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Tsirkin" , Ziyang Zhang , Stefan Hajnoczi , io-uring@vger.kernel.org, linux-block@vger.kernel.org, linux-kernel , "Denis V. Lunev" , Xiaoguang Wang Subject: Re: ublk-qcow2: ublk-qcow2 is available Message-ID: References: <50827796-af93-4af5-4121-dc13c31a67fc@linux.alibaba.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Oct 18, 2022 at 10:54:45AM -0400, Stefan Hajnoczi wrote: > On Tue, 18 Oct 2022 at 09:17, Yongji Xie wrote: > > > > On Tue, Oct 18, 2022 at 2:59 PM Ming Lei wrote: > > > > > > On Mon, Oct 17, 2022 at 07:11:59PM +0800, Yongji Xie wrote: > > > > On Fri, Oct 14, 2022 at 8:57 PM Ming Lei wrote: > > > > > > > > > > On Thu, Oct 13, 2022 at 02:48:04PM +0800, Yongji Xie wrote: > > > > > > On Wed, Oct 12, 2022 at 10:22 PM Stefan Hajnoczi wrote: > > > > > > > > > > > > > > On Sat, 8 Oct 2022 at 04:43, Ziyang Zhang wrote: > > > > > > > > > > > > > > > > On 2022/10/5 12:18, Ming Lei wrote: > > > > > > > > > On Tue, Oct 04, 2022 at 09:53:32AM -0400, Stefan Hajnoczi wrote: > > > > > > > > >> On Tue, 4 Oct 2022 at 05:44, Ming Lei wrote: > > > > > > > > >>> > > > > > > > > >>> On Mon, Oct 03, 2022 at 03:53:41PM -0400, Stefan Hajnoczi wrote: > > > > > > > > >>>> On Fri, Sep 30, 2022 at 05:24:11PM +0800, Ming Lei wrote: > > > > > > > > >>>>> ublk-qcow2 is available now. > > > > > > > > >>>> > > > > > > > > >>>> Cool, thanks for sharing! > > > > > > > > >>>> > > > > > > > > >>>>> > > > > > > > > >>>>> So far it provides basic read/write function, and compression and snapshot > > > > > > > > >>>>> aren't supported yet. The target/backend implementation is completely > > > > > > > > >>>>> based on io_uring, and share the same io_uring with ublk IO command > > > > > > > > >>>>> handler, just like what ublk-loop does. > > > > > > > > >>>>> > > > > > > > > >>>>> Follows the main motivations of ublk-qcow2: > > > > > > > > >>>>> > > > > > > > > >>>>> - building one complicated target from scratch helps libublksrv APIs/functions > > > > > > > > >>>>> become mature/stable more quickly, since qcow2 is complicated and needs more > > > > > > > > >>>>> requirement from libublksrv compared with other simple ones(loop, null) > > > > > > > > >>>>> > > > > > > > > >>>>> - there are several attempts of implementing qcow2 driver in kernel, such as > > > > > > > > >>>>> ``qloop`` [2], ``dm-qcow2`` [3] and ``in kernel qcow2(ro)`` [4], so ublk-qcow2 > > > > > > > > >>>>> might useful be for covering requirement in this field > > > > > > > > >>>>> > > > > > > > > >>>>> - performance comparison with qemu-nbd, and it was my 1st thought to evaluate > > > > > > > > >>>>> performance of ublk/io_uring backend by writing one ublk-qcow2 since ublksrv > > > > > > > > >>>>> is started > > > > > > > > >>>>> > > > > > > > > >>>>> - help to abstract common building block or design pattern for writing new ublk > > > > > > > > >>>>> target/backend > > > > > > > > >>>>> > > > > > > > > >>>>> So far it basically passes xfstest(XFS) test by using ublk-qcow2 block > > > > > > > > >>>>> device as TEST_DEV, and kernel building workload is verified too. Also > > > > > > > > >>>>> soft update approach is applied in meta flushing, and meta data > > > > > > > > >>>>> integrity is guaranteed, 'make test T=qcow2/040' covers this kind of > > > > > > > > >>>>> test, and only cluster leak is reported during this test. > > > > > > > > >>>>> > > > > > > > > >>>>> The performance data looks much better compared with qemu-nbd, see > > > > > > > > >>>>> details in commit log[1], README[5] and STATUS[6]. And the test covers both > > > > > > > > >>>>> empty image and pre-allocated image, for example of pre-allocated qcow2 > > > > > > > > >>>>> image(8GB): > > > > > > > > >>>>> > > > > > > > > >>>>> - qemu-nbd (make test T=qcow2/002) > > > > > > > > >>>> > > > > > > > > >>>> Single