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

[Ritesh Harjani (IBM) <ritesh.list@gmail.com>]

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