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

lizetao
2025-02-13T15:12:50Z

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.
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.

---
Li Zetao