Re: [PATCH RFC 0/9] nxt propagation + locking optimisation

[Jens Axboe <axboe@kernel.dk>]

On 3/1/20 9:18 AM, Pavel Begunkov wrote:
> There are several independent parts in the patchset, but bundled
> to make a point.
> 1-2: random stuff, that implicitly used later.
> 3-5: restore @nxt propagation
> 6-8: optimise locking in io_worker_handle_work()
> 9: optimise io_uring refcounting
> 
> The next propagation bits are done similarly as it was before, but
> - nxt stealing is now at top-level, but not hidden in handlers
> - ensure there is no with REQ_F_DONT_STEAL_NEXT
> 
> [6-8] is the reason to dismiss the previous @nxt propagation appoach,
> I didn't found a good way to do the same. Even though it looked
> clearer and without new flag.
> 
> Performance tested it with link-of-nops + IOSQE_ASYNC:
> 
> link size: 100
> orig:  501 (ns per nop)
> 0-8:   446
> 0-9:   416
> 
> link size: 10
> orig:  826
> 0-8:   776
> 0-9:   756

This looks nice, I'll take a closer look tomorrow or later today. Seems
that at least patch 2 should go into 5.6 however, so may make sense to
order the series like that.

BTW, Andres brought up a good point, and that's hashed file write works.
Generally they complete super fast (just copying into the page cache),
which means that that worker will be hammering the wq lock a lot. Since
work N+1 can't make any progress before N completes (since that's how
hashed work works), we should pull a bigger batch of these work items
instead of just one at the time. I think that'd potentially make a huge
difference for the performance of buffered writes.

Just throwing it out there, since you're working in that space anyway
and the rewards will be much larger.

-- 
Jens Axboe