queue? > > > > > > > > >>> > > > > > > > > >>> Yeah. > > > > > > > > >>> > > > > > > > > >>>> > > > > > > > > >>>>> randwrite(4k): jobs 1, iops 24605 > > > > > > > > >>>>> randread(4k): jobs 1, iops 30938 > > > > > > > > >>>>> randrw(4k): jobs 1, iops read 13981 write 14001 > > > > > > > > >>>>> rw(512k): jobs 1, iops read 724 write 728 > > > > > > > > >>>> > > > > > > > > >>>> Please try qemu-storage-daemon's VDUSE export type as well. The > > > > > > > > >>>> command-line should be similar to this: > > > > > > > > >>>> > > > > > > > > >>>> # modprobe virtio_vdpa # attaches vDPA devices to host kernel > > > > > > > > >>> > > > > > > > > >>> Not found virtio_vdpa module even though I enabled all the following > > > > > > > > >>> options: > > > > > > > > >>> > > > > > > > > >>> --- vDPA drivers > > > > > > > > >>> vDPA device simulator core > > > > > > > > >>> vDPA simulator for networking device > > > > > > > > >>> vDPA simulator for block device > > > > > > > > >>> VDUSE (vDPA Device in Userspace) support > > > > > > > > >>> Intel IFC VF vDPA driver > > > > > > > > >>> Virtio PCI bridge vDPA driver > > > > > > > > >>> vDPA driver for Alibaba ENI > > > > > > > > >>> > > > > > > > > >>> BTW, my test environment is VM and the shared data is done in VM too, and > > > > > > > > >>> can virtio_vdpa be used inside VM? > > > > > > > > >> > > > > > > > > >> I hope Xie Yongji can help explain how to benchmark VDUSE. > > > > > > > > >> > > > > > > > > >> virtio_vdpa is available inside guests too. Please check that > > > > > > > > >> VIRTIO_VDPA ("vDPA driver for virtio devices") is enabled in "Virtio > > > > > > > > >> drivers" menu. > > > > > > > > >> > > > > > > > > >>> > > > > > > > > >>>> # modprobe vduse > > > > > > > > >>>> # qemu-storage-daemon \ > > > > > > > > >>>> --blockdev file,filename=test.qcow2,cache.direct=of|off,aio=native,node-name=file \ > > > > > > > > >>>> --blockdev qcow2,file=file,node-name=qcow2 \ > > > > > > > > >>>> --object iothread,id=iothread0 \ > > > > > > > > >>>> --export vduse-blk,id=vduse0,name=vduse0,num-queues=$(nproc),node-name=qcow2,writable=on,iothread=iothread0 > > > > > > > > >>>> # vdpa dev add name vduse0 mgmtdev vduse > > > > > > > > >>>> > > > > > > > > >>>> A virtio-blk device should appear and xfstests can be run on it > > > > > > > > >>>> (typically /dev/vda unless you already have other virtio-blk devices). > > > > > > > > >>>> > > > > > > > > >>>> Afterwards you can destroy the device using: > > > > > > > > >>>> > > > > > > > > >>>> # vdpa dev del vduse0 > > > > > > > > >>>> > > > > > > > > >>>>> > > > > > > > > >>>>> - ublk-qcow2 (make test T=qcow2/022) > > > > > > > > >>>> > > > > > > > > >>>> There are a lot of other factors not directly related to NBD vs ublk. In > > > > > > > > >>>> order to get an apples-to-apples comparison with qemu-* a ublk export > > > > > > > > >>>> type is needed in qemu-storage-daemon. That way only the difference is > > > > > > > > >>>> the ublk interface and the rest of the code path is identical, making it > > > > > > > > >>>> possible to compare NBD, VDUSE, ublk, etc more precisely. > > > > > > > > >>> > > > > > > > > >>> Maybe not true. > > > > > > > > >>> > > > > > > > > >>> ublk-qcow2 uses io_uring to handle all backend IO(include meta IO) completely, > > > > > > > > >>> and so far single io_uring/pthread is for handling all qcow2 IOs and IO > > > > > > > > >>> command. > > > > > > > > >> > > > > > > > > >> qemu-nbd doesn't use io_uring to handle the backend IO, so we don't > > > > > > > > > > > > > > > > > > I tried to use it via --aio=io_uring for setting up qemu-nbd, but not succeed. > > > > > > > > > > > > > > > > > >> know whether the benchmark demonstrates that ublk is faster than NBD, > > > > > > > > >> that the ublk-qcow2 implementation is faster than qemu-nbd's qcow2, > > > > > > > > >> whether there are miscellaneous implementation