Re: [PATCH 10/11] io_uring/rsrc: cache struct io_rsrc_node

Pavel Begunkov
2023-04-04T13:22:38Z

On 4/1/23 01:04, Gabriel Krisman Bertazi wrote:
> Pavel Begunkov <asml.silence@gmail.com> writes:
>
>> On 3/31/23 15:09, Gabriel Krisman Bertazi wrote:
>>> Pavel Begunkov <asml.silence@gmail.com> writes:
>>>
>>>> Add allocation cache for struct io_rsrc_node, it's always allocated and
>>>> put under ->uring_lock, so it doesn't need any extra synchronisation
>>>> around caches.
>>> Hi Pavel,
>>> I'm curious if you considered using kmem_cache instead of the custom
>>> cache for this case? I'm wondering if this provokes visible difference in
>>> performance in your benchmark.
>>
>> I didn't try it, but kmem_cache vs kmalloc, IIRC, doesn't bring us
>> much, definitely doesn't spare from locking, and the overhead
>> definitely wasn't satisfactory for requests before.
>
> There is no locks in the fast path of slub, as far as I know. it has a
> per-cpu cache that is refilled once empty, quite similar to the fastpath
> of this cache. I imagine the performance hit in slub comes from the
> barrier and atomic operations?

Yeah, I mean all kinds of synchronisation. And I don't think
that's the main offender here, the test is single threaded without
contention and the system was mostly idle.

> kmem_cache works fine for most hot paths of the kernel. I think this

It doesn't for io_uring. There are caches for the net side and now
in the block layer as well. I wouldn't say it necessarily halves
performance but definitely takes a share of CPU.

> custom cache makes sense for the request cache, where objects are
> allocated at an incredibly high rate. but is this level of update
> frequency a valid use case here?

I can think of some. For example it was of interest before to
install a file for just 2-3 IO operations and also fully bypassing
the normal file table. I rather don't see why we wouldn't use it.

> If it is indeed a significant performance improvement, I guess it is
> fine to have another user of the cache. But I'd be curious to know how
> much of the performance improvement you mentioned in the cover letter is
> due to this patch!

It was definitely sticking out in profiles, 5-10% of cycles, maybe more

--
Pavel Begunkov