On Wed, 30 Oct 2002, Jamie Lokier wrote:
> > 1) "issuing a command to an IDE disk" == "using read/write until EAGAIN"
> > 2) "adding yourself on the IDE disk wait queue" == "calling sys_epoll_wait()"
>
> That is quite a good analogy. epoll is like a waitqueue - which is
> also like a futex. To use a waitqueue properly you have do these
> things in the order shown:
>
> 1. Set the task state to stopped.
> 2. Register yourself on the waitqueue.
> 3. Check the condition.
> 4. If condition is not met, schedule.
>
> With epoll it is very similar. To wait for a condition on a file
> descriptor, such as readability, you must do these things in the order
> shown:
>
> 1. Register your interest using epoll_ctl.
> 2. Check the condition by actually calling read().
> 3. If the condition is not met (i.e. read() returned EAGAIN),
> call epoll_wait (i.e. equivalent to schedule).
>
> With epoll, you can optimise by registering interest just once. In
> other words, steps 2 and 3 may be repeated without repeating step 1.
>
> And if you are concerned about starvation -- that is, one of your file
> descriptors always has new data so others don't get a chance to be
> serviced -- don't be. You don't have to completely read one fd until
> you see EGAIN. All that matters is that until you see the EAGAIN,
> your user space data structure should have a flag that says the fd is
> still readable, so another epoll event is not expected or required for
> that fd.
I would say more about this. I did not understand the interface until I
read this one :-)
Hanna, is it possible for you to make Jamie description to fit the epoll.2
man page ?
If you can do that I'll send you the source of the man page ...
- Davide
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This archive was generated by hypermail 2b29 : Thu Oct 31 2002 - 22:00:46 EST