Re: [PATCH 1/3] io_uring: split req init from submit

[Jens Axboe <axboe@kernel.dk>]

On 7/31/23 6:53?AM, Pavel Begunkov wrote:
> On 7/28/23 21:14, Keith Busch wrote:
>> From: Keith Busch <kbusch@kernel.org>
>>
>> Split the req initialization and link handling from the submit. This
>> simplifies the submit path since everything that can fail is separate
>> from it, and makes it easier to create batched submissions later.
> 
> Keith, I don't think this prep patch does us any good, I'd rather
> shove the link assembling code further out of the common path. I like
> the first version more (see [1]). I'd suggest to merge it, and do
> cleaning up after.
> 
> I'll also say that IMHO the overhead is well justified. It's not only
> about having multiple nvmes, the problem slows down cases mixing storage
> with net and the rest of IO in a single ring.
> 
> [1] https://lore.kernel.org/io-uring/20230504162427.1099469-1-kbusch@meta.com/

The downside of that one, to me, is that it just serializes all of it
and we end up looping over the submission list twice. With alloc+init
split, at least we get some locality wins by grouping the setup side of
the requests.

-- 
Jens Axboe