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Re: [minor] "precision" of $SECONDS

From: Linda Walsh
Subject: Re: [minor] "precision" of $SECONDS
Date: Thu, 25 Feb 2016 03:03:41 -0800
User-agent: Thunderbird

Stephane Chazelas wrote:
$ time bash -c 'while ((SECONDS < 1)); do :; done'
bash -c 'while ((SECONDS < 1)); do :; done'  0.39s user 0.00s system 99% cpu 
0.387 total

That can take in between 0 and 1 seconds. Or in other words,
$SECONDS becomes 1 in between 0 and 1 second after the shell was
The format you are using to display output of 'time' doesn't show
real time -- only CPU seconds.


TIMEFORMAT='%2Rsec %2Uusr %2Ssys (%P%% cpu)'

It shows the real, "clock" time, as well as
the standard % formula of cpu-secs/real-secs -- which can give
up to #Cores x 100"%" as value for %cpu  stretches the standard
def of "percent", but is at least more honest than grouping
all cpu's together as "1 cpu", and showing 100% usage of 1 core
on a 12-core machine as "8.3%".

mksh and zsh behave like bash (I'll raise the issue there as

With zsh (like in ksh93), one can do "typeset -F SECONDS" to
make $SECONDS floating point, which can be used as a work around
of the "issue".
   With linux, one can read /proc/uptime to 100th's of a sec, or
use date to get more digits.  A middle of the road I used for
trace timing was something like:

function __age { declare ns=$(date +"%N"); declare -i ms=${ns##+(0)}/1000000;
 printf "%4d.%03d\n" $SECONDS $ms
Then I add that in my PS4 prompt:
## for *relative* script exec speed, only!
##   (slows down normal speed significantly):
#export PS4='>[$(__age)]${BASH_SOURCE:+${BASH_SOURCE[0]/$HOME/\~}}#${LINENO}${FUNCNAME:+(${FUNCNAME[0]})}> '

On cygwin, calling 'date' was a deal breaker, so I just used /proc/uptime
to get the relative time down to centiseconds.
As you can see, I wanted the times
relative to the start of a given script, thus used SECONDS for that.

Given the overhead of calling 'date', the times printed are easily 20-50x slower
than w/o the time -- BUT, as the comment says above, it's good for
determining relative differences -- not only times for each command, but
also algorithmic differences, as the overhead is relatively constant.

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