Top not showing cpu usage even remotely accurately

[Giorgos Keramidas]

On 2006-09-14 00:48, "Tamouh H." <hakmi at rogers.com> wrote:
> I think TOP and load averages are no longer accurate on FBSD 5.x and
> 6.x with SMP kernel. As far as I've seen. Load averages hit sometimes
> 8.0 without a noticable degradation in performance.
>
> This is one TOP that freaked me out, notice Idle CPU is 70% while the
> process is showing it is using 99% of CPU. systat draws more accurate
> picture, however, load average is still useless as far as performance
> monitoring :
>
> last pid: 10174;  load averages:  1.63,  1.44,  1.20  up 4+00:25:19  00:39:20
> 169 processes: 2 running, 166 sleeping, 1 zombie
> CPU states: 25.8% user,  0.0% nice,  0.7% system,  0.1% interrupt, 73.4% idle
> Mem: 1316M Active, 1445M Inact, 297M Wired, 127M Cache, 112M Buf, 79M Free
> Swap: 8762M Total, 2096K Used, 8760M Free
>
>   PID USERNAME PRI NICE   SIZE    RES STATE  C   TIME   WCPU    CPU COMMAND
> 13362 root     111    0 36444K 34196K CPU3   3  50:06 98.88% 98.88% perl5.8.7
> 90391 root      96    0 27356K 26236K select 2   0:06  0.54%  0.54% perl5.8.7
> 79619 nobody     4    0   209M 84640K sbwait 1   0:09  0.39%  0.39% httpd
> 10161 root      97    0  6712K  4752K select 2   0:00  1.40%  0.20% exim-4.62-0
> 79649 nobody    20    0   210M 84464K lockf  0   0:06  0.15%  0.15% httpd

Apparently, you have a 4-CPU system :-)

What you see displayed as "CPU" is for one of the processors, not for
all of them.  Load average is not an easy thing to update for an SMP
system, I guess.  There are two options:

  - Set load-average to >= 1.0 if at least one process wants to run on
    at least one processor

  - Calculate an aggregate load-average for all CPUs

None of these is 100% correct, though.  One of them is useful in some
cases.  The other in other cases :-(

I don't remember off-hand how 5.X or 6.X calculate their load-average,
but I'd be interested to know what you expected it to show, or what it
shows on Linux systems.

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