epoll improvements

[Greg KH]

On Thu, Oct 16, 2014 at 02:09:05PM +0200, Nev Ikte wrote:
> Hi,
> 
> I've a server using epoll and I've noticed that when
> the traffic is low, the epoll_wait() latency goes up.
> 
> I've tried to reproduce it with a single client
> and basically, if ep_poll() is able to find an event or the timeout is 0,
> the latency is down to 5usec, otherwise if it enters the waitqueue
> the latency goes up to 10-25usec, which impact the application performance.
> 
> Looking at the code there is something like a todo ("Is it worth to try to dig for events ?")
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/fs/eventpoll.c?id=refs/tags/v3.17#n1650
> 
> Anyone knows how the situation can be improved?
> in theory having something like a 50usec spin loop 
> checking for events before entering in the waitqueue should improve the situation.
> 
> suggestion? patches?

The epoll code is subtle, tricky, and very very difficult to modify.  I
suggest trying your suggestion and seeing if it helps any.  But be
prepared to go down a long rabbit-hole in this area of the kernel.

Enjoy the trip!  :)

greg k-h