linux-kernel - Re: user-space concurrent pipe buffer scheduler interactions

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Date: Thu, 4 Apr 2024 15:59:36 +1300
From: Michael Clark <michael@...aparadigm.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Jens Axboe <axboe@...nel.dk>, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org
Subject: Re: user-space concurrent pipe buffer scheduler interactions

On 4/4/24 09:57, Linus Torvalds wrote:
> On Wed, 3 Apr 2024 at 13:52, Michael Clark <michael@...aparadigm.com> wrote:
>>
>> On 4/4/24 05:56, Linus Torvalds wrote:
>>> On Tue, 2 Apr 2024 at 13:54, Michael Clark <michael@...aparadigm.com> wrote:
>>>>
>>>> I am working on a low latency cross-platform concurrent pipe buffer
>>>> using C11 threads and atomics.
>>>
>>> You will never get good performance doing spinlocks in user space
>>> unless you actually tell the scheduler about the spinlocks, and have
>>> some way to actually sleep on contention.
>>>
>>> Which I don't see you as having.
>>
>> We can work on this.
> 
> It's been tried.
> 
> Nobody ever found a use-case that is sufficiently convincing, but see
> the write-up at
> 
>     https://lwn.net/Articles/944895/

It's certainly interesting!

I wouldn't throw in the towel so soon until folks tried a bit harder. 
Even if we can't make it faster it would be neat to have an Austin Group 
meeting to give us user-space devels a waitlist_t so we don't hit that 
brain damaged deadlocky cond_wait foot-gun and realize its totally 
broken and that we must use cond_timedwait with some stupid delay until 
we can recheck the condition again in user-space in our "portable code".

Michael.