Re: [RFC] sched: implement the exclusive wait queue as a LIFO queue

[Changli Gao <xiaosuo@gmail.com>]

On Wed, Apr 28, 2010 at 9:21 PM, Jamie Lokier <jamie@shareable.org> wrote:
> Changli Gao wrote:
>>
>> fs/eventpoll.c: 1443.
>>                 wait.flags |= WQ_FLAG_EXCLUSIVE;
>>                 __add_wait_queue(&ep->wq, &wait);
>
> The same thing about assumptions applies here.  The userspace process
> may be waiting for an epoll condition to get access to a resource,
> rather than being a worker thread interchangeable with others.

Oh, the lines above are the current ones. So the assumptions applies
and works here.

>
> For example, userspace might be using a pipe as a signal-safe lock, or
> signal-safe multi-token semaphore, and epoll to wait for that pipe.
>
> WQ_FLAG_EXCLUSIVE means there is no point waking all tasks, to avoid a
> pointless thundering herd.  It doesn't mean unfairness is ok.

The users should not make any assumption about the waking up sequence,
neither LIFO nor FIFO.

>
> The LIFO idea _might_ make sense for interchangeable worker-thread
> situations - including userspace.  It would make sense for pipe
> waiters, socket waiters (especially accept), etc.

Yea, and my following patches are for socket waiters.

>
> Do you have any measurements which showing the LIFO mode performing
> better than FIFO, and by how much?
>

I didn't do any test yet. But some work done by LSE project years ago
showed that it is better.

http://lse.sourceforge.net/io/aionotes.txt

" Also in view of
better cache utilization the wake queue mechanism is LIFO by default.
(A new exclusive LIFO wakeup option has been introduced for this purpose)"

-- 
Regards，
Changli Gao(xiaosuo@gmail.com)