Re: [Qemu-block] [PATCH v2 1/1] virtio: fix IO request length in virtio SCSI/block

[Denis V. Lunev]

On 12/14/2017 08:52 PM, Paolo Bonzini wrote:
> On 14/12/2017 15:09, 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.
> The patch is still missing compat properties (and 2.12 machine types).
> If you don't want to add the machine types, feel free to wait until
> someone else does it. :)
>
> Paolo

sorry :( my fault. Will re-spin tomorrow.