Re: [Libevent-users] Threading concerns

[Nick Mathewson <nickm@xxxxxxxxxxxxx>]

On Sun, Nov 29, 2009 at 11:18:19PM -0500, Donghua Xu wrote:
> Hi,
> 
> Sorry if this is a stupid question. But I have not found a clear
> answer in the documentations. I am just beginning to look into using
> libevent for our project. We will need to schedule millions of timed
> events, and thousands of them may be triggered at the same time. So we
> have to use thread pool to handle all these events. Let's say we have
> 10 threads running at the same time, so how do I know which thread
> will the call-back function be running when an event is triggered? Is
> it always the thread that calls event_base_set() for the event? Or
> could it be some other thread?

When you schedule an event, it will be run in the thread that is
running the loop for that event base.  So if you say

        event_assign(event2, base99, ...);

And later, you say

        event_base_dispatch(base99);

Then the thread that called event_base_dispatch() for base99 will be
the one to run event2's callback.

Note that Libevent 1.4.x does not provide thread-safety for access to
an event_base from another thread: if you want to be able to add or
remove an event from an event_base that another thread is using,
please check out the 2.0.x series.

Note also that if you have millions of timer events scheduled (wow!),
Libevent's default min-heap timer implementation might not be as
fast as you want.  The "common timeout" logic added in 2.0.3-alpha might
be a viable choice for this kind of scenario, if it applies to your
actual use-case.

Right now Libevent has no built-in thread-pool support.  Generally
speaking, if you want a thread-pool style structure with today's
Libevent, I think you have two basic options:
    - Run an event_base loop in each thread, and try to assign events
      to bases fairly
and/or
    - Have the callback from each work-intensive event assign the
      work to a worker thread rather than running it immediately.
though other people can probably add more info about what has worked
for them.

(I'd like to get better thread pool support in a future version, but
as things stand, it is probably too big a feature for the 2.0.x
series.  Maybe 2.1.x, if somebody wants to write it.)

yrs,
-- 
Nick





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