Re: Increased memory usage with scsi-mq

Paolo Bonzini
2017-08-07T12:11:44Z

On 05/08/2017 17:51, Richard W.M. Jones wrote:
> 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?

You could also add a module parameter to the driver, and set it to 64 on
the kernel command line (there is an example in
drivers/scsi/vmw_pvscsi.c of how to do it).

Paolo