied to be used as a guest on 4 vcpus
> > >
> > > - using a micron RealSSD P320h as the underlying local storage on a Dell
> > > PowerEdge R720 with 2 Xeon E5-2643 v2 cpus.
> > >
> > > - fio 2.2.7-22-g36870 as the generator of synthetic loads in the guest.
> > > We used direct_io to skip caching in the guest and ran fio for 60s
> > > reading a number of block sizes ranging from 512 bytes to 4MiB. Queue
> > > depth of 32 for each queue was used to saturate individual vcpus in the
> > > guest.
> > >
> > > We were interested in observing storage iops for different values of
> > > block sizes. Our expectation was that iops would improve when increasing
> > > the number of queues, because both the guest and dom0 would be able to
> > > make use of more vcpus to handle these requests.
> > >
> > > These are the results (as aggregate iops for all the fio threads) that
> > > we got for the conditions above with sequential reads:
> > >
> > > fio_threads  io_depth  block_size   1-queue_iops  8-queue_iops
> > >       8           32       512           158K         264K
> > >       8           32        1K           157K         260K
> > >       8           32        2K           157K         258K
> > >       8           32        4K           148K         257K
> > >       8           32        8K           124K         207K
> > >       8           32       16K            84K         105K
> > >       8           32       32K            50K          54K
> > >       8           32       64K            24K          27K
> > >       8           32      128K            11K          13K
> > >
> > > 8-queue iops was better than single queue iops for all the block sizes.
> > > There were very good improvements as well for sequential writes with
> > > block size 4K (from 80K iops with single queue to 230K iops with 8
> > > queues), and no regressions were visible in any measurement performed.
> > Great results! And I don't know why this code has lingered for so long,
> > so thanks for helping get some attention to this again.
> >
> > Personally I'd be really interested in the results for the same set of
> > tests, but without the blk-mq patches. Do you have them, or could you
> > potentially run them?
> Hello,
>
> We rerun the tests for sequential reads with the identical settings but with 
> Bob Liu's multiqueue patches reverted from dom0 and guest kernels.
> The results we obtained were *better* than the results we got with multiqueue 
> patches applied:
>
> fio_threads  io_depth  block_size   1-queue_iops  8-queue_iops  
> *no-mq-patches_iops*
>       8           32       512           158K         264K         321K
>       8           32        1K           157K         260K         328K
>       8           32        2K           157K         258K         336K
>       8           32        4K           148K         257K         308K
>       8           32        8K           124K         207K         188K
>       8           32       16K            84K         105K         82K
>       8           32       32K            50K          54K         36K
>       8           32       64K            24K          27K         16K
>       8           32      128K            11K          13K         11K
>
> We noticed that the requests are not merged by the guest when the multiqueue 
> patches are applied,
> which results in a regression for small block sizes (RealSSD P320h's optimal 
> block size is around 32-64KB).
>
> We observed similar regression for the Dell MZ-5EA1000-0D3 100 GB 2.5" Internal 
> SSD
>
> As I understand blk-mq layer bypasses I/O scheduler which also effectively 
> disables merges.
> Could you explain why it is difficult to enable merging in the blk-mq layer?
> That could help closing the performance gap we observed.
>
> Otherwise, the tests shows that the multiqueue patches does not improve the 
> performance,
> at least when it comes to sequential read/writes operations.
blk-mq still provides merging, there should be no difference there. Does 
the xen patches set BLK_MQ_F_SHOULD_MERGE?
--
Jens Axboe


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