context switch vs. signal delivery [was: Re: Accelerating user mode

[Ingo Molnar]

On 2 Aug 2002, Alan Cox wrote:

> The numbers look very different on a real processor. Signal delivery is
> indeed not stunningly fast but relative to a context switch its very low
> indeed.

actually the opposite is true, on a 2.2 GHz P4:

  $ ./lat_sig catch
  Signal handler overhead: 3.091 microseconds

  $ ./lat_ctx -s 0 2
  2 0.90

ie. *process to process* context switches are 3.4 times faster than signal
delivery. Ie. we can switch to a helper thread and back, and still be
faster than a *single* signal.

signals are in essence 'lightweight' threads created and destroyed for the
purpose of a single asynchronous event, it's IMO a very inefficient and
baroque concept for almost anything (but debugging and a number of very
special uses). I'd guess that with a sane threading library a helper
thread is faster for almost everything.

	Ingo