differences between > > > > > > > > >> ublk-qcow2 and qemu-nbd (like using the same io_uring context for both > > > > > > > > >> ublk and backend IO), or something else. > > > > > > > > > > > > > > > > > > The theory shouldn't be too complicated: > > > > > > > > > > > > > > > > > > 1) io uring passthough(pt) communication is fast than socket, and io command > > > > > > > > > is carried over io_uring pt commands, and should be fast than virio > > > > > > > > > communication too. > > > > > > > > > > > > > > > > > > 2) io uring io handling is fast than libaio which is taken in the > > > > > > > > > test on qemu-nbd, and all qcow2 backend io(include meta io) is handled > > > > > > > > > by io_uring. > > > > > > > > > > > > > > > > > > https://github.com/ming1/ubdsrv/blob/master/tests/common/qcow2_common > > > > > > > > > > > > > > > > > > 3) ublk uses one single io_uring to handle all io commands and qcow2 > > > > > > > > > backend IOs, so batching handling is common, and it is easy to see > > > > > > > > > dozens of IOs/io commands handled in single syscall, or even more. > > > > > > > > > > > > > > > > > >> > > > > > > > > >> I'm suggesting measuring changes to just 1 variable at a time. > > > > > > > > >> Otherwise it's hard to reach a conclusion about the root cause of the > > > > > > > > >> performance difference. Let's learn why ublk-qcow2 performs well. > > > > > > > > > > > > > > > > > > Turns out the latest Fedora 37-beta doesn't support vdpa yet, so I built > > > > > > > > > qemu from the latest github tree, and finally it starts to work. And test kernel > > > > > > > > > is v6.0 release. > > > > > > > > > > > > > > > > > > Follows the test result, and all three devices are setup as single > > > > > > > > > queue, and all tests are run in single job, still done in one VM, and > > > > > > > > > the test images are stored on XFS/virito-scsi backed SSD. > > > > > > > > > > > > > > > > > > The 1st group tests all three block device which is backed by empty > > > > > > > > > qcow2 image. > > > > > > > > > > > > > > > > > > The 2nd group tests all the three block devices backed by pre-allocated > > > > > > > > > qcow2 image. > > > > > > > > > > > > > > > > > > Except for big sequential IO(512K), there is still not small gap between > > > > > > > > > vdpa-virtio-blk and ublk. > > > > > > > > > > > > > > > > > > 1. run fio on block device over empty qcow2 image > > > > > > > > > 1) qemu-nbd > > > > > > > > > running qcow2/001 > > > > > > > > > run perf test on empty qcow2 image via nbd > > > > > > > > > fio (nbd(/mnt/data/ublk_null_8G_nYbgF.qcow2), libaio, bs 4k, dio, hw queues:1)... > > > > > > > > > randwrite: jobs 1, iops 8549 > > > > > > > > > randread: jobs 1, iops 34829 > > > > > > > > > randrw: jobs 1, iops read 11363 write 11333 > > > > > > > > > rw(512k): jobs 1, iops read 590 write 597 > > > > > > > > > > > > > > > > > > > > > > > > > > > 2) ublk-qcow2 > > > > > > > > > running qcow2/021 > > > > > > > > > run perf test on empty qcow2 image via ublk > > > > > > > > > fio (ublk/qcow2( -f /mnt/data/ublk_null_8G_s761j.qcow2), libaio, bs 4k, dio, hw queues:1, uring_comp: 0, get_data: 0). > > > > > > > > > randwrite: jobs 1, iops 16086 > > > > > > > > > randread: jobs 1, iops 172720 > > > > > > > > > randrw: jobs 1, iops read 35760 write 35702 > > > > > > > > > rw(512k): jobs 1, iops read 1140 write 1149 > > > > > > > > > > > > > > > > > > 3) vdpa-virtio-blk > > > > > > > > > running debug/test_dev > > > > > > > > > run io test on specified device > > > > > > > > > fio (vdpa(/dev/vdc), libaio, bs 4k, dio, hw queues:1)... > > > > > > > > > randwrite: jobs 1, iops 8626 > > > > > > > > > randread: jobs 1, iops 126118 > > > > > > > > > randrw: jobs 1, iops read 17698 write 17665 > > > > > > > > > rw(512k): jobs 1, iops read 1023 write 1031 > > > > > > > > > > > > > > > > > > > > > > > > > > > 2. run