M. Warner Losh wrote:
In message: <[EMAIL PROTECTED]>
"M. L. Dodson" <[EMAIL PROTECTED]> writes:
: On a computational chemistry list I subscribe to there is a
: current thread about multi-cpu systems needing to have the cpu
: frequencies synced (this is in a Linux context). This is
: evidently not just having the cpus running at nominally the same
: frequency but something else in addition. A posting in the thread
: said variations less than 0.1% were not problematic. However, the
: poster said it was an issue in a dual cpu, dual core system he had
: set up.
:
: My questions are:
: 1. Is this real or an urban legend?
: 2. If real, is this a Linuxism or is FreeBSD affected as well?
: 3. How do you "sync" the cpus, if it is needed?
: 4. anything else some one wants to expound on along this line.
Linux keeps the cpu's frequencies 'synchronized' so that it can use
the fast time keeping hardware (TSC). FreeBSD uses different
mechanisms for its timekeeping, so doesn't need to keep them in sync
at all, and doesn't even try at this point. Maybe this is what they
are talking about...
Warner
One thing I have noticed with FreeBSD is that if I am running a program
that multithreads and creates and destroys threads a lot (e.g. the fftw3
port), then top underreports significantly - that is on a 4 processor
system it might report 60% (or even 0%) cpu usage, when it is clear from
the TIME field that it is closer to 250% cpu usage.
The other thing I have noticed is that when I split jobs using threads
so that I can use several processors, the speed up to the program is far
less than one might expect - indeed sometimes it even gets slower.
Stephen
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