RE: [PATCHv2 0/6] ublk zero-copy support

lizetao
2025-02-14T04:21:16Z

Hi,

> -----Original Message-----
> From: Ming Lei <ming.lei@redhat.com>
> Sent: Friday, February 14, 2025 10:41 AM
> To: lizetao <lizetao1@huawei.com>
> Cc: Keith Busch <kbusch@meta.com>; io-uring@vger.kernel.org;
> axboe@kernel.dk; Ming Lei <ming.lei@redhat.com>
> Subject: Re: [PATCHv2 0/6] ublk zero-copy support
>
> On Thu, Feb 13, 2025 at 11:24 PM lizetao <lizetao1@huawei.com> wrote:
> >
> > Hi,
> >
> > > -----Original Message-----
> > > From: Keith Busch <kbusch@meta.com>
> > > Sent: Tuesday, February 11, 2025 8:57 AM
> > > To: ming.lei@redhat.com; asml.silence@gmail.com; axboe@kernel.dk;
> > > linux- block@vger.kernel.org; io-uring@vger.kernel.org
> > > Cc: bernd@bsbernd.com; Keith Busch <kbusch@kernel.org>
> > > Subject: [PATCHv2 0/6] ublk zero-copy support
> > >
> > > From: Keith Busch <kbusch@kernel.org>
> > >
> > > Previous version was discussed here:
> > >
> > > https://lore.kernel.org/linux-block/20250203154517.937623-1-
> > > kbusch@meta.com/
> > >
> > > The same ublksrv reference code in that link was used to test the
> > > kernel side changes.
> > >
> > > Before listing what has changed, I want to mention what is the same:
> > > the reliance on the ring ctx lock to serialize the register ahead of
> > > any use. I'm not ignoring the feedback; I just don't have a solid
> > > answer right now, and want to progress on the other fronts in the
> meantime.
> > >
> > > Here's what's different from the previous:
> > >
> > > - Introduced an optional 'release' callback when the resource node is
> > > no longer referenced. The callback addresses any buggy applications
> > > that may complete their request and unregister their index while IO
> > > is in flight. This obviates any need to take extra page references
> > > since it prevents the request from completing.
> > >
> > > - Removed peeking into the io_cache element size and instead use a
> > > more intuitive bvec segment count limit to decide if we're caching
> > > the imu (suggested by Pavel).
> > >
> > > - Dropped the const request changes; it's not needed.
> >
> > I tested this patch set. When I use null as the device, the test results are like
> your v1.
> > When the bs is 4k, there is a slight improvement; when the bs is 64k, there is
> a significant improvement.
>
> Yes, the improvement is usually more obvious with a big IO size(>= 64K).
>
> > However, when I used loop as the device, I found that there was no
> improvement, whether using 4k or 64k. As follow:
> >
> > ublk add -t loop -f ./ublk-loop.img
> > ublk add -t loop -f ./ublk-loop-zerocopy.img
> >
> > fio -filename=/dev/ublkb0 -direct=1 -rw=read -iodepth=1 -ioengine=io_uring
> -bs=128k -size=5G
> > read: IOPS=2015, BW=126MiB/s (132MB/s)(1260MiB/10005msec)
> >
> > fio -filename=/dev/ublkb1 -direct=1 -rw=read -iodepth=1 -ioengine=io_uring
> -bs=128k -size=5G
> > read: IOPS=1998, BW=125MiB/s (131MB/s)(1250MiB/10005msec)
> >
> >
> > So, this patch set is optimized for null type devices? Or if I've missed any key
> information, please let me know.
>
> Latency may have decreased a bit.
>
> System sources can't be saturated in single queue depth, please run the same
> test with high queue depth per Keith's suggestion:
>
> --iodepth=128 --iodepth_batch_submit=16 --
> iodepth_batch_complete_min=16

I tested it with these settings, but the result is similar to iodepth=1:

fio -filename=/dev/ublkb0 -direct=1 -rw=read --iodepth=128 --iodepth_batch_submit=16 --iodepth_batch_complete_min=16 -ioengine=io_uring -bs=64k -size=8G -numjobs=10
read: IOPS=2182, BW=136MiB/s (143MB/s)(1440MiB/10558msec)

fio -filename=/dev/ublkb1 -direct=1 -rw=read --iodepth=128 --iodepth_batch_submit=16 --iodepth_batch_complete_min=16 -ioengine=io_uring -bs=64k -size=8G -numjobs=10
read: IOPS=2174, BW=136MiB/s (143MB/s)(1438MiB/10580msec)

So I believe this is limited by the performance limitations of the file system where ./ublk-loop.img is located.

>
> Also if you set up the backing file as ramfs image, the improvement should be
> pretty obvious, I observed IOPS doubled in this way.

This is true, I tested it in /tmp/ and got a large optimizations. The results as follow:

fio -filename=/dev/ublkb0 -direct=1 -rw=read --iodepth=128 --iodepth_batch_submit=16 --iodepth_batch_complete_min=16 -ioengine=io_uring -bs=64k -size=8G -numjobs=10
read: IOPS=95.8k, BW=5985MiB/s (6276MB/s)(58.5GiB/10014msec)

fio -filename=/dev/ublkb1 -direct=1 -rw=read --iodepth=128 --iodepth_batch_submit=16 --iodepth_batch_complete_min=16 -ioengine=io_uring -bs=64k -size=8G -numjobs=10
read: IOPS=170k, BW=10.4GiB/s (11.1GB/s)(80.0GiB/7721msec)

So this test result is in line with expectations.

---
Li Zetao