top -d1 behavior

[David Wolfskill]

On Sun, Nov 23, 2014 at 06:11:02PM +0100, Fernando Apesteguía wrote:
> Hi hackers,
> 
> While writing a small script for a friend, I run into a peculiar top behavior:
> 
> top -d1 shows only one 'iteration' of the information screen, but in
> it, the CPU percentages line is not well formed: there are 5 columns
> with the '%' symbol, but no values are shown. This only happens for
> -d1 and this behavior is deliberately done in the sources (see
> 'dostates' variable).
> ...
> My point is: why are we doing this? If we remove that constraint, top
> would show the values for -d1. I don't know if they would be really
> accurate, but not printing anything doesn't seem a nice behavior
> either (especially when this is not documented in the man page)
> ....

At the time of the inital display, the program only has access to the
counters (from kern.cp_time) from one instant.

In order to calculate the relative proportion of CPU time spent in each
of the 5 states, it is necessary to determine the extent to which those
counters changed over an interval.

If we call the time when the first sample is taken "T0" and the time
when the second sample is takne "T1", we get something like:

Time          user    nice      sys    intr    idle
  T0        user_0  nice_0    sys_0  intr_0  idle_0
  T1        user_1  nice_1    sys_1  intr_1  idle_1

To determine the relative proportions for the interval from T0:T1, we
first determnine the differences (or deltas):

user_delta <== user_1 - user_0
nice_delta <== nice_1 - nice_0
sys_delta  <== sys_1 - sys_0
intr_delta <== intr_1 - intr_0
idle_delta <== idle_1 - idle_0

Then sum the deltas:

Interval_total = user_delta + nice_delta + sys_delta + intr_delta + idle_delta

Then, for each of user, nice, sys, intr, and idle, the percentage for
the interval is

100 * _delta / Interval_total.

Here's a sample (produced by a little shell script):

albert(10.1-S)[6] get_sys_info -N 0 -c 2 -w 10 kern.cp_time
time:1416763949 kern.cp_time:20934 0 28872 5664 1027225 
time:1416763959 kern.cp_time:20980 0 28933 5674 1029686 

Given those numbers, we have:

Time          user    nice      sys    intr    idle
1416763949   20934       0    28872    5664 1027225
1416763959   20980       0    28933    5674 1029686

The differences:
              user    nice      sys    intr    idle
                46       0       61      10    2461

The total of the deltas is 46+0+61+10+2461 ==> 2578.

The percentages:
              user    nice      sys    intr    idle
              1.78    0.00     2.37    0.39   95.46


(System's uptime, in this case, was a little over an hour -- I had just
updated in to stable/10 @r274909.)

Peace,
david
-- 
David H. Wolfskill				david at catwhisker.org
Taliban: Evil cowards with guns afraid of truth from a 14-year old girl.

See http://www.catwhisker.org/~david/publickey.gpg for my public key.
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