Date: Wed, 08 Oct 2003 00:19:40 -0700 From: Terry Lambert <tlambert2@mindspring.com> To: Luigi Rizzo <rizzo@icir.org> Cc: "akanwar@digitarchy.com" <akanwar@digitarchy.com> Subject: Re: HZ = 1000 slows down application Message-ID: <3F83BA8C.7149BB52@mindspring.com> References: <410-22003102722174477@M2W077.mail2web.com> <20031007163552.A92652@xorpc.icir.org>
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Luigi Rizzo wrote: > On Tue, Oct 07, 2003 at 06:17:04PM -0400, akanwar@digitarchy.com wrote: > > We did some intensive profiling of our application. It does not seem like > > we are depending on clock ticks for any calculations. > > > > On the other hand we notice that our slow iterations happen almost at the > > same instant as "microuptime went backward" messages in the system log. We > > if this is the case, probably your code at some point computes a > time difference which turns out negative (or if it is unsigned, it > becomes very very large) upon those events, thus causing some loop > to explode. > It should be easy to check if this is the case, and just ignore > those outliers rather than trying to figure out why the clock > goes backward. I used to see the same "microuptime went backwards" > msg on some of my 400MHz boxes, even without NTP enabled. > Maybe a buggy timer, not sure which timecounter was used on that > box (i read some time ago that the cpu on the soekris4801 has a > weird TSC implementation where the upper 32 bits change when the > lower 32 bits are 0xfffffffd, who knows what other bugs might be > in other hardware...) FWIW: Internally, MacOS X supports "monotime", which is a monotonically increasing time counter, guaranteed to not go backwards. That avoids problems exactly like what you are describing. FreeBSD should consider supporting a "monotime". -- Terry
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