On 21.10.2019 16:24, Stefan Hajnoczi wrote: > On Fri, Oct 18, 2019 at 02:55:47PM +0300, Denis Plotnikov wrote: >> From: "Denis V. Lunev" <d...@openvz.org> >> >> 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. >> >> Unfortunately the commit can not be applied as is. For the real cure we >> need guest to be fixed to accomodate that queue length, which is done >> only in the latest 4.14 kernel. Thus we are going to expose the property >> and tweak it on machine type level. >> >> The problem with the old kernels is that they have >> max_segments <= virtqueue_size restriction which cause the guest >> crashing in the case of violation. >> To fix the case described above in the old kernels we can increase >> virtqueue_size to 256 and max_segments to 254. The pitfall here is >> that seabios allows the virtqueue_size-s < 128, however, the seabios >> patch extending that value to 256 is pending. > If I understand correctly you are relying on Indirect Descriptor support > in the guest driver in order to exceed the Virtqueue Descriptor Table > size. > > Unfortunately the "max_segments <= virtqueue_size restriction" is > required by the VIRTIO 1.1 specification: > > 2.6.5.3.1 Driver Requirements: Indirect Descriptors > > A driver MUST NOT create a descriptor chain longer than the Queue > Size of the device. > > So this idea seems to be in violation of the specification? > > There is a bug in hw/block/virtio-blk.c:virtio_blk_update_config() and > hw/scsi/virtio-scsi.c:virtio_scsi_get_config(): > > virtio_stl_p(vdev, &blkcfg.seg_max, 128 - 2); > > This number should be the minimum of blk_get_max_iov() and > virtio_queue_get_num(), minus 2 for the header and footer.
Stefan, It seems VitrioSCSI don't have a direct link to blk, apart of VirtIOBlock->blk, and the link to a blk comes with each scsi request. I suspect that idea here is that a single virtioscsi can serve several blk-s. If my assumption is corect, then we can't get blk_get_max_iov() on virtioscsi configuration stage and we shouldn't take into account max_iov and limit max_segments with virtio_queue_get_num()-2 only. Is it so, or is there any other details to take into account? Thanks! Denis > > I looked at the Linux SCSI driver code and it seems each HBA has a > single max_segments number - it does not vary on a per-device basis. > This could be a problem if two host block device with different > max_segments are exposed to the guest through the same virtio-scsi > controller. Another bug? :( > > Anyway, if you want ~1024 descriptors you should set Queue Size to 1024. > I don't see a spec-compliant way of doing it otherwise. Hopefully I > have overlooked something and there is a nice way to solve this. > > Stefan