On Wed, Oct 05, 2022 at 02:50:06PM +0300, Andrey Zhadchenko wrote: > > > On 10/4/22 22:00, Stefan Hajnoczi wrote: > > On Mon, Jul 25, 2022 at 11:55:26PM +0300, Andrey Zhadchenko wrote: > > > Although QEMU virtio-blk is quite fast, there is still some room for > > > improvements. Disk latency can be reduced if we handle virito-blk requests > > > in host kernel so we avoid a lot of syscalls and context switches. > > > > > > The biggest disadvantage of this vhost-blk flavor is raw format. > > > Luckily Kirill Thai proposed device mapper driver for QCOW2 format to > > > attach > > > files as block devices: > > > https://www.spinics.net/lists/kernel/msg4292965.html > > > > > > Also by using kernel modules we can bypass iothread limitation and finaly > > > scale > > > block requests with cpus for high-performance devices. This is planned to > > > be > > > implemented in next version. > > > > > > Linux kernel module part: > > > https://lore.kernel.org/kvm/20220725202753.298725-1-andrey.zhadche...@virtuozzo.com/ > > > > > > test setups and results: > > > fio --direct=1 --rw=randread --bs=4k --ioengine=libaio --iodepth=128 > > > QEMU drive options: cache=none > > > filesystem: xfs > > > > > > SSD: > > > | randread, IOPS | randwrite, IOPS | > > > Host | 95.8k | 85.3k | > > > QEMU virtio | 57.5k | 79.4k | > > > QEMU vhost-blk | 95.6k | 84.3k | > > > > > > RAMDISK (vq == vcpu): > > > | randread, IOPS | randwrite, IOPS | > > > virtio, 1vcpu | 123k | 129k | > > > virtio, 2vcpu | 253k (??) | 250k (??) | > > > virtio, 4vcpu | 158k | 154k | > > > vhost-blk, 1vcpu | 110k | 113k | > > > vhost-blk, 2vcpu | 247k | 252k | > > > vhost-blk, 4vcpu | 576k | 567k | > > > > > > Andrey Zhadchenko (1): > > > block: add vhost-blk backend > > > > > > configure | 13 ++ > > > hw/block/Kconfig | 5 + > > > hw/block/meson.build | 1 + > > > hw/block/vhost-blk.c | 395 ++++++++++++++++++++++++++++++++++ > > > hw/virtio/meson.build | 1 + > > > hw/virtio/vhost-blk-pci.c | 102 +++++++++ > > > include/hw/virtio/vhost-blk.h | 44 ++++ > > > linux-headers/linux/vhost.h | 3 + > > > 8 files changed, 564 insertions(+) > > > create mode 100644 hw/block/vhost-blk.c > > > create mode 100644 hw/virtio/vhost-blk-pci.c > > > create mode 100644 include/hw/virtio/vhost-blk.h > > > > vhost-blk has been tried several times in the past. That doesn't mean it > > cannot be merged this time, but past arguments should be addressed: > > > > - What makes it necessary to move the code into the kernel? In the past > > the performance results were not very convincing. The fastest > > implementations actually tend to be userspace NVMe PCI drivers that > > bypass the kernel! Bypassing the VFS and submitting block requests > > directly was not a huge boost. The syscall/context switch argument > > sounds okay but the numbers didn't really show that kernel block I/O > > is much faster than userspace block I/O. > > > > I've asked for more details on the QEMU command-line to understand > > what your numbers show. Maybe something has changed since previous > > times when vhost-blk has been tried. > > > > The only argument I see is QEMU's current 1 IOThread per virtio-blk > > device limitation, which is currently being worked on. If that's the > > only reason for vhost-blk then is it worth doing all the work of > > getting vhost-blk shipped (kernel, QEMU, and libvirt changes)? It > > seems like a short-term solution. > > > > - The security impact of bugs in kernel vhost-blk code is more serious > > than bugs in a QEMU userspace process. > > > > - The management stack needs to be changed to use vhost-blk whereas > > QEMU can be optimized without affecting other layers. > > > > Stefan > > Indeed there was several vhost-blk attempts, but from what I found in > mailing lists only the Asias attempt got some attention and discussion. > Ramdisk performance results were great but ramdisk is more a benchmark than > a real use. I didn't find out why Asias dropped his version except vague "He > concluded performance results was not worth". The storage speed is very > important for vhost-blk performance, as there is no point to cut cpu costs > from 1ms to 0,1ms if the request need 50ms to proceed in the actual disk. I > think that 10 years ago NVMI was non-existent and SSD + SATA was probably a > lot faster than HDD but still not enough to utilize this technology.
Yes, it's possible that latency improvements are more noticeable now. Thank you for posting the benchmark results. I will also run benchmarks so we can compare vhost-blk with today's QEMU as well as multiqueue IOThreads QEMU (for which I only have a hacky prototype) on a local NVMe PCI SSD. > The tests I did give me 60k IOPS randwrite for VM and 95k for host. And the > vhost-blk is able to negate the difference even using only 1 thread/vq/vcpu. > And unlinke current QEMU single IOThread it can be easily scaled with number > of cpus/vcpus. For sure this can be solved by liftimg IOThread limitations > but this will probably be even more disastrous amount of changes (and adding > vhost-blk won't break old setups!). > > Probably the only undisputed advantage of vhost-blk is syscalls reduction. > And again the benefit really depends on a storage speed, as it should be > somehow comparable with syscalls time. Also I must note that this may be > good for high-density servers with a lot of VMs. But for now I did not have > the exact numbers which show how much time we are really winning for a > single request at average. > > Overall vhost-blk will only become better along with the increase of storage > speed. > > Also I must note that all arguments above apply to vdpa-blk. And unlike > vhost-blk, which needs it's own QEMU code, vdpa-blk can be setup with > generic virtio-vdpa QEMU code (I am not sure if it is merged yet but still). > Although vdpa-blk have it's own problems for now. Yes, I think that's why Stefano hasn't pushed for a software vpda-blk device yet despite having played with it and is more focussed on hardware enablement. vdpa-blk has the same issues as vhost-blk. Stefan
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