Re: [PATCH v4 4/5] virtio-blk: default num_queues to -smp N

[Pankaj Gupta]

> Automatically size the number of virtio-blk-pci request virtqueues to
> match the number of vCPUs.  Other transports continue to default to 1
> request virtqueue.
>
> A 1:1 virtqueue:vCPU mapping ensures that completion interrupts are
> handled on the same vCPU that submitted the request.  No IPI is
> necessary to complete an I/O request and performance is improved.
>
> Performance improves from 78k to 104k IOPS on a 32 vCPU guest with 101
> virtio-blk-pci devices (ioengine=libaio, iodepth=1, bs=4k, rw=randread
> with NVMe storage).
>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> Reviewed-by: Cornelia Huck <cohuck@redhat.com>
> ---
>  include/hw/virtio/virtio-blk.h | 2 ++
>  hw/block/virtio-blk.c          | 6 +++++-
>  hw/core/machine.c              | 1 +
>  hw/virtio/virtio-blk-pci.c     | 7 ++++++-
>  4 files changed, 14 insertions(+), 2 deletions(-)
>
> diff --git a/include/hw/virtio/virtio-blk.h b/include/hw/virtio/virtio-blk.h
> index 1e62f869b2..4e5e903f4a 100644
> --- a/include/hw/virtio/virtio-blk.h
> +++ b/include/hw/virtio/virtio-blk.h
> @@ -30,6 +30,8 @@ struct virtio_blk_inhdr
>      unsigned char status;
>  };
>
> +#define VIRTIO_BLK_AUTO_NUM_QUEUES UINT16_MAX
> +
>  struct VirtIOBlkConf
>  {
>      BlockConf conf;
> diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
> index f5f6fc925e..3c36b38255 100644
> --- a/hw/block/virtio-blk.c
> +++ b/hw/block/virtio-blk.c
> @@ -1135,6 +1135,9 @@ static void virtio_blk_device_realize(DeviceState *dev, 
> Error **errp)
>          error_setg(errp, "Device needs media, but drive is empty");
>          return;
>      }
> +    if (conf->num_queues == VIRTIO_BLK_AUTO_NUM_QUEUES) {
> +        conf->num_queues = 1;
> +    }
>      if (!conf->num_queues) {
>          error_setg(errp, "num-queues property must be larger than 0");
>          return;
> @@ -1274,7 +1277,8 @@ static Property virtio_blk_properties[] = {
>  #endif
>      DEFINE_PROP_BIT("request-merging", VirtIOBlock, conf.request_merging, 0,
>                      true),
> -    DEFINE_PROP_UINT16("num-queues", VirtIOBlock, conf.num_queues, 1),
> +    DEFINE_PROP_UINT16("num-queues", VirtIOBlock, conf.num_queues,
> +                       VIRTIO_BLK_AUTO_NUM_QUEUES),
>      DEFINE_PROP_UINT16("queue-size", VirtIOBlock, conf.queue_size, 256),
>      DEFINE_PROP_BOOL("seg-max-adjust", VirtIOBlock, conf.seg_max_adjust, 
> true),
>      DEFINE_PROP_LINK("iothread", VirtIOBlock, conf.iothread, TYPE_IOTHREAD,
> diff --git a/hw/core/machine.c b/hw/core/machine.c
> index df7664bc8d..4aba3bdd3c 100644
> --- a/hw/core/machine.c
> +++ b/hw/core/machine.c
> @@ -29,6 +29,7 @@
>  #include "migration/vmstate.h"
>
>  GlobalProperty hw_compat_5_0[] = {
> +    { "virtio-blk-device", "num-queues", "1"},
>      { "virtio-scsi-device", "num_queues", "1"},
>      { "vhost-scsi", "num_queues", "1"},
>      { "vhost-user-scsi", "num_queues", "1"},
> diff --git a/hw/virtio/virtio-blk-pci.c b/hw/virtio/virtio-blk-pci.c
> index 28838fa958..2f0ede3863 100644
> --- a/hw/virtio/virtio-blk-pci.c
> +++ b/hw/virtio/virtio-blk-pci.c
> @@ -50,9 +50,14 @@ static void virtio_blk_pci_realize(VirtIOPCIProxy 
> *vpci_dev, Error **errp)
>  {
>      VirtIOBlkPCI *dev = VIRTIO_BLK_PCI(vpci_dev);
>      DeviceState *vdev = DEVICE(&dev->vdev);
> +    VirtIOBlkConf *conf = &dev->vdev.conf;
> +
> +    if (conf->num_queues == VIRTIO_BLK_AUTO_NUM_QUEUES) {
> +        conf->num_queues = virtio_pci_optimal_num_queues(0);
> +    }
>
>      if (vpci_dev->nvectors == DEV_NVECTORS_UNSPECIFIED) {
> -        vpci_dev->nvectors = dev->vdev.conf.num_queues + 1;
> +        vpci_dev->nvectors = conf->num_queues + 1;
>      }
>
>      qdev_set_parent_bus(vdev, BUS(&vpci_dev->bus));

Looks good to me.
Reviewed-by: Pankaj Gupta <pankaj.gupta.linux@gmail.com>