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A system is
CPU bound
(has a CPU
bottleneck)
if the processor cannot execute
fast enough to keep the number of processes on the run
queue consistently low.
To determine if a system is CPU bound, run sar -u
(or cpusar -u for each processor on a system with an
SCO SMP License) and examine the %idle value.
If %idle is consistently less than 5% (for all
CPUs) on a heavily loaded database server system,
then the system may be lacking in processing power.
On a heavily loaded system with many logged-in users, a
%idle value that is persistently less than 20%
suggests that the system not be able to cope
with a much larger load.
Examination of the number of processes on the
run queue shows whether there is
an unacceptable buildup of runnable processes.
If processes are not building up on the run
queue, a low idle time need not indicate an immediate
problem provided that the other subsystems (memory and
I/O) can cope with the demands placed upon them.
Run queue activity can be considered heavy if
sar -q (mpsar -q for SMP)
reports that runq-sz is consistently greater than 2
(and %runocc is greater than 90% for SMP).
If low %idle values are combined with heavy
run queue activity then the system is CPU bound.
If low %idle values are combined with low or
non-existent run queue activity, it is possible that
the system is running CPU-intensive processes.
This in itself is not a problem
unless an increase in the number of executing processes
causes a buildup of numbers of processes on the run queue.
If %wio values are consistently high (greater than 15%),
this is more likely to indicate a potential I/O
bottleneck than a problem with CPU resources.
See
``Tuning I/O resources''
for more information on identifying I/O
bottlenecks.
High values of %wio may also be seen if
the system is swapping and paging.
Memory shortages can also lead to
a disk I/O bottleneck
because the system spends so much time moving
processes and
pages
between memory and
swap areas
on disk.
If the value of %sys is high relative
to %usr, and %idle is close to zero,
this could indicate that the
kernel
is consuming a large amount of CPU time running the
swapping
and
page stealing
daemons(
sched
and
vhand).
These daemons are part of the kernel and
cannot be context switched;
this may lead to several processes
being stuck on the run queue waiting to run.
For details of how to identify and tune
memory-bound systems, see
``Tuning memory resources''
and
``Tuning memory-bound systems''.
The following table summarizes the commands that you can use to determine if a system is CPU bound:
Identifying a CPU-bound system
| Command | Field | Description |
|---|---|---|
| sar -u | %idle | percentage of time that the CPU was idle |
| mpsar -u | %idle | average percentage of time all CPUs are idle (SMP only) |
| cpusar -u | %idle | percentage of time the specified CPU was idle (SMP only) |
| [mp]sar -q | %runocc | percentage of time the run queue is occupied |
| runq-sz | number of processes on the run queue |