Date:   Fri, 21 Oct 2022 14:30:36 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Yongji Xie <xieyongji@...edance.com>,
        Stefan Hajnoczi <stefanha@...il.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Ming Lei <tom.leiming@...il.com>,
        Ziyang Zhang <ZiyangZhang@...ux.alibaba.com>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        io-uring@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "Denis V. Lunev" <den@...nvz.org>,
        Xiaoguang Wang <xiaoguang.wang@...ux.alibaba.com>
Subject: Re: ublk-qcow2: ublk-qcow2 is available


在 2022/10/21 13:33, Yongji Xie 写道:
> On Tue, Oct 18, 2022 at 10:54 PM Stefan Hajnoczi <stefanha@...il.com> wrote:
>> On Tue, 18 Oct 2022 at 09:17, Yongji Xie <xieyongji@...edance.com> wrote:
>>> On Tue, Oct 18, 2022 at 2:59 PM Ming Lei <tom.leiming@...il.com> 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 <tom.leiming@...il.com> 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 <stefanha@...il.com> wrote:
>>>>>>>> On Sat, 8 Oct 2022 at 04:43, Ziyang Zhang <ZiyangZhang@...ux.alibaba.com> 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 <tom.leiming@...il.com> 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
>>>>>>>>>>>>            <M>   vDPA device simulator core
>>>>>>>>>>>>            <M>     vDPA simulator for networking device
>>>>>>>>>>>>            <M>     vDPA simulator for block device
>>>>>>>>>>>>            <M>   VDUSE (vDPA Device in Userspace) support
>>>>>>>>>>>>            <M>   Intel IFC VF vDPA driver
>>>>>>>>>>>>            <M>   Virtio PCI bridge vDPA driver
>>>>>>>>>>>>            <M>   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
>> 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?

Thanks


>
>> 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
>>
> Good to know that!
>
> Thanks,
> Yongji
>

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