• Richard W.M. Jones's avatar
    virtio-scsi: Add virtqueue_size parameter allowing virtqueue size to be set. · 5c0919d0
    Richard W.M. Jones authored
    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>
    5c0919d0
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