Re: [Qemu-devel] [PATCH 2/2] virtio: fix IO request length in virtio SCSI/block

[Stefan Hajnoczi]

On Fri, Dec 15, 2017 at 06:02:50PM +0300, Denis V. Lunev wrote:
> Linux guests submit IO requests no longer than PAGE_SIZE * max_seg
> field reported by SCSI controler. Thus typical sequential read with
> 1 MB size results in the following pattern of the IO from the guest:
>   8,16   1    15754     2.766095122  2071  D   R 2095104 + 1008 [dd]
>   8,16   1    15755     2.766108785  2071  D   R 2096112 + 1008 [dd]
>   8,16   1    15756     2.766113486  2071  D   R 2097120 + 32 [dd]
>   8,16   1    15757     2.767668961     0  C   R 2095104 + 1008 [0]
>   8,16   1    15758     2.768534315     0  C   R 2096112 + 1008 [0]
>   8,16   1    15759     2.768539782     0  C   R 2097120 + 32 [0]
> The IO was generated by
>   dd if=/dev/sda of=/dev/null bs=1024 iflag=direct
> 
> This effectively means that on rotational disks we will observe 3 IOPS
> for each 2 MBs processed. This definitely negatively affects both
> guest and host IO performance.
> 
> The cure is relatively simple - we should report lengthy scatter-gather
> ability of the SCSI controller. Fortunately the situation here is very
> good. VirtIO transport layer can accomodate 1024 items in one request
> while we are using only 128. This situation is present since almost
> very beginning. 2 items are dedicated for request metadata thus we
> should publish VIRTQUEUE_MAX_SIZE - 2 as max_seg.
> 
> The following pattern is observed after the patch:
>   8,16   1     9921     2.662721340  2063  D   R 2095104 + 1024 [dd]
>   8,16   1     9922     2.662737585  2063  D   R 2096128 + 1024 [dd]
>   8,16   1     9923     2.665188167     0  C   R 2095104 + 1024 [0]
>   8,16   1     9924     2.665198777     0  C   R 2096128 + 1024 [0]
> which is much better.
> 
> The dark side of this patch is that we are tweaking guest visible
> parameter, though this should be relatively safe as above transport
> layer support is present in QEMU/host Linux for a very long time.
> The patch adds configurable property for VirtIO SCSI with a new default
> and hardcode option for VirtBlock which does not provide good
> configurable framework.
> 
> Signed-off-by: Denis V. Lunev <address@hidden>
> CC: "Michael S. Tsirkin" <address@hidden>
> CC: Stefan Hajnoczi <address@hidden>
> CC: Kevin Wolf <address@hidden>
> CC: Max Reitz <address@hidden>
> CC: Paolo Bonzini <address@hidden>
> CC: Richard Henderson <address@hidden>
> CC: Eduardo Habkost <address@hidden>
> ---
>  include/hw/compat.h             | 17 +++++++++++++++++
>  include/hw/virtio/virtio-blk.h  |  1 +
>  include/hw/virtio/virtio-scsi.h |  1 +
>  hw/block/virtio-blk.c           |  4 +++-
>  hw/scsi/vhost-scsi.c            |  2 ++
>  hw/scsi/vhost-user-scsi.c       |  2 ++
>  hw/scsi/virtio-scsi.c           |  4 +++-
>  7 files changed, 29 insertions(+), 2 deletions(-)
> 
> diff --git a/include/hw/compat.h b/include/hw/compat.h
> index 026fee9..b9be5d7 100644
> --- a/include/hw/compat.h
> +++ b/include/hw/compat.h
> @@ -2,6 +2,23 @@
>  #define HW_COMPAT_H
>  
>  #define HW_COMPAT_2_11 \
> +    {\
> +        .driver   = "virtio-blk-device",\
> +        .property = "max_segments",\
> +        .value    = "126",\
> +    },{\
> +        .driver   = "vhost-scsi",\
> +        .property = "max_segments",\
> +        .value    = "126",\
> +    },{\
> +        .driver   = "vhost-user-scsi",\
> +        .property = "max_segments",\
> +        .value    = "126",\

Existing vhost-user-scsi slave programs might not expect up to 1022
segments.  Hopefully we can get away with this change since there are
relatively few vhost-user-scsi slave programs.

CCed Felipe (Nutanix) and Jim (SPDK) in case they have comments.

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