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

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

On 3/4/25 12:12, Stefan Metzmacher wrote:
> Hi Pavel,
> 
>>> 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.
>>>
>>> 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
>>> examples/send-zerocopy.c default send size (64KB):
>>>    82558 -> 123855 MB/s
>>
>> Thanks for implementing this, it's great to be able to combine these 2
>> optimizations! Though I suspect many applications will want to perform
>> vectorized I/O using iovecs that come from different registered
>> buffers (e.g. separate header and data allocations). Perhaps a future
>> improvement could allow a list of buffer indices to be specified.
> 
> I'm wondering about the same. And it's not completely
> clear to me what the value of iov_base is in this case,
> is it the offset into the buffer, or the real pointer address
> that must within the range of the registered buffer?

Same as with other registered buffer requests. It's a pointer into
the initial buffer you specified when registering it, which serves
to calculate the offset.

See the io_uring_register(2) man, addr and len are iov_base and
iov_len and there are multiple of them. You can call it confusing,
and I'd agree, but that's how it was done from the very beginning,
so staying consistent here.

https://github.com/axboe/liburing/blob/master/man/io_uring_register.2#L87

  
> It might also be very useful to have some vector elements pointing
> into one of the registered buffer, while others refer to non-registered
> buffers.

See the other reply.

-- 
Pavel Begunkov