Date: Sun, 23 Nov 2014 09:45:55 -0800 From: David Wolfskill <david@catwhisker.org> To: Fernando =?iso-8859-1?Q?Apestegu=EDa?= <fernando.apesteguia@gmail.com> Cc: FreeBSD Hackers <freebsd-hackers@freebsd.org> Subject: Re: top -d1 behavior Message-ID: <20141123174555.GB1228@albert.catwhisker.org> In-Reply-To: <CAGwOe2Zp2cHhCb%2Br_m5HZYJFyc=3DDsPEi7v_7ZVxJ0fkO1jEA@mail.gmail.com> References: <CAGwOe2Zp2cHhCb%2Br_m5HZYJFyc=3DDsPEi7v_7ZVxJ0fkO1jEA@mail.gmail.com>
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On Sun, Nov 23, 2014 at 06:11:02PM +0100, Fernando Apestegu=EDa wrote:
> Hi hackers,
>=20
> While writing a small script for a friend, I run into a peculiar top beha=
vior:
>=20
> 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 <=3D=3D user_1 - user_0
nice_delta <=3D=3D nice_1 - nice_0
sys_delta <=3D=3D sys_1 - sys_0
intr_delta <=3D=3D intr_1 - intr_0
idle_delta <=3D=3D idle_1 - idle_0
Then sum the deltas:
Interval_total =3D 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=20
time:1416763959 kern.cp_time:20980 0 28933 5674 1029686=20
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 =3D=3D> 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
--=20
David H. Wolfskill david@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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