Greg Stark wrote:
Alan Stange <[EMAIL PROTECTED]> writes:
Iowait is time spent waiting on blocking io calls. As another poster
pointed out, you have a two CPU system, and during your scan, as predicted,
one CPU went 100% busy on the seq scan. During iowait periods, the CPU can
be context switched to other users, but as I pointed out earlier, that's not
useful for getting response on decision support queries.
I don't think that's true. If the syscall was preemptable then it wouldn't
show up under "iowait", but rather "idle". The time spent in iowait is time in
uninterruptable sleeps where no other process can be scheduled.
That would be wrong. The time spent in iowait is idle time. The
iowait stat would be 0 on a machine with a compute bound runnable
process available for each cpu.
Come on people, read the man page or look at the source code. Just
stop making stuff up.
iowait time is idle time. Period. This point has been debated endlessly for
Solaris and other OS's as well.
Here's the man page:
%iowait
Show the percentage of time that the CPU or CPUs were
idle during which the system had an outstanding disk I/O
request.
If the system had some other cpu bound work to perform you wouldn't ever see
any iowait time. Anyone claiming the cpu was 100% busy on the sequential scan
using the one set of numbers I posted is misunderstanding the actual metrics.
That's easy to test. rerun the test with another process running a simple C
program like "main() {while(1);}" (or two invocations of that on your system
because of the extra processor). I bet you'll see about half the percentage of
iowait because postres will get half as much opportunity to schedule i/o. If
what you are saying were true then you should get 0% iowait.
Yes, I did this once about 10 years ago. But instead of saying "I bet"
and guessing at the result, you should try it yourself. Without
guessing, I can tell you that the iowait time will go to 0%. You can do
this loop in the shell, so there's no code to write. Also, it helps to
do this with the shell running at a lower priority.
-- Alan
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