On Sat, Aug 05, 2017 at 10:44:36AM +0200, Christoph Hellwig wrote:
> On Fri, Aug 04, 2017 at 10:00:47PM +0100, Richard W.M. Jones wrote:
> > I read your slides about scsi-mq and it seems like a significant
> > benefit to large machines, but could the out of the box defaults be
> > made more friendly for small memory machines?
> The default inumber of queues and queue depth and thus memory usage is
> set by the LLDD.
> Try to reduce the can_queue value in virtio_scsi and/or make sure
> you use the single queue variant in your VM (which should be tunable
> in qemu).

Thanks, this is interesting.

Virtio-scsi seems to have a few settable parameters that might be
related to this:

  DEFINE_PROP_UINT32("num_queues", VirtIOSCSI, parent_obj.conf.num_queues, 1),
  DEFINE_PROP_UINT32("max_sectors", VirtIOSCSI, parent_obj.conf.max_sectors,
  DEFINE_PROP_UINT32("cmd_per_lun", VirtIOSCSI, parent_obj.conf.cmd_per_lun,

Unfortunately (assuming I'm setting them right - see below), none of
them have any effect on the number of disks that I can add to the VM.

I am testing them by placing them in the ‘-device virtio-scsi-pci’
parameter, ie. as a property of the controller, not a property of the
LUN, eg:

    -device virtio-scsi-pci,cmd_per_lun=32,id=scsi \
    -device scsi-hd,drive=hd0 \

The debugging output is a bit too large to attach to this email, but I
have placed it at the link below.  It contains (if you scroll down a
bit) the full qemu command line and full kernel output.


I can add some extra debugging into the kernel if you like.  Just
point me to the right place.


Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-builder quickly builds VMs from scratch

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