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

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Date:   Mon, 7 Aug 2017 15:07:48 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Richard W.M. Jones" <rjones@...hat.com>
Cc:     Christoph Hellwig <hch@....de>, linux-scsi@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        "Martin K. Petersen" <martin.petersen@...cle.com>
Subject: Re: Increased memory usage with scsi-mq

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