Re: Increased memory usage with scsi-mq

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From	Paolo Bonzini <pbonzini@redhat.com>
Newsgroups	linux.kernel
Subject	Re: Increased memory usage with scsi-mq
Date	Mon, 07 Aug 2017 15:10:03 +0200
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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

Thread

Increased memory usage with scsi-mq "Richard W.M. Jones" <rjones@redhat.com> - 2017-08-04 23:10 +0200
  Re: Increased memory usage with scsi-mq Christoph Hellwig <hch@lst.de> - 2017-08-05 10:50 +0200
    Re: Increased memory usage with scsi-mq "Richard W.M. Jones" <rjones@redhat.com> - 2017-08-05 11:30 +0200
      Re: Increased memory usage with scsi-mq Christoph Hellwig <hch@lst.de> - 2017-08-05 15:50 +0200
        Re: Increased memory usage with scsi-mq "Richard W.M. Jones" <rjones@redhat.com> - 2017-08-05 18:00 +0200
          Re: Increased memory usage with scsi-mq Paolo Bonzini <pbonzini@redhat.com> - 2017-08-07 14:20 +0200
            Re: Increased memory usage with scsi-mq "Richard W.M. Jones" <rjones@redhat.com> - 2017-08-07 14:30 +0200
              Re: Increased memory usage with scsi-mq Paolo Bonzini <pbonzini@redhat.com> - 2017-08-07 15:10 +0200
                Re: Increased memory usage with scsi-mq Christoph Hellwig <hch@lst.de> - 2017-08-09 18:10 +0200
                Re: Increased memory usage with scsi-mq Paolo Bonzini <pbonzini@redhat.com> - 2017-08-09 19:00 +0200
                Re: Increased memory usage with scsi-mq "Richard W.M. Jones" <rjones@redhat.com> - 2017-08-10 14:30 +0200
                Re: Increased memory usage with scsi-mq Paolo Bonzini <pbonzini@redhat.com> - 2017-08-10 15:00 +0200
                Re: Increased memory usage with scsi-mq "Richard W.M. Jones" <rjones@redhat.com> - 2017-08-10 16:20 +0200
                Re: Increased memory usage with scsi-mq Paolo Bonzini <pbonzini@redhat.com> - 2017-08-10 16:40 +0200
                Re: Increased memory usage with scsi-mq "Richard W.M. Jones" <rjones@redhat.com> - 2017-08-10 17:50 +0200
                Re: Increased memory usage with scsi-mq Paolo Bonzini <pbonzini@redhat.com> - 2017-08-10 18:10 +0200

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