Re: [PATCH v2 2/2] epoll: introduce EPOLLEXCLUSIVE and EPOLLROUNDROBIN

Ingo Molnar
2015-02-18T16:33:09Z

* Jason Baron <jbaron@akamai.com> wrote:

> > This has two main advantages: firstly it solves the
> > O(N) (micro-)problem, but it also more evenly
> > distributes events both between task-lists and within
> > epoll groups as tasks as well.
>
> Its solving 2 issues - spurious wakeups, and more even
> loading of threads. The event distribution is more even
> between 'epoll groups' with this patch, however, if
> multiple threads are blocking on a single 'epoll group',
> this patch does not affect the the event distribution
> there. [...]

Regarding your last point, are you sure about that?

If we have say 16 epoll threads registered, and if the list
is static (no register/unregister activity), then the
wakeup pattern is in strict order of the list: threads
closer to the list head will be woken more frequently, in a
wake-once fashion. So if threads do just quick work and go
back to sleep quickly, then typically only the first 2-3
threads will get any runtime in practice - the wakeup
iteration never gets 'deep' into the list.

With the round-robin shuffling of the list, the threads get
shuffled to the tail on wakeup, which distributes events
evenly: all 16 epoll threads will accumulate an even
distribution of runtime, statistically.

Have I misunderstood this somehow?

Thanks,

Ingo