Date: Thu, 14 Sep 2006 13:28:03 -0400 From: Gary Corcoran <gcorcoran@rcn.com> To: =?ISO-8859-1?Q?Dag-Erling_Sm=F8rgrav?= <des@des.no> Cc: freebsd-hackers@freebsd.org Subject: Re: numbers don't lie ... Message-ID: <45099123.4000500@rcn.com> In-Reply-To: <863bauk3gp.fsf@dwp.des.no> References: <E1GNOLq-000DC2-1Q@cs1.cs.huji.ac.il> <863bauk3gp.fsf@dwp.des.no>
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Dag-Erling Smørgrav wrote: > Danny Braniss <danny@cs.huji.ac.il> 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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