Messages in this thread | | | Date | Thu, 6 May 1999 17:52:10 -0700 | From | (Jim Gettys) | Subject | Re: Overscheduling DOES happen with high web server load. |
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> From: Andrea Arcangeli <andrea@e-mind.com> > Date: Fri, 7 May 1999 00:06:41 +0200 (CEST) > To: Phillip Ezolt <ezolt@perf.zko.dec.com> > Cc: linux-kernel@vger.rutgers.edu, jg@pa.dec.com, greg.tarsa@digital.com > Subject: Re: Overscheduling DOES happen with high web server load. > ----- > On Thu, 6 May 1999, Phillip Ezolt wrote: > > >On Thu, 6 May 1999, Andrea Arcangeli wrote: > > > >> On Wed, 5 May 1999, Phillip Ezolt wrote: > >> > >> Alt. Alpha uses HZ at 1024 so you get a scheduling rate by default 10 > >> times higher than in all other archs. > > > >This is interesting. Is there any reason why alpha is at 1024? >
I have a theory: (only Linus, Jay Estebrook, or Jim Paradis may know if this theory is good...)
In short: historical (hysterical) reasons... I have memories, when Ultrix was ported to MIPS, and when ports to Alpha were done, that we performed experiments to determine at what point running at a higher HZ rate cost significant overhead; we turned it up until overhead was no more than 1% of the CPU (on the slowest system we had of the new type). This allows for better response times and performance measurements in various parts of the system (e.g. tasking systems relying on select timeouts, etc.), and if there were any performance problems which resulted, they certainly got fixed as Ultrix and/or Digital UNIX were shipped. Given some of the newer performance registers on recent processors, some of the reason for liking higher clock tick rates may have gone away.
So theory is therefore that the HZ rate was taken from what Digital UNIX was using and applied without further thought.
In the days of 1 MIP vaxen, HZ was 60, if I remember: this means that the HZ rate is still lower relative to processor speed than it once was on old UNIX systems, even at HZ == 1024. - Jim
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