Re: [PATCH RESEND v3 0/4] virtio-pci: enable blk and scsi multi-queue by default

[Michael S. Tsirkin]

On Fri, Mar 20, 2020 at 10:30:37AM +0000, Stefan Hajnoczi wrote:
> v3:
>  * Add new performance results that demonstrate the scalability
>  * Mention that this is PCI-specific [Cornelia]
> v2:
>  * Let the virtio-DEVICE-pci device select num-queues because the optimal
>    multi-queue configuration may differ between virtio-pci, virtio-mmio, and
>    virtio-ccw [Cornelia]


So this needs to be rebased wrt compat properties. I also see some
comments from Cornelia, worth addressing.

> Enabling multi-queue on virtio-pci storage devices improves performance on SMP
> guests because the completion interrupt is handled on the vCPU that submitted
> the I/O request.  This avoids IPIs inside the guest.
> 
> Note that performance is unchanged in these cases:
> 1. Uniprocessor guests.  They don't have IPIs.
> 2. Application threads might be scheduled on the sole vCPU that handles
>    completion interrupts purely by chance.  (This is one reason why benchmark
>    results can vary noticably between runs.)
> 3. Users may bind the application to the vCPU that handles completion
>    interrupts.
> 
> Set the number of queues to the number of vCPUs by default on virtio-blk and
> virtio-scsi PCI devices.  Older machine types continue to default to 1 queue
> for live migration compatibility.
> 
> Random read performance:
>       IOPS
> q=1    78k
> q=32  104k  +33%
> 
> Boot time:
>       Duration
> q=1        51s
> q=32     1m41s  +98%
> 
> Guest configuration: 32 vCPUs, 101 virtio-blk-pci disks
> 
> Previously measured results on a 4 vCPU guest were also positive but showed a
> smaller 1-4% performance improvement.  They are no longer valid because
> significant event loop optimizations have been merged.
> 
> Stefan Hajnoczi (4):
>   virtio-scsi: introduce a constant for fixed virtqueues
>   virtio-scsi: default num_queues to -smp N
>   virtio-blk: default num_queues to -smp N
>   vhost-user-blk: default num_queues to -smp N
> 
>  hw/block/vhost-user-blk.c          |  6 +++++-
>  hw/block/virtio-blk.c              |  6 +++++-
>  hw/core/machine.c                  |  5 +++++
>  hw/scsi/vhost-scsi.c               |  3 ++-
>  hw/scsi/vhost-user-scsi.c          |  5 +++--
>  hw/scsi/virtio-scsi.c              | 13 +++++++++----
>  hw/virtio/vhost-scsi-pci.c         | 10 ++++++++--
>  hw/virtio/vhost-user-blk-pci.c     |  6 ++++++
>  hw/virtio/vhost-user-scsi-pci.c    | 10 ++++++++--
>  hw/virtio/virtio-blk-pci.c         |  9 ++++++++-
>  hw/virtio/virtio-scsi-pci.c        | 10 ++++++++--
>  include/hw/virtio/vhost-user-blk.h |  2 ++
>  include/hw/virtio/virtio-blk.h     |  2 ++
>  include/hw/virtio/virtio-scsi.h    |  5 +++++
>  14 files changed, 76 insertions(+), 16 deletions(-)
> 
> -- 
> 2.24.1
>