I have a question about the "wa" fields in vmstat, top, and the
like. I and someone else I know have both read a great deal
about its meaning, and have come to two different conclusions.
Here are our interpretations. Could someone please tell me
which interpretation is right? Thanks!
Interpretation A:
"wa" reflects time that your cpu is waiting on hard disks and
network. It is basically the same as "idle" time. If your CPU
had more work to do, it could do it. So in a situation like
this.... (I'm simplifying the example to get the point across)
10%us 10%sy 30%id 50%wa
The CPU is really truly 80% idle. It's only using 20% of its
capacity, and if it had enough work to fill the remaining 80%,
it could and would. You'd see 100% allocated between us and sy,
and 0% on id and wa.
In this interpretation, "wa" is really just helpful because it
tells you how much more of your CPU would be allocated between
us and sy if your hard disks and network could keep up with your
CPU. It gives you some impression of the bottleneck your I/O
currently is on your PC.
Interpretation B:
"wa" reflects the time your CPU is waiting on hard disks and
network. But it is NOT the same as "id". If your CPU had more
work to do, it could not "allocate" wa time to that work, only
id time. So in a situation like this....
10%us 10%sy 30%id 50%wa
The CPU is really only 30% idle. It's actually using a whopping
70% of its capacity, and only has another 30% to allocate to
more work. If the CPU had a ton of work to do, the best you'd
see is 50% allocated between us and sy, and 50% allocated to wa
with 0% id.
In this interpretation, "wa" is really like "blocked" CPU time.
The CPU has processes that are not really doing anything,
because they are waiting on slow hard disks and network I/O,
*but* these waiting processes still prevent that much CPU from
being used.
So which is it? (Or is it neither?)
Thanks in advance everyone,
- GM
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