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