Date: Wed, 4 Oct 2000 18:46:00 -0700 From: Alfred Perlstein <bright@wintelcom.net> To: Bruce Evans <bde@zeta.org.au> Cc: Matt Dillon <dillon@earth.backplane.com>, Peter Wemm <peter@netplex.com.au>, Chuck Paterson <cp@bsdi.com>, John Baldwin <jhb@FreeBSD.ORG>, arch@FreeBSD.ORG, John Polstra <jdp@polstra.com>, Daniel Eischen <eischen@vigrid.com>, Greg Lehey <grog@lemis.com> Subject: Re: Mutexes and semaphores Message-ID: <20001004184559.Q27736@fw.wintelcom.net> In-Reply-To: <Pine.BSF.4.21.0010051216130.12069-100000@besplex.bde.org>; from bde@zeta.org.au on Thu, Oct 05, 2000 at 12:40:16PM %2B1100 References: <200010041639.e94GdVR24384@earth.backplane.com> <Pine.BSF.4.21.0010051216130.12069-100000@besplex.bde.org>
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* Bruce Evans <bde@zeta.org.au> [001004 18:40] wrote:
> On Wed, 4 Oct 2000, Matt Dillon wrote:
>
> [Writing reordered to reply after quoted material]
>
> [Peter Wemm wrote]
>
> > :int
> > :getppid(p, uap)
> > : struct proc *p;
> > : struct getppid_args *uap;
> > :{
> > : struct proc *parent;
> > : pid_t pid;
> > :
> > : parent = p->p_pptr;
> > : pid = parent->p_pid;
> > :#ifdef SMP
> > : for (;;) {
> > : __asm __volatile (": : : memory"); /* mb(); on x86 */
> > : if (parent == p->p_pptr)
> > : break;
> > : /* lost a race, our parent died and we reparented - redo */
> > : parent = p->p_pptr;
> > : pid = parent->p_pid;
> > : }
> > :#endif
> > : p->p_retval[0] = (register_t)pid;
> > : return 0;
> > :}
>
> > This is one of the few things I really hate in the linux source base.
>
> This seems to be only to win getppid() benchmarks. Complications like it
> might be justified in inner loops of syscalls that are called somewhat
> more than once at most in normal programs. It's too hard to do things
> this in the thousands of places that would be necessary to get a uniform
> speedup.
the idea is to reduce inter-cpu communication, lock-less systems are a
terrific idea but hard to get right, we have two choices:
1) do it right and _not_ cause extra lock/cpu/bus contention
2) do it right and cause extra lock/cpu/bus contention
It's my opinion that something simple like getpid shouldn't cause a
locked bus cycle if at all possible.
--
-Alfred Perlstein - [bright@wintelcom.net|alfred@freebsd.org]
"I have the heart of a child; I keep it in a jar on my desk."
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