kern/17842: Erratic user time reports for long running processes
bde at zeta.org.au
Wed Feb 2 02:00:44 PST 2005
The following reply was made to PR kern/17842; it has been noted by GNATS.
From: Bruce Evans <bde at zeta.org.au>
To: Kees Hendrikse <kees at echelon.nl>
Cc: freebsd-gnats-submit at FreeBSD.ORG
Subject: Re: kern/17842: Erratic user time reports for long running processes
Date: Wed, 2 Feb 2005 20:56:50 +1100 (EST)
On Wed, 26 Apr 2000, Bruce Evans meant to Cc this to freebsd-gnats-submit
but only Cced it to freebsd-bugs:
[The submitter of the PR wrote:]
> > What baffles me most: why stop counting at 379265,687860 seconds?
> The multiplication in "uu = (tu * ut) / tt;" in kern_resource.c
> overflows near there. Here tu is the total time in usec, ut is the
> user tick count and tt is the total tick count. If all ticks are user
> ticks, then overflow occurs for tu = sqrt(2^64 / 10^6 / hz) = 429496
> seconds when hz = 100. Normally there are some interrupt and system
> ticks, so overflow occurs for a somewhat larger tu and a somewhat
> smaller ut -- apparently about 379 / 429 times smaller in your case.
> 4.0 enforces monotonicity of the resource times, so truncated values
> resulting from the overflow don't cause the resource times to stick
^^^^^ oops, delete "don't" here
> at the first value where overflow occurs.
The factor of 379 / 429 was because I got hz and stathz mixed up.
sqrt(2^64 / 10^6 / 128) is 379265.06...
See PR 76972 for more details, except about the stickyness of the value.
The value might not stick at the first overflowed value -- overflow just
gives garbage values, and monotonicity limits the jitter caused by the
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