Re: Increased memory usage with scsi-mq

Richard W.M. Jones
2017-08-05T15:51:07Z

On Sat, Aug 05, 2017 at 03:39:54PM +0200, Christoph Hellwig wrote:
> For now can you apply this testing patch to the guest kernel?
>
> diff --git a/drivers/scsi/virtio_scsi.c b/drivers/scsi/virtio_scsi.c
> index 9be211d68b15..0cbe2c882e1c 100644
> --- a/drivers/scsi/virtio_scsi.c
> +++ b/drivers/scsi/virtio_scsi.c
> @@ -818,7 +818,7 @@ static struct scsi_host_template virtscsi_host_template_single = {
> .eh_timed_out = virtscsi_eh_timed_out,
> .slave_alloc = virtscsi_device_alloc,
>
> - .can_queue = 1024,
> + .can_queue = 64,
> .dma_boundary = UINT_MAX,
> .use_clustering = ENABLE_CLUSTERING,
> .target_alloc = virtscsi_target_alloc,
> @@ -839,7 +839,7 @@ static struct scsi_host_template virtscsi_host_template_multi = {
> .eh_timed_out = virtscsi_eh_timed_out,
> .slave_alloc = virtscsi_device_alloc,
>
> - .can_queue = 1024,
> + .can_queue = 64,
> .dma_boundary = UINT_MAX,
> .use_clustering = ENABLE_CLUSTERING,
> .target_alloc = virtscsi_target_alloc,
> @@ -983,7 +983,7 @@ static int virtscsi_probe(struct virtio_device *vdev)
> shost->max_id = num_targets;
> shost->max_channel = 0;
> shost->max_cmd_len = VIRTIO_SCSI_CDB_SIZE;
> - shost->nr_hw_queues = num_queues;
> + shost->nr_hw_queues = 1;
>
> #ifdef CONFIG_BLK_DEV_INTEGRITY
> if (virtio_has_feature(vdev, VIRTIO_SCSI_F_T10_PI)) {

Yes, that's an improvement, although it's still a little way off the
density possible the old way:

With scsi-mq enabled: 175 disks
* With this patch: 319 disks *
With scsi-mq disabled: 1755 disks

Also only the first two hunks are necessary. The kernel behaves
exactly the same way with or without the third hunk (ie. num_queues
must already be 1).

Can I infer from this that qemu needs a way to specify the can_queue
setting to the virtio-scsi driver in the guest kernel?

Rich.

--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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