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

[Davide Libenzi]

On 24-Sep-2001 Jamie Lokier wrote:
> Davide Libenzi wrote:
>> > Well, memory move consists of 2 words: (a) file descriptor; (b) poll
>> > state/edge flags.
>> 
>> 2-words * number-of-ready-fds == pretty-high-cache-drain
> 
> Perhaps there is a cache issue, but note it is the number of _new_ ready
> fds (since the last sample), not the number currently ready.
> 
>> > That will be completely swamped by the system calls and so on needed to
>> > processes each of the file descriptors.  I.e. no scalability problem here.
>> 
>> The other issue is that by keeping infos in file* you'll have to scan each fd
>> to report the ready ones, that will make the method to fall back in O(n).
> 
> No, that would be silly.  You would queue signals exactly as they are
> queued now (but collapsing multiple signals per fd into one).
> 
>> Anyway there's a pretty good patch ( http://www.luban.org/GPL/gpl.html ),
>> that has been tested here :
>> 
>> http://www.xmailserver.org/linux-patches/nio-improve.html
>> 
>> that implement the signal-per-fd mechanism and it achieves a very good
>> scalability too.
> 
> It has the bonus of requiring no userspace changes too.  Lovely!

Sure you can avoid the scan, if you pick up one event at a time.
To be compared to /dev/epoll you need the signal-per-fd patch plus a method to
collect the whole event-set in a single system call ( see perfs ).

- Davide