Re: [External] Re: [PATCH] iomap: allow iomap using the per-cpu bio cache

[Fengnan Chang <changfengnan@bytedance.com>]

Sorry, Need to wait a few more days for this, too busy recently.

Ritesh Harjani <ritesh.list@gmail.com> 于2025年8月27日周三 01:26写道：
>
> Fengnan Chang <changfengnan@bytedance.com> writes:
>
> > Christoph Hellwig <hch@infradead.org> 于2025年8月25日周一 17:21写道：
> >>
> >> On Mon, Aug 25, 2025 at 04:51:27PM +0800, Fengnan Chang wrote:
> >> > No restrictions for now, I think we can enable this by default.
> >> > Maybe better solution is modify in bio.c?  Let me do some test first.
>
> If there are other implications to consider, for using per-cpu bio cache
> by default, then maybe we can first get the optimizations for iomap in
> for at least REQ_ALLOC_CACHE users and later work on to see if this
> can be enabled by default for other users too.
> Unless someone else thinks otherwise.
>
> Why I am thinking this is - due to limited per-cpu bio cache if everyone
> uses it for their bio submission, we may not get the best performance
> where needed. So that might require us to come up with a different
> approach.
>
> >>
> >> Any kind of numbers you see where this makes a different, including
> >> the workloads would also be very valuable here.
> > I'm test random direct read performance on  io_uring+ext4, and try
> > compare to io_uring+ raw blkdev,  io_uring+ext4 is quite poor, I'm try to
> > improve this, I found ext4 is quite different with blkdev when run
> > bio_alloc_bioset. It's beacuse blkdev ext4  use percpu bio cache, but ext4
> > path not. So I make this modify.
>
> I am assuming you meant to say - DIO with iouring+raw_blkdev uses
> per-cpu bio cache where as iouring+(ext4/xfs) does not use it.
> Hence you added this patch which will enable the use of it - which
> should also improve the performance of iouring+(ext4/xfs).
>
> That make sense to me.
>
> > My test command is:
> > /fio/t/io_uring -p0 -d128 -b4096 -s1 -c1 -F1 -B1 -R1 -X1 -n1 -P1 -t0
> > /data01/testfile
> > Without this patch:
> > BW is 1950MB
> > with this patch
> > BW is 2001MB.
>
> Ok. That's around 2.6% improvement.. Is that what you were expecting to
> see too? Is that because you were testing with -p0 (non-polled I/O)?
>
> Looking at the numbers here [1] & [2], I was hoping this could give
> maybe around 5-6% improvement ;)
>
> [1]: https://lore.kernel.org/io-uring/cover.1666347703.git.asml.silence@gmail.com/
> [2]: https://lore.kernel.org/all/20220806152004.382170-3-axboe@kernel.dk/
>
>
> -ritesh