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Old 02-24-2008, 04:34 AM   #1
exscape
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What is iowait, exactly?


My question is quite simple: If I have a iowait at (theoretically) 95%, and tried to run something CPU intensive (pure calculations), would the calcs run very slow or not? Reason I'm asking is that the man page (for mpstat) says "Show the percentage of time that the CPU or CPUs were idle during which the system had an outstanding disk I/O request".
Does this mean, they were actually idle and available for use elsewhere, or not?

For the record, here's a capture of mpstat:
Code:
11:27:53 AM  CPU   %user   %nice    %sys %iowait    %irq   %soft  %steal   %idle    intr/s
11:28:08 AM  all    4.47    0.00    2.07   53.73    0.53    1.73    0.00   37.47   1050.93
11:28:23 AM  all    1.20    0.00    1.07   51.30    0.53    0.93    0.00   44.97   1176.22
11:28:38 AM  all    1.40    0.00    1.40   55.40    0.53    1.27    0.00   40.00   1184.07
11:28:53 AM  all    1.67    0.00    0.93   52.56    0.47    0.93    0.00   43.44   1135.58
Average:     all    2.18    0.00    1.37   53.25    0.52    1.22    0.00   41.47   1136.70
The high wait is most likely caused by bittorrent. I'm trying to figure out if I should care about it or not.
 
Old 02-24-2008, 04:49 AM   #2
jlliagre
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Quote:
Originally Posted by exscape View Post
My question is quite simple: If I have a iowait at (theoretically) 95%, and tried to run something CPU intensive (pure calculations), would the calcs run very slow or not?
Not any slower or faster.
Quote:
Reason I'm asking is that the man page (for mpstat) says "Show the percentage of time that the CPU or CPUs were idle during which the system had an outstanding disk I/O request".
Does this mean, they were actually idle and available for use elsewhere, or not?
Yes.
Quote:
The high wait is most likely caused by bittorrent. I'm trying to figure out if I should care about it or not.
Just don't. I/O Wait is just a confusing metric, and mostly meaningless in multi-core / multi-CPU systems.
 
Old 02-24-2008, 05:18 AM   #3
exscape
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Great, thanks. I've been a bit annoyed with my CPU graph (rrdtool), it shows an average users of 30-60%, while the top process in 'top' usually consumes about 2%.
 
  


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