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From: Pavel Begunkov <[email protected]>
To: Gabriel Krisman Bertazi <[email protected]>
Cc: [email protected], Jens Axboe <[email protected]>,
	[email protected]
Subject: Re: [PATCH 10/11] io_uring/rsrc: cache struct io_rsrc_node
Date: Tue, 4 Apr 2023 14:21:41 +0100	[thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>

On 4/1/23 01:04, Gabriel Krisman Bertazi wrote:
> Pavel Begunkov <[email protected]> writes:
> 
>> On 3/31/23 15:09, Gabriel Krisman Bertazi wrote:
>>> Pavel Begunkov <[email protected]> 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

  reply	other threads:[~2023-04-04 13:22 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-03-30 14:53 [RFC 00/11] optimise registered buffer/file updates Pavel Begunkov
2023-03-30 14:53 ` [PATCH 01/11] io_uring/rsrc: use non-pcpu refcounts for nodes Pavel Begunkov
2023-03-30 14:53 ` [PATCH 02/11] io_uring/rsrc: keep cached refs per node Pavel Begunkov
2023-03-30 14:53 ` [PATCH 03/11] io_uring: don't put nodes under spinlocks Pavel Begunkov
2023-03-30 14:53 ` [PATCH 04/11] io_uring: io_free_req() via tw Pavel Begunkov
2023-03-30 14:53 ` [PATCH 05/11] io_uring/rsrc: protect node refs with uring_lock Pavel Begunkov
2023-03-30 14:53 ` [PATCH 06/11] io_uring/rsrc: kill rsrc_ref_lock Pavel Begunkov
2023-03-30 14:53 ` [PATCH 07/11] io_uring/rsrc: rename rsrc_list Pavel Begunkov
2023-03-30 14:53 ` [PATCH 08/11] io_uring/rsrc: optimise io_rsrc_put allocation Pavel Begunkov
2023-03-30 14:53 ` [PATCH 09/11] io_uring/rsrc: don't offload node free Pavel Begunkov
2023-03-30 14:53 ` [PATCH 10/11] io_uring/rsrc: cache struct io_rsrc_node Pavel Begunkov
2023-03-31 14:09   ` Gabriel Krisman Bertazi
2023-03-31 16:27     ` Pavel Begunkov
2023-04-01  0:04       ` Gabriel Krisman Bertazi
2023-04-04 13:21         ` Pavel Begunkov [this message]
2023-04-04 15:48           ` Gabriel Krisman Bertazi
2023-04-04 15:52             ` Jens Axboe
2023-04-04 16:53               ` Gabriel Krisman Bertazi
2023-04-04 18:26                 ` Pavel Begunkov
2023-03-30 14:53 ` [PATCH 11/11] io_uring/rsrc: add lockdep sanity checks Pavel Begunkov
2023-03-31 13:35 ` [RFC 00/11] optimise registered buffer/file updates Gabriel Krisman Bertazi
2023-03-31 16:21   ` Pavel Begunkov
2023-03-31 15:18 ` Jens Axboe

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