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

From: Stefan Hajnoczi
Subject: [PATCH RESEND v3 0/4] virtio-pci: enable blk and scsi multi-queue by default
Date: Fri, 20 Mar 2020 10:30:37 +0000

 * Add new performance results that demonstrate the scalability
 * Mention that this is PCI-specific [Cornelia]
 * 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]

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

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:
q=1    78k
q=32  104k  +33%

Boot time:
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  They are no longer valid because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(-)


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