Dual-core & Parallel Compile Issues 2005-12-07 - By Peter Ruprecht
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>
> I'm grasping at straws here hoping someone else has run into issues
> running dual-core Opterons (Sun V40z specifically). When we run a large
> parallel compile with various j values (i.e. -j8, -j16, -j24, -j32,
> -j40), we are able to compile the end product in much less time on a
> quad single-core V40z (quad Opteron 852 / 2.6GHz) than on a quad
> dual-core V40z (quad Opteron 875 / 2.2GHz). For example, with -j20, 7.4
> mins vs. 9.6 mins without ClearCase and 13.7 mins. vs. 18.6 mins with
> ClearCase). We have tried on the U5 and U6 kernel with zero improvement
> seen between the two.
>
> When compiling with large -j values on the dual-core system, the CPUs
> stay 80% idle the majority of the time (as measured by sar on 10-sec
> interval). On the single-core system, the CPU is much more utilized
> throughout the build. I can provide graphs if anyone is interested. We
> have not yet tried to duplicate the issue on RHEL 4 U2, but plan to try
> in the next few days.
>
> I would appreciate any pointers on how we might further investigate what
> is causing the bottleneck. Are there known issues with the RHEL 3
> kernel scheduler or anything else I should be aware of? Thanks.
Brian,
We had somewhat similar problems with a quad dual-core opteron system
and the initial release of RHEL 4. Four simultaneous threads or
processes worked well; more than that would not. It was like the
2nd cores weren't well utilized. The problem seems to have gone away,
or at least is much diminished, with the latest RHEL 4 update.
Unfortunately I don't have detailed benchmarks, but the symptoms we
had originally sound similar to yours.
-Peter Ruprecht
JILA / U. of Colorado
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