Re: numbers don't lie ...

Dag-Erling Smørgrav wrote:
Danny Braniss <danny@xxxxxxxxxxxxx> writes:
Im testing these 2 boxes, Sun X4100 and Dell-2950, and:

SUN X4100: Dual Core AMD Opteron(tm) Processor 280 (2393.19-MHz K8-class CPU)
one 70g sata disk
DELL 2950: Intel(R) Xeon(TM) CPU 3.20GHz (3192.98-MHz K8-class CPU)
4 sata disks + raid0

they both run identical 6.1-STABLE.

my 'cpu benchmark' shows the amd being much better than the intel.
but, doing a make buildworld give interesting results:

dell-2950 : make -j16 TARGET_ARCH=amd64 buildworld : 24m17.41s real 1h3m3.26s user 17m15.07s sys
dell-2950 : make -j8 TARGET_ARCH=amd64 buildworld : 24m8.28s real 1h2m59.38s user 16m16.20s sys

sunfire : make -j16 TARGET_ARCH=amd64 buildworld : 24m21.38s real 49m6.68s user 14m22.64s sys
sunfire : make -j8 TARGET_ARCH=amd64 buildworld : 23m47.69s real 48m53.58s user 13m44.81s sys

which probably says something about my 'cpu benchmark' :-(
but why is the user time so much different between the boxes?

I don't see what's so surprising. User time reflects time actually
spent compiling stuff; you can see there that the Opteron is much
faster than the Xeon. Sys time is time spent executing kernel code on
behalf of the build, which is mostly time spent processing I/O
requests (but does not include time spent actually reading from or
writing to disks).

The reason why there is no significant difference in wall time between
the two is that buildworld is mostly bound by I/O and memory
bandwidth, not by CPU power. If you have enough memory, place
/usr/src and /usr/obj on malloc()-backed RAM disks and see if it makes
any difference.

The confusing thing is that I thought 'real' time should be >= 'user' + 'sys'.
But here 'user' is much greater than 'real' for both machines! The sense I
got from the other messages in this thread is that 'user' time is somewhat
meaningless (i.e. unreliable as a measure) in a multi-CPU and/or hyperthreading
environment. Can you clarify?

Thanks,
Gary
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