vhost: multiple worker support

Stefan Hajnoczi
Thu, 03 Jun 2021 10:13:57 -0000

On Tue, May 25, 2021 at 01:05:51PM -0500, Mike Christie
wrote:
> Results:
> --------
> When running with the null_blk driver and vhost-scsi I can get 1.2
> million IOPs by just running a simple
>
> fio --filename=/dev/sda --direct=1 --rw=randrw --bs=4k --ioengine=libaio
> --iodepth=128  --numjobs=8 --time_based --group_reporting --name=iops
> --runtime=60 --eta-newline=1
>
> The VM has 8 vCPUs and sda has 8 virtqueues and we can do a total of
> 1024 cmds per devices. To get 1.2 million IOPs I did have to tune and
> ran the virsh emulatorpin command so the vhost threads were running
> on different CPUs than the VM. If the vhost threads share CPUs then I
> get around 800K.
>
> For a more real device that are also CPU hogs like iscsi, I can still
> get 1 million IOPs using 1 dm-multipath device over 8 iscsi paths
> (natively it gets 1.1 million IOPs).

There is no comparison against a baseline, but I guess it would be the
same 8 vCPU guest with single queue vhost-scsi?

>
> Results/TODO Note:
>
> - I ported the vdpa sim code to support multiple workers and as-is now
> it made perf much worse. If I increase vdpa_sim_blk's num queues to 4-8
> I get 700K IOPs with the fio command above. However with the multiple
> worker support it drops to 400K. The problem is the vdpa_sim lock
> and the iommu_lock. If I hack (like comment out locks or not worry about
> data corruption or crashes) then I can get around 1.2M - 1.6M IOPs with
> 8 queues and fio command above.
>
> So these patches could help other drivers, but it will just take more
> work to remove those types of locks. I was hoping the 2 items could be
> done indepentently since it helps vhost-scsi immediately.

Cool, thank you for taking a look. That's useful info for Stefano. vDPA
and vhost can be handled independently though in the long term hopefully
they can share code.
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