linux-kernel - Re: Increased memory usage with scsi-mq

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Date:   Sat, 5 Aug 2017 16:51:03 +0100
From:   "Richard W.M. Jones" <rjones@...hat.com>
To:     Christoph Hellwig <hch@....de>
Cc:     linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        pbonzini@...hat.com
Subject: Re: Increased memory usage with scsi-mq

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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