thomson@zazen.macc.wisc.edu (Don Thomson) (04/19/91)
My manager is trying to analyze performance on our DECstation 3100 running
Ultrix 4.0 and has asked me to clarify the meaning of the percentage CPU time
that is displayed. We're looking at percentage idle and guessing that
percentage CPU time used + idle time should add up to 100%, but are rarely
seeing CPU percentages wander over 1 or 2%. I have a feeling we're comparing
apples and oranges here, so am looking for clarification of what exactly we're
looking at:
last pid: 8233; load averages: 0.64, 0.56, 0.00 12:03:09
59 processes: 55 sleeping, 1 running, 3 stopped
Cpu states: 5.2% user, 0.0% nice, 21.0% system, 73.8% idle zazen
Memory: 9536K ( 5360K) real, 27388K (17316K) virtual, 1296K free
PID USERNAME PRI NICE SIZE RES STATE TIME WCPU CPU COMMAND
8213 thomson 25 0 612K 396K run 0:00 1.95% 1.95% top
8232 root 1 0 172K 64K sleep 0:00 0.39% 0.39% nntpxmit
21060 thomson 26 0 7828K 2648K stop 4:39 0.00% 0.00% emacs
The top man page says:
WCPU is the weighted cpu percentage (this is the same value that
ps(1) displays as CPU), CPU is the raw percentage and is the
field that is sorted to determine the order of the processes
and the ps man page says:
%CPU CPU utilization of the process. This is a decaying
average over a minute or less of previous (real) time.
Because the time base over which this is computed
varies since processes may be very young, it is possi-
ble for the sum of all %CPU fields to exceed 200%.
Can anyone help shed any light on exactly what kind of measurements we're
seeing here, and/or clarify the distinction between weighted and raw CPU
percentages? Thanks!
--
----- Don Thomson ----- MACC, 1210 W. Dayton, Madison, WI 53706 -------------
(608) 262-0138 thomson@macc.wisc.edu / thomson@wiscmacc.bitnetbarmar@think.com (Barry Margolin) (04/19/91)
I've redirected followups to comp.unix.internals, since there is nothing in the question that is specific to Ultrix. In fact, much of my response here is based on a very non-Unix-like OS (Genera on Symbolics Lisp Machines), but I strongly believe that my answer is pretty correct for Unix as well. In article <THOMSON.91Apr18122955@zazen.macc.wisc.edu> thomson@zazen.macc.wisc.edu (Don Thomson) writes: >My manager is trying to analyze performance on our DECstation 3100 running >Ultrix 4.0 and has asked me to clarify the meaning of the percentage CPU time >that is displayed. We're looking at percentage idle and guessing that >percentage CPU time used + idle time should add up to 100%, but are rarely >seeing CPU percentages wander over 1 or 2%. I have a feeling we're comparing >apples and oranges here > %CPU CPU utilization of the process. This is a decaying > average over a minute or less of previous (real) time. > Because the time base over which this is computed > varies since processes may be very young, it is possi- > ble for the sum of all %CPU fields to exceed 200%. The problem is that on a single-CPU computer, at any instant at most one process can be running, so an instantaneous CPU time percentage would show one process having 100% CPU time and all the rest having 0%. On the other hand, if the CPU percentage were simply the amount of CPU time the process has used during the last real minute divided by 1 minute, processes that were started less than a minute ago would have artificially low CPU utilization, and processes that became idle in the middle of the minute would have misleadingly high CPU utilitization; for instance, a process that started monopolizing the system 30 seconds ago would only show 50% usage, even though it has actually taken over the system. The common solution to this is to use a weighted average, with decaying weights. Averages are computed more often than just once a minute, and the more recent averages are multiplied by weighting factors to make them more significant in the final number. Thus, if only two processes ran during the last minute, and process A was the only process during the first 30 seconds and process B was the only process during the second 30 seconds, process B would have a higher %CPU than A. However, the algorithms for computing this generally don't actually keep around all the little per-period averages. Instead, running weighted averages are computed at process switch time. Furthermore, the weighting factors are generally arbitrary, and decay exponentially rather than linearly, with very recent activity having a very high weight. It's basically a heuristic, and it's a white lie to call these things percentages. Like car mileage, the numbers should be used for comparison purposes only. -- Barry Margolin, Thinking Machines Corp. barmar@think.com {uunet,harvard}!think!barmar