On 07/08/2017 14:27, Richard W.M. Jones wrote: > Also I noticed this code in virtio_scsi.c: > > cmd_per_lun = virtscsi_config_get(vdev, cmd_per_lun) ?: 1; > shost->cmd_per_lun = min_t(u32, cmd_per_lun, shost->can_queue); > > but setting cmd_per_lun (as a qemu virtio-scsi-pci parameter) didn't > seem to make any difference to memory usage.
That's because memory depends on shost->can_queue, not shost->cmd_per_lun (see scsi_mq_setup_tags). can_queue should depend on the virtqueue size, which unfortunately can vary for each virtio-scsi device in theory. The virtqueue size is retrieved by drivers/virtio prior to calling vring_create_virtqueue, and in QEMU it is the second argument to virtio_add_queue. For virtio-scsi this is VIRTIO_SCSI_VQ_SIZE, which is 128, so changing can_queue to 128 should be a no brainer. Thanks, Paolo
