Re: [PATCH RFC 5/6] epoll: Add implementation for epoll_mod_wait

[Fam Zheng <famz@redhat.com>]

On Wed, 01/21 11:37, Paolo Bonzini wrote:
> 
> 
> On 21/01/2015 09:58, Fam Zheng wrote:
> >> > See my comment in the earlier mail. If you split this into two 
> >> > APIs, and epoll_ctl_batch() is guaranteed to execute 'cmds' in order, 
> >> > then the return value of epoll_ctl_batch() could be used to tell
> >> > user space how many commands succeeded. Much simpler!
> > Yes it is much simpler. However the reason to add batching in the first place is
> > to make epoll faster, by reducing syscalls. Splitting makes the result
> > sub-optimal: we still need at least 2 calls instead of 1.  Each one of the three
> > proposed new call *is* a step forward, but I don't think we will have everything
> > solved even by implementing them all. Compromise needed between performance or
> > complexity.
> > 
> > My take for simplicity will be leaving epoll_ctl as-is, and my take for
> > performance will be epoll_pwait1. And I don't really like putting my time on
> > epoll_ctl_batch, thinking it as a ambivalent compromise in between.
> 
> I agree with Michael actually.  The big change is going from O(n)
> epoll_ctl calls to O(1), and epoll_ctl_batch achieves that just fine.
> Changing 2 syscalls to 1 is the icing on the cake, but we're talking of
> a fraction of a microsecond.
> 

Maybe I'm missing something, but in common cases, the set of fds for epoll_wait
doesn't change that radically from one iteration to another, does it?

Fam