Re: [PATCH 0/8] Add support for vectored registered buffers

[Pavel Begunkov <asml.silence@gmail.com>]

On 3/3/25 21:03, Andres Freund wrote:
> Hi,
> 
> On 2025-03-03 15:50:55 +0000, Pavel Begunkov wrote:
>> Add registered buffer support for vectored io_uring operations. That
>> allows to pass an iovec, all entries of which must belong to and
>> point into the same registered buffer specified by sqe->buf_index.
> 
> This is very much appreciated!'

Glad to hear. I do remember you mentioning the contention issue
in the list. A bunch of other people who were interested as well.

>> The series covers zerocopy sendmsg and reads / writes. Reads and
>> writes are implemented as new opcodes, while zerocopy sendmsg
>> reuses IORING_RECVSEND_FIXED_BUF for the api.
>>
>> Results are aligned to what one would expect from registered buffers:
>>
>> t/io_uring + nullblk, single segment 16K:
>>    34 -> 46 GiB/s
> 
> FWIW, I'd expect bigger wins with real IO when using 1GB huge pages. I

I didn't even benchmark it meaningfully as we should be able to
extrapolate results from registered buffer test, but I agree, such
contention might make it even more desirable.


> encountered when there were a lot of reads from a large nvme raid into a small
> set of shared huge pages (database buffer pool), by many proceses
> concurrently. The constant pinning/unpinning of the relevant folio caused a
> lot of contention.
> 
> Unfortunately switching to registered buffers would, until now, have required
> using non-vectored IO, which causes significant performance regressions in
> other cases...

-- 
Pavel Begunkov