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SubjectRe: Overscheduling DOES happen with high web server load.

> From: davem@redhat.com
> Date: Fri, 7 May 1999 04:53:49 -0700
> To: jg@pa.dec.com
> Cc: andrea@e-mind.com, ezolt@perf.zko.dec.com, linux-kernel@vger.rutgers.edu,
> jg@pa.dec.com, greg.tarsa@digital.com
> Subject: Re: Overscheduling DOES happen with high web server load.
> -----
> Date: Thu, 6 May 1999 17:52:10 -0700
> From: jg@pa.dec.com (Jim Gettys)
>
> So theory is therefore that the HZ rate was taken from what Digital
> UNIX was using and applied without further thought.
>
> One further theory, from the networking perspective, is that a higher
> resolution HZ allows you to use more accurate timestamping in TCP and
> thus get more accurate round trip estimates. I don't know if the
> Digital people had specifically thought about this at the time.

I don't specifically remember discussions of TCP RTT estimates, but getting
better performance data everywhere, including network code, and reducing
latency in scheduler dependent wakeups of processes (better "interactive
feel"), as I remember the discussions. Some of the need for this is less
now that we have better performance tools and performance counters. I
was at least periperally involved early on in both MIPS and Alpha ports,
where the topic came up.

100hz == 10 ms == about 1/3 - 1/6 of human perception or your latency
budget for actions to be percieved as truly "instantaneous"; you
never can get latency back.

Given Jay's recent mail on the topic, I think things jibe with my memory:
the console is set that high in large part since we run that fast on other
operating systems; this got set as part of the initial experiments when
porting to new architectures; the cost of running at a high basic
rate is low enough that this was a rational design decision.
- Jim

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