Re: [PATCH] /dev/epoll update ...

[Davide Libenzi]

On 19-Sep-2001 Christopher K. St. John wrote:
> Davide Libenzi wrote:
>> 
>> 1)      select()/poll();
>> 2)      recv()/send();
>> 
>> vs :
>> 
>> 1)      if (recv()/send() == FAIL)
>> 2)              ioctl(EP_POLL);
>> 
>> When there's no data/tx buffer full these will result in 2 syscalls while
>> if data is available/tx buffer ok the first method will result in 2 syscalls
>> while the second will never call the ioctl().
>> It looks very linear to me, with select()/poll() you're asking for a state while
>> with /dev/epoll you're asking for a state change.
>> 
> 
>  Ok, if we're just disagreeing about the best api,
> then I can live with that. But it appears we're
> talking at cross-purposes, so I want to try this one
> more time. I'll lay my though processes out in detail,
> and you can tell me at which step I'm going wrong:
> 
> 
>  Normally, you'd spend most of your time sitting in
> ioctl(EP_POLL) waiting for something to happen. So
> that's one syscall.
> 
>  If you get an event that indicates you can accept()
> a new connection, then you do an accept(). Assume it
> succeeds. That's two syscalls. Then you register
> interest in the fd with a write to /dev/poll, that's
> three.
> 
>  With the current /dev/epoll, you must try to read()
> the new socket before you go back to ioctl(EP_POLL),
> just in case there is data available. You expect
> there isn't, but you have to try. This is the step
> I'm talking about. That's four.
> 
>  Assume data was not available, so you loop back
> to ioctl(EP_POLL) and wait for an event. That's five
> syscalls. The event comes in, you do another read()
> on the socket, and probably get some data. That's
> six syscalls to finally get your data.
> 
>  ioctl(kpfd, EP_POLL) 1     wait for events
>  s = accept()           2     accept a new socket
>  write(kpfd, s)         3     register interest
>  n = read(s)            4 <-- annoying test-read
>  ioctl(kpfd, EP_POLL)   5     wait for events
>  n = read(s)            6     get some data

You continue to put the state check ( ioctl() ) before the system call,
that require you to use select()/poll()//dev/poll interfaces that are
state inquiry interfaces.
The /dev/epoll is, like i said before, a state change notification interface.
That's how have been designed and that how it completely avoid fds scan.
If you're looking for a state inquiry interface it's better for you to seek /dev/poll.

- Davide