[H-GEN] Hyperthreading and CPU utilisation

[Michael Anthon]

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I am in the process of setting up a new machine.  It is a dual 2.8G Xeon
and I'm a bit confused about some aspects of HT.  The bit that is
confusing me the most is the reporting of CPU utilisation.  For example,
if there is only one active process on the system that is able to
actully use 100% of one processor then top reports this processes
utilisation as 100% and reports the overall utilisation as 25%.  I kinda
understand this but I'm a bit uncertain if this is reporting the true
state of things.  Here is a trivial example obtained by running 'cat
/dev/zero > /dev/null'

top - 13:41:42 up 6 days,  3:29,  6 users,  load average: 0.45, 0.18,
0.40
Tasks: 246 total,   2 running, 244 sleeping,   0 stopped,   0 zombie
 Cpu0 :   0.3% user,   0.0% system,   0.0% nice,  99.7% idle
 Cpu1 :   0.3% user,   3.6% system,   0.0% nice,  96.1% idle
 Cpu2 :   0.7% user,   0.0% system,   0.0% nice,  99.3% idle
 Cpu3 :  23.9% user,  76.1% system,   0.0% nice,   0.0% idle
Mem:   2587980k total,  2436836k used,   151144k free,    80660k buffers
Swap:   999592k total,   133676k used,   865916k free,  2148236k cached

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
25073 root      18   0   328  328  272 R 99.9  0.0   0:33.50 cat
25074 root      14   0  1092 1092  780 R  2.0  0.0   0:00.94 top


Now, perhaps I am totally misunderstanding HT but is this truly going to
utilise only 50% of the CPU's capacity when only running a single active
process?  I guess what I am trying to understand is if this is giving me
a true understanding of system preformance or is it making some
assumptions about the usage measurement due to thinking it has 4
processors when it really only has 2?

Cheers,
Michael

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