RE: Linux SMP performance vs. UP: not a laurel

From: Mario Vanoni (vanonim@dial.eunet.ch)
Date: Fri Jul 21 2000 - 16:45:56 EST


WU's are, with (few) exceptions I agree, very constant!
          -----------------------------

UP PII450 512MB RAM, last 40 WU's between 7:32 and 8:11.
And this is a production machine with max 13 users.

UP PIII550 512MB RAM, last 40 WU's between 6:48 and 6:55.
No load machine.

SMP 2xPIII550 1024MB RAM, times between 9:15 and 10:45,
depending on kernel and/or load before. Badest are 2.4.0test!

Rebooting the SMP, starting seti gives the longest times.

Rebooting the SMP, e.g. memtest (Doug Ledford) before
starting any seti, then start seti, times go <=8:45.
But not always reproducible, I ignore why.
This on kernel 2.2.17pre6aa2, same as both UP's.

Other loads do the same effect, it seems when the used
memory is at the upper level _AND_ there was swap (>1000K),
the performance of the seti's is _MUCH_ _MUCH_ better.

My questions:
=============

a) Why an SMP takes >2h more time than an UP with same CPU/RAM?

b) Why starting an SMP the times are so dependent of the
   hypothetical load before running the setis?

c) Why after a _NOT_ _OBVIOUS_, not planned load,
   the next 24h seti are faster, SMP running 24/7.
   Then with another load, e.g. mmap002, they slow down?

I register the values of cputime=??? in state.sah when
a WU is finished, via crontab each minute, on all machines.

Regards
Mario

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