Re: X15 alpha release: as fast as TUX but in user space

[Lincoln Dale]

Hi,

At 10:50 AM 2/05/2001 +0200, Ingo Molnar wrote:
>i think Zach's phhttpd is an important milestone as well, it's the first
>userspace webserver that shows how to use event-based, sigio-based async
>networking IO and sendfile() under Linux. (I believe it had some
>performance problems related to sigio queue overflow, these issues might
>be solved in the latest kernels.) The zerocopy enhancements should help
>phhttpd as well.

my experience with sigio-based event-handlers is that the net-gain of 
event-driven i/o is mitigated by the fact that SIGIO is based on signals.

the problem with signals for this purpose are:
  (a) you go thru a syncronization point in the kernel.  signals are protected
      by a spinlock.
      it doesn't scale with SMP.
  (b) SI_PAD_SIZE

explicitly, (b) means that you have an awful lot of memory-accesses going 
on for every signal.
my experience with the overhead is that it mitigates the advantages when 
you become bottlenecked on memory-bus-accesses.

cheers,

lincoln.