From owner-freebsd-smp  Tue Feb 26 20:30:29 2002
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Date: Tue, 26 Feb 2002 20:29:46 -0800
From: Terry Lambert <tlambert2@mindspring.com>
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To: Bob Van Valzah <Bob@Talarian.Com>
Cc: Robert Watson <rwatson@FreeBSD.ORG>,
	Jorge Aldana <jorge@salk.edu>, Garance A Drosihn <drosih@rpi.edu>,
	smp@FreeBSD.ORG
Subject: Re: Performance vs. Stable
References: <Pine.NEB.3.96L.1020226224905.38595O-100000@fledge.watson.org> <1014782526.15635.22.camel@Relent.Bob.WhiteBarn.Com>
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Bob Van Valzah wrote:
> On Tue, 2002-02-26 at 21:50, Robert Watson wrote:
> > What is the 'null system call'?
> 
> It looks like Larry was a bit fast and loose with language in writing
> the summary report (maybe to make it fit in the header). It appears to
> be the time needed to write one byte to /dev/null. I don't see why
> that's any more "null" than say, getpid().

It makes you cross the user/kernel boundary.

Smart systems implement things that never change, like
getpid, as:


	static struct _saved {
		pid_t	s_pid;
	} cached_state = {
		-1
	};

	pid_t
	getpid(void)
	{
		pid_t	rv = cached_state.s_pid;

		if( s_pid == -1)
			rv = system_getpid();
		return rv;
	}

	...

	fork()
	...
		cached_state.s_pid = -1;	/* reset cache on fork */

Thus such calls have zero system call overhead.

Similar shortcuts can be had for other read-values, such as
getgid, getgroups, etc., etc. (hacks required in the set
call wrapper for fork, etc., obviously).

Using a write of /dev/null is an attempt to work around this;
of course, you could special case that, as well, in user space,
but it'd be more work than Larry thinks most people will go to
to cheat on the benchmark (Hi Larry!  8-)).

-- Terry

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