From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 33238C04A95 for ; Tue, 25 Oct 2022 08:17:54 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231659AbiJYIRw (ORCPT ); Tue, 25 Oct 2022 04:17:52 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:35836 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231408AbiJYIRu (ORCPT ); Tue, 25 Oct 2022 04:17:50 -0400 Received: from mail-ed1-x533.google.com (mail-ed1-x533.google.com [IPv6:2a00:1450:4864:20::533]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 209E4C2C8E for ; Tue, 25 Oct 2022 01:17:47 -0700 (PDT) Received: by mail-ed1-x533.google.com with SMTP id a5so21982142edb.11 for ; Tue, 25 Oct 2022 01:17:47 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=bytedance-com.20210112.gappssmtp.com; s=20210112; h=content-transfer-encoding:cc:to:subject:message-id:date:from :in-reply-to:references:mime-version:from:to:cc:subject:date :message-id:reply-to; bh=uReB/Ptp2ISgZua1vja00OdAAxuE9UvI1yi/3W7gtmQ=; b=zmZxcv2MRXqmhGwk5VoX9FaEB5pjRbgd480ucHf55c1ey0oqC1+ivsQ2IYL5X5LmIa 84yPBCLppWdqoO9leWdLQLEdspVAu09sW6UCUfCYg+uYc7WLXzsqfH9vYRveUqraIUER s8xCbKHdXxg/NotjSHqmCZ6kkWfMGRljvh7mHj12gYBZS69uzrpxDkBLWUTPXE7PA7ZT AfTLXG6nAQwuotphkHMs6PC/pz7NRBu7/qOgTK3j59Rr+BJ/YGhGWQsTmRH+pl8yOZok E7Wwbpa4UElyY07zULsZEn+DqKgxZ5wg8VpAbCNaiUVeuh88BSP/N2IgiYrlMyuJ4X8V d4QA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=content-transfer-encoding:cc:to:subject:message-id:date:from :in-reply-to:references:mime-version:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=uReB/Ptp2ISgZua1vja00OdAAxuE9UvI1yi/3W7gtmQ=; b=GG0BO8THLlmjg9/xCz/jQ5Xe1TIjoBiQaRgokJ+aVBAYrK8J2YaEhBAvhKKYheWOAn poeJ6WHS+RAXR6W4kv5AYQhlawDj49cp20oHga3vAP4PBP1dg3UBRPNK0cVgaaUHHWqh aDNQN7qje5otZbgQ5B4nw0qzDoqNb7u7tWvGxBJznYfSFFcTdyuRE9C0ERP72ZnEcSGz CS2N6NWBpTx4BMSvO5SP839iqw1fEjo5S0VBMlhmUSUP5QGnwm3Y9XOY+4+kO70wjkId GJg6Vu0/Gu3GKZ+s0938ajR9I0YE2VDVY2MLhDterwLWQhaTjHtEfd8JMcshPakkHPxx ar8A== X-Gm-Message-State: ACrzQf2o+pBC9vYI1AGMa+pEN8OXskR/ai0FEDyIlt2rFbXTp3onDZKG RYi6mGA1ftZjidrh01znX8hSCHJue/Mv3JxVCKd9 X-Google-Smtp-Source: AMsMyM7k3i+5MyvD21oT5YFCSShz5blyU7LTl69jLdf0/UpWKR2ZQbsnkZCL0oBLI99FcMBYoeiXyleErl3jcmf1jwM= X-Received: by 2002:a05:6402:5024:b0:440:e4ad:f7b6 with SMTP id p36-20020a056402502400b00440e4adf7b6mr35123998eda.358.1666685865550; Tue, 25 Oct 2022 01:17:45 -0700 (PDT) MIME-Version: 1.0 References: <50827796-af93-4af5-4121-dc13c31a67fc@linux.alibaba.com> <1d051d63-ce34-1bb3-2256-4ced4be6d690@redhat.com> In-Reply-To: <1d051d63-ce34-1bb3-2256-4ced4be6d690@redhat.com> From: Yongji Xie Date: Tue, 25 Oct 2022 16:17:33 +0800 Message-ID: Subject: Re: ublk-qcow2: ublk-qcow2 is available To: Jason Wang Cc: Stefan Hajnoczi , "Michael S. Tsirkin" , Ming Lei , Ziyang Zhang , Stefan Hajnoczi , io-uring@vger.kernel.org, linux-block@vger.kernel.org, linux-kernel , "Denis V. Lunev" , Xiaoguang Wang Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On Fri, Oct 21, 2022 at 2:30 PM Jason Wang wrote: > > > =E5=9C=A8 2022/10/21 13:33, Yongji Xie =E5=86=99=E9=81=93: > > On Tue, Oct 18, 2022 at 10:54 PM Stefan Hajnoczi w= rote: > >> On Tue, 18 Oct 2022 at 09:17, Yongji Xie wro= te: > >>> On Tue, Oct 18, 2022 at 2:59 PM Ming Lei wrot= e: > >>>> On Mon, Oct 17, 2022 at 07:11:59PM +0800, Yongji Xie wrote: > >>>>> On Fri, Oct 14, 2022 at 8:57 PM Ming Lei wr= ote: > >>>>>> 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 wrot= e: > >>>>>>>>>>> On Tue, 4 Oct 2022 at 05:44, Ming Lei = wrote: > >>>>>>>>>>>> On Mon, Oct 03, 2022 at 03:53:41PM -0400, Stefan Hajnoczi wr= ote: > >>>>>>>>>>>>> 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 compress= ion and snapshot > >>>>>>>>>>>>>> aren't supported yet. The target/backend implementation is= completely > >>>>>>>>>>>>>> based on io_uring, and share the same io_uring with ublk I= O command > >>>>>>>>>>>>>> handler, just like what ublk-loop does. > >>>>>>>>>>>>>> > >>>>>>>>>>>>>> Follows the main motivations of ublk-qcow2: > >>>>>>>>>>>>>> > >>>>>>>>>>>>>> - building one complicated target from scratch helps libub= lksrv APIs/functions > >>>>>>>>>>>>>> become mature/stable more quickly, since qcow2 is compl= icated 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(r= o)`` [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 ubl= k-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 verifi= ed too. Also > >>>>>>>>>>>>>> soft update approach is applied in meta flushing, and meta= data > >>>>>>>>>>>>>> integrity is guaranteed, 'make test T=3Dqcow2/040' 