fio on block device over pre-allocated qcow2 image > > > > > > > > > 1) qemu-nbd > > > > > > > > > running qcow2/002 > > > > > > > > > run perf test on pre-allocated qcow2 image via nbd > > > > > > > > > fio (nbd(/mnt/data/ublk_data_8G_sc0SB.qcow2), libaio, bs 4k, dio, hw queues:1)... > > > > > > > > > randwrite: jobs 1, iops 21439 > > > > > > > > > randread: jobs 1, iops 30336 > > > > > > > > > randrw: jobs 1, iops read 11476 write 11449 > > > > > > > > > rw(512k): jobs 1, iops read 718 write 722 > > > > > > > > > > > > > > > > > > 2) ublk-qcow2 > > > > > > > > > running qcow2/022 > > > > > > > > > run perf test on pre-allocated qcow2 image via ublk > > > > > > > > > fio (ublk/qcow2( -f /mnt/data/ublk_data_8G_yZiaJ.qcow2), libaio, bs 4k, dio, hw queues:1, uring_comp: 0, get_data: 0). > > > > > > > > > randwrite: jobs 1, iops 98757 > > > > > > > > > randread: jobs 1, iops 110246 > > > > > > > > > randrw: jobs 1, iops read 47229 write 47161 > > > > > > > > > rw(512k): jobs 1, iops read 1416 write 1427 > > > > > > > > > > > > > > > > > > 3) vdpa-virtio-blk > > > > > > > > > running debug/test_dev > > > > > > > > > run io test on specified device > > > > > > > > > fio (vdpa(/dev/vdc), libaio, bs 4k, dio, hw queues:1)... > > > > > > > > > randwrite: jobs 1, iops 47317 > > > > > > > > > randread: jobs 1, iops 74092 > > > > > > > > > randrw: jobs 1, iops read 27196 write 27234 > > > > > > > > > rw(512k): jobs 1, iops read 1447 write 1458 > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Hi All, > > > > > > > > > > > > > > > > We are interested in VDUSE vs UBLK, too. And I have tested them with nullblk backend. > > > > > > > > Let me share some results here. > > > > > > > > > > > > > > > > I setup UBLK with: > > > > > > > > ublk add -t loop -f /dev/nullb0 -d QUEUE_DEPTH -q NR_QUEUE > > > > > > > > > > > > > > > > I setup VDUSE with: > > > > > > > > qemu-storage-daemon \ > > > > > > > > --chardev socket,id=charmonitor,path=/tmp/qmp.sock,server=on,wait=off \ > > > > > > > > --monitor chardev=charmonitor \ > > > > > > > > --blockdev driver=host_device,cache.direct=on,filename=/dev/nullb0,node-name=disk0 \ > > > > > > > > --export vduse-blk,id=test,node-name=disk0,name=vduse_test,writable=on,num-queues=NR_QUEUE,queue-size=QUEUE_DEPTH > > > > > > > > > > > > > > > > Here QUEUE_DEPTH is 1, 32 or 128 and NR_QUEUE is 1 or 4. > > > > > > > > > > > > > > > > Note: > > > > > > > > (1) VDUSE requires QUEUE_DEPTH >= 2. I cannot setup QUEUE_DEPTH to 1. > > > > > > > > (2) I use qemu 7.1.0-rc3. It supports vduse-blk. > > > > > > > > (3) I do not use ublk null target so that the test is fair. > > > > > > > > (4) I setup fio with direct=1, bs=4k. > > > > > > > > > > > > > > > > ------------------------------ > > > > > > > > 1 job 1 iodepth, lat（usec) > > > > > > > > vduse ublk > > > > > > > > seq-read 22.55 11.15 > > > > > > > > rand-read 22.49 11.17 > > > > > > > > seq-write 25.67 10.25 > > > > > > > > rand-write 24.13 10.16 > > > > > > > > > > > > > > Thanks for sharing. Any idea what the bottlenecks are for vduse and ublk? > > > > > > > > > > > > > > > > > > > I think one reason for the latency gap of sync I/O is that vduse uses > > > > > > workqueue in the I/O completion path but ublk doesn't. > > > > > > > > > > > > And one bottleneck for the async I/O in vduse is that vduse will do > > > > > > memcpy inside the critical section of virtqueue's spinlock in the > > > > > > virtio-blk driver. That will hurt the performance heavily when > > > > > > virtio_queue_rq() and virtblk_done() run concurrently. And it can be > > > > > > mitigated by the advance DMA mapping feature [1] or irq binding > > > > > > support [2]. > > > > > > > > > > Hi Yongji, > > > > > > > > > > Yeah, that is the cost you paid for virtio. Wrt. userspace block device > > > > > or other sort of userspace devices, cmd completion is driven by > > > > > userspace, not sure if one such 'irq' is needed. > > > > > > > > I'm not sure, it can be an optional feature in the future if needed. > > > > > > > > > Even not sure if virtio > > > > > ring is one good choice for such use case, given io_uring has been proved > > > > > as very efficient(should be better than virtio ring, IMO). > > > > > > > > > > > > > Since vduse is aimed at creating a generic userspace device framework, > > > > virtio should be the right way IMO. > > > > > > OK, it is the right way, but may not be the effective one. > > > > > > > Maybe, but I think we can try to optimize it. > > > > > > And with the vdpa framework, the > > > > userspace device can serve both virtual machines and containers. > > > > > > virtio is good for VM, but not sure it is good enough for other > > > cases. > > > > > > > > > > > Regarding the performance issue, actually I can't measure how much of > > > > the performance loss is due to the difference between virtio ring and > > > > iouring. But I think it should be very small. The main costs come from > > > > the two bottlenecks I mentioned before which could be mitigated in the > > > > future. > > > > > > Per my understanding, at least there are two places where virtio ring is > > > less efficient than io_uring: > > > > > > > I might have misunderstood what you mean by virtio ring before. My > > previous understanding of the virtio ring does not include the > > virtio-blk driver. > > > > > 1) io_uring uses standalone submission queue(SQ) and completion queue(CQ), > > > so no contention exists between submission and completion; but virtio queue > > > requires per-vq lock in both submission and completion. > > > > > > > Yes, this is the bottleneck of the virtio-blk driver, even in the VM > > case. We are also trying to optimize this lock. > > > > One way to mitigate it is making submission and completion happen in > > the same core. > > QEMU sizes virtio-blk device num-queues to match the vCPU count. The num-queues is configurable via qemu-storage-daemon command line, and single queue is usually common case, more queues often means more resources. > virtio-blk driver is a blk-mq driver, so submissions and completions > for a given virtqueue should already be processed by the same vCPU. > > Unless the device is misconfigured or the guest software chooses a > custom vq:vCPU mapping, there should be no vq lock contention between > vCPUs. Single queue or nr_queue less than nr_cpus can't be thought as mis-configured, so every vCPU can submit request, but only one or a few vCPUs complete all. > > I can think of a reason why submission and completion require > coordination: descriptors are occupied until completion. The > submission logic chooses free descriptors from the table. The > completion logic returns free descriptors so they can be used in > future submissions. Shared descriptors is one fundamental design of virtio ring, and looks the reason why vq spin lock is needed in both sides. > > Other ring designs expose the submission ring head AND tail index so > that it's clear which submissions have been processed by the other > side. Once processed, the descriptors are no longer occupied and can > be reused for future submissions immediately. This means that > submission and completion do not share state. > > This is for the split virtqueue layout. For the packed layout I think > there is a similar dependency because descriptors are used for both > submission and completion. > > I have CCed Michael Tsirkin in case he has any thoughts on the > independence of submission and completion in the vring design. > > BTW I have written about difference in the VIRTIO, NVMe, and io_uring > descriptor ring designs here: > https://blog.vmsplice.net/2022/06/comparing-virtio-nvme-and-iouring-queue.html Except for ring, notification could be another difference. Thanks, Ming