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

[Paolo Bonzini]

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.

Paolo
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