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-al= located qcow2 > >>>>>>>>>>>>>> image(8GB): > >>>>>>>>>>>>>> > >>>>>>>>>>>>>> - qemu-nbd (make test T=3Dqcow2/002) > >>>>>>>>>>>>> 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 k= ernel > >>>>>>>>>>>> Not found virtio_vdpa module even though I enabled all the f= ollowing > >>>>>>>>>>>> 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 i= n 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=3Dtest.qcow2,cache.direct= =3Dof|off,aio=3Dnative,node-name=3Dfile \ > >>>>>>>>>>>>> --blockdev qcow2,file=3Dfile,node-name=3Dqcow2 \ > >>>>>>>>>>>>> --object iothread,id=3Diothread0 \ > >>>>>>>>>>>>> --export vduse-blk,id=3Dvduse0,name=3Dvduse0,num-que= ues=3D$(nproc),node-name=3Dqcow2,writable=3Don,iothread=3Diothread0 > >>>>>>>>>>>>> # vdpa dev add name vduse0 mgmtdev vduse > >>>>>>>>>>>>> > >>>>>>>>>>>>> A virtio-blk device should appear and xfstests can be run o= n it > >>>>>>>>>>>>> (typically /dev/vda unless you already have other virtio-bl= k devices). > >>>>>>>>>>>>> > >>>>>>>>>>>>> Afterwards you can destroy the device using: > >>>>>>>>>>>>> > >>>>>>>>>>>>> # vdpa dev del vduse0 > >>>>>>>>>>>>> > >>>>>>>>>>>>>> - ublk-qcow2 (make test T=3Dqcow2/022) > >>>>>>>>>>>>> There are a lot of other factors not directly related to NB= D vs ublk. In > >>>>>>>>>>>>> order to get an apples-to-apples comparison with qemu-* a u= blk export > >>>>>>>>>>>>> type is needed in qemu-storage-daemon. That way only the di= fference is > >>>>>>>>>>>>> the ublk interface and the rest of the code path is identic= al, 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 me= ta 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=3Dio_uring for setting up qemu-nbd= , but not succeed. > >>>>>>>>>> > >>>>>>>>>>> know whether the benchmark demonstrates that ublk is faster t= han NBD, > >>>>>>>>>>> that the ublk-qcow2 implementation is faster than qemu-nbd's = qcow2, > >>>>>>>>>>> whether there are miscellaneous implementation differences be= tween > >>>>>>>>>>> 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 m= ore. > >>>>>>>>>> > >>>>>>>>>>> I'm suggesting measuring changes to just 1 variable at a time= . > >>>>>>>>>>> Otherwise it's hard to reach a conclusion about the root caus= e of the > >>>>>>>>>>> performance difference. Let's learn why ublk-qcow2 performs w= ell. > >>>>>>>>>> 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 wor= k. And test kernel > >>>>>>>>>> is v6.0 release. > >>>>>>>>>> > >>>>>>>>>> Follows the test result, and all three devices are setup as si= ngle > >>>>>>>>>> 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 g= ap 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, b= s 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, b= s 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=3Dcharmonitor,path=3D/tmp/qmp.sock,= server=3Don,wait=3Doff \ > >>>>>>>>> --monitor chardev=3Dcharmonitor \ > >>>>>>>>> --blockdev driver=3Dhost_device,cache.direct=3Don,filen= ame=3D/dev/nullb0,node-name=3Ddisk0 \ > >>>>>>>>> --export vduse-blk,id=3Dtest,node-name=3Ddisk0,name=3Dv= duse_test,writable=3Don,num-queues=3DNR_QUEUE,queue-size=3DQUEUE_DEPTH > >>>>>>>>> > >>>>>>>>> Here QUEUE_DEPTH is 1, 32 or 128 and NR_QUEUE is 1 or 4. > >>>>>>>>> > >>>>>>>>> Note: > >>>>>>>>> (1) VDUSE requires QUEUE_DEPTH >=3D 2. I cannot setup QUEUE_DEP= TH 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=3D1, bs=3D4k. > >>>>>>>>> > >>>>>>>>> ------------------------------ > >>>>>>>>> 1 job 1 iodepth, lat=EF=BC=88usec) > >>>>>>>>> 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 d= evice > >>>>>> 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 framewo= rk, > >>>>> 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 a= nd > >>>>> iouring. But I think it should be very small. The main costs come f= rom > >>>>> the two bottlenecks I mentioned before which could be mitigated in = the > >>>>> future. > >>>> Per my understanding, at least there are two places where virtio rin= g 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 queu= e(CQ), > >>>> so no contention exists between submission and completion; but virti= o 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 > >> 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. > >> > >> 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. > >> > > Yes, we need to maintain a head pointer of the free descriptors in > > both submission and completion path. > > > Not necessarily after IN_ORDER? > Sounds like a good idea. Thanks, Yongji