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Subjectsmp cputime issues
hello,

we are encountering some weird timing behaviour on our linux cluster.

specifically: when running 2 copies of selected programs on a
dual-cpu system, the cputime reported for each process is up to 25%
higher than when the processes are run on their own. however, if running
two different jobs on the same machine, both complete with a cputime
equal to when run individually. sample timing output attached.

profiling confirms that everything slows down approximately to scale.
the results reproduce on a range of different machines (see below).

additional specifications:
- kernel version 2.4.16 (with apic enabled)
- chipsets: apollo pro 133, apollo pro 266,
intel i860, serverworks LE
- all jobs requires less than 1/10 of physical memory
- no significant disk i/o takes place
- timing with dtime(), /usr/bin/time and shell built-in time
- this behavior is NOT seen for all applications. the worst
"offender" spends most of its time doing linear algebra.

ideas or info-pointers appreciated. more specs available on request.

regards,
--
Steinar Hauan, dept of ChemE -- hauan@cmu.edu
Carnegie Mellon University, Pittsburgh PA, USA
output from running a single image copy

[reported by dtime()]

CPU seconds spent in IPOPT and function evaluations = 131.9999982

[reported by /usr/bin/time -v ]

Command being timed: "./ipopt robot_2000.nl"
User time (seconds): 134.01
System time (seconds): 0.36
Percent of CPU this job got: 99%
Elapsed (wall clock) time (h:mm:ss or m:ss): 2:14.42
Average shared text size (kbytes): 0
Average unshared data size (kbytes): 0
Average stack size (kbytes): 0
Average total size (kbytes): 0
Maximum resident set size (kbytes): 0
Average resident set size (kbytes): 0
Major (requiring I/O) page faults: 293
Minor (reclaiming a frame) page faults: 23352
Voluntary context switches: 0
Involuntary context switches: 0
Swaps: 0
File system inputs: 0
File system outputs: 0
Socket messages sent: 0
Socket messages received: 0
Signals delivered: 0
Page size (bytes): 4096
Exit status: 0
output from running two images simultaneously

[reported by dtime()]

CPU seconds spent in IPOPT and function evaluations = 157.7000024

[reported by /usr/bin/time -v ]
Command being timed: "./ipopt robot_2000.nl"
User time (seconds): 159.81
System time (seconds): 0.50
Percent of CPU this job got: 99%
Elapsed (wall clock) time (h:mm:ss or m:ss): 2:40.41
Average shared text size (kbytes): 0
Average unshared data size (kbytes): 0
Average stack size (kbytes): 0
Average total size (kbytes): 0
Maximum resident set size (kbytes): 0
Average resident set size (kbytes): 0
Major (requiring I/O) page faults: 293
Minor (reclaiming a frame) page faults: 23352
Voluntary context switches: 0
Involuntary context switches: 0
Swaps: 0
File system inputs: 0
File system outputs: 0
Socket messages sent: 0
Socket messages received: 0
Signals delivered: 0
Page size (bytes): 4096
Exit status: 0
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