Commit 5c0919d0 authored by Richard W.M. Jones's avatar Richard W.M. Jones Committed by Paolo Bonzini

virtio-scsi: Add virtqueue_size parameter allowing virtqueue size to be set.

Since Linux switched to blk-mq as the default in Linux commit
5c279bd9e406 ("scsi: default to scsi-mq"), virtio-scsi LUNs consume
about 10x as much guest kernel memory.

This commit allows you to choose the virtqueue size for each
virtio-scsi-pci controller like this:

  -device virtio-scsi-pci,id=scsi,virtqueue_size=16

The default is still 128 as before.  Using smaller virtqueue_size
allows many more disks to be added to small memory virtual machines.
For a 1 vCPU, 500 MB, no swap VM I observed:

  With scsi-mq enabled (upstream kernel):              175 disks
    -"- ditto -"-   virtqueue_size=64:                 318 disks
    -"- ditto -"-   virtqueue_size=16:                 775 disks
  With scsi-mq disabled (kernel before 5c279bd9e406): 1755 disks

Note that to have any effect, this requires a kernel patch:

  https://lkml.org/lkml/2017/8/10/689Signed-off-by: 's avatarRichard W.M. Jones <rjones@redhat.com>
Message-Id: <20170810165255.20865-1-rjones@redhat.com>
Signed-off-by: 's avatarPaolo Bonzini <pbonzini@redhat.com>
parent aa406fea
......@@ -867,10 +867,10 @@ void virtio_scsi_common_realize(DeviceState *dev,
s->sense_size = VIRTIO_SCSI_SENSE_DEFAULT_SIZE;
s->cdb_size = VIRTIO_SCSI_CDB_DEFAULT_SIZE;
s->ctrl_vq = virtio_add_queue(vdev, VIRTIO_SCSI_VQ_SIZE, ctrl);
s->event_vq = virtio_add_queue(vdev, VIRTIO_SCSI_VQ_SIZE, evt);
s->ctrl_vq = virtio_add_queue(vdev, s->conf.virtqueue_size, ctrl);
s->event_vq = virtio_add_queue(vdev, s->conf.virtqueue_size, evt);
for (i = 0; i < s->conf.num_queues; i++) {
s->cmd_vqs[i] = virtio_add_queue(vdev, VIRTIO_SCSI_VQ_SIZE, cmd);
s->cmd_vqs[i] = virtio_add_queue(vdev, s->conf.virtqueue_size, cmd);
}
}
......@@ -917,6 +917,8 @@ static void virtio_scsi_device_unrealize(DeviceState *dev, Error **errp)
static Property virtio_scsi_properties[] = {
DEFINE_PROP_UINT32("num_queues", VirtIOSCSI, parent_obj.conf.num_queues, 1),
DEFINE_PROP_UINT32("virtqueue_size", VirtIOSCSI,
parent_obj.conf.virtqueue_size, 128),
DEFINE_PROP_UINT32("max_sectors", VirtIOSCSI, parent_obj.conf.max_sectors,
0xFFFF),
DEFINE_PROP_UINT32("cmd_per_lun", VirtIOSCSI, parent_obj.conf.cmd_per_lun,
......
......@@ -32,7 +32,6 @@
#define VIRTIO_SCSI(obj) \
OBJECT_CHECK(VirtIOSCSI, (obj), TYPE_VIRTIO_SCSI)
#define VIRTIO_SCSI_VQ_SIZE 128
#define VIRTIO_SCSI_MAX_CHANNEL 0
#define VIRTIO_SCSI_MAX_TARGET 255
#define VIRTIO_SCSI_MAX_LUN 16383
......@@ -48,6 +47,7 @@ typedef struct virtio_scsi_config VirtIOSCSIConfig;
struct VirtIOSCSIConf {
uint32_t num_queues;
uint32_t virtqueue_size;
uint32_t max_sectors;
uint32_t cmd_per_lun;
#ifdef CONFIG_VHOST_SCSI
......
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