Re: [RFC 0/4] futex request support

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

On 6/3/21 7:59 PM, Andres Freund wrote:
> Hi,
> 
> On 2021-06-01 15:58:25 +0100, Pavel Begunkov wrote:
>> Should be interesting for a bunch of people, so we should first
>> outline API and capabilities it should give. As I almost never
>> had to deal with futexes myself, would especially love to hear
>> use case, what might be lacking and other blind spots.
> 
> I did chat with Jens about how useful futex support would be in io_uring, so I
> should outline our / my needs. I'm off work this week though, so I don't think
> I'll have much time to experiment.
> 
> For postgres's AIO support (which I am working on) there are two, largely
> independent, use-cases for desiring futex support in io_uring.
> 
> The first is the ability to wait for locks (queued r/w locks, blocking
> implemented via futexes) and IO at the same time, within one task. Quickly and
> efficiently processing IO completions can improve whole-system latency and
> throughput substantially in some cases (journalling, indexes and other
> high-contention areas - which often have a low queue depth). This is true
> *especially* when there also is lock contention, which tends to make efficient
> IO scheduling harder.

Can you give a quick pointer to futex uses in the postgres code or
where they are? Can't find in master but want to see what types of
futex operations are used and how.

> The second use case is the ability to efficiently wait in several tasks for
> one IO to be processed. The prototypical example here is group commit/journal
> flush, where each task can only continue once the journal flush has
> completed. Typically one of waiters has to do a small amount of work with the
> completion (updating a few shared memory variables) before the other waiters
> can be released. It is hard to implement this efficiently and race-free with
> io_uring right now without adding locking around *waiting* on the completion
> side (instead of just consumption of completions). One cannot just wait on the
> io_uring, because of a) the obvious race that another process could reap all
> completions between check and wait b) there is no good way to wake up other
> waiters once the userspace portion of IO completion is through.

IIRC, the idea is to have a link "I/O -> fut_wake(master_task or nr=1)",
and then after getting some work done the woken task does wake(nr=all),
presumably via sys_futex or io_uring. Is that right?

As with this option userspace can't modify the memory on which futex
sits, the wake in the patchset allows to do an atomic add similarly
to FUTEX_WAKE_OP. However, I still have general concerns if that's
a flexible enough way.

When io_uring-BPF is added it can be offloaded to BPF programs
probably together with "updating a few shared memory variables",
but these are just thoughts for the future.

> All answers for postgres:
> 
>> 1) Do we need PI?
> 
> Not right now.
> 
> Not related to io_uring: I do wish there were a lower overhead (and lower
> guarantees) version of PI futexes. Not for correctness reasons, but
> performance. Granting the waiter's timeslice to the lock holder would improve
> common contention scenarios with more runnable tasks than cores.
> 
> 
>> 2) Do we need requeue? Anything else?
> 
> I can see requeue being useful, but I haven't thought it through fully.
> 
> Do the wake/wait ops as you have them right now support bitsets?

No, but trivial to add

>> 3) How hot waits are? May be done fully async avoiding io-wq, but
>> apparently requires more changes in futex code.
> 
> The waits can be quite hot, most prominently on low latency storage, but not
> just.

Thanks Andres, that clears it up. The next step would be to verify
that FUTEX_WAKE_OP-style waking is enough.

-- 
Pavel Begunkov