child time accounting is different from other shells

child time accounting is different from other shells

[Vincent Lefevre]

With all shells I could test except zsh,

  time sh -c 'pi 500000 > /dev/null & sleep 2'

where "pi 500000 > /dev/null" lasts for less than 2 seconds
(or replace it with any command that takes CPU time less than
2 seconds), includes the time taken by this child process in
the user/system times, but not with zsh:

xvii% time sh -c 'pi 500000 > /dev/null & sleep 2'
sh -c 'pi 500000 > /dev/null & sleep 2'  1.16s user 0.02s system 58% cpu 2.017 total
xvii% time zsh -c 'pi 500000 > /dev/null & sleep 2'
zsh -c 'pi 500000 > /dev/null & sleep 2'  0.00s user 0.00s system 0% cpu 2.023 total

Is this a bug? Shouldn't this behavior be changed to make zsh
consistent with the other shells?

-- 
Vincent Lefèvre <vincent@vinc17.net> - Web: <https://www.vinc17.net/>
100% accessible validated (X)HTML - Blog: <https://www.vinc17.net/blog/>
Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)

Re: child time accounting is different from other shells

[Bart Schaefer]

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On Wed, Sep 10, 2014 at 5:30 AM, Vincent Lefevre <vincent@vinc17.net> wrote:

>
> xvii% time sh -c 'pi 500000 > /dev/null & sleep 2'
> sh -c 'pi 500000 > /dev/null & sleep 2'  1.16s user 0.02s system 58% cpu
> 2.017 total
> xvii% time zsh -c 'pi 500000 > /dev/null & sleep 2'
> zsh -c 'pi 500000 > /dev/null & sleep 2'  0.00s user 0.00s system 0% cpu
> 2.023 total
>
> Is this a bug?


No, it isn't.

Zsh does a sort of tail-call optimization:  In the expression "left &
right", zsh can tell that there is no further need for job-control of
"left", so it does an implicit "exec right".  Other shells fork off a new
process for "right" and wait for it.

You can see the difference if you forcibly prevent the optimization by
appending a no-op command, e.g.

schaefer[101] time zsh -c 'ps & sleep 2; :'
  PID TTY           TIME CMD
47681 ttys000    0:00.47 -zsh
50366 ttys000    0:00.01 zsh -c ps & sleep 2; :
50370 ttys000    0:00.00 sleep 2
zsh -c 'ps & sleep 2; :'  0.01s user 0.01s system 1% cpu 2.019 total
schaefer[102] time zsh -c 'ps & sleep 2'
  PID TTY           TIME CMD
47681 ttys000    0:00.48 -zsh
50372 ttys000    0:00.01 sleep 2
zsh -c 'ps & sleep 2'  0.01s user 0.01s system 1% cpu 2.018 total

You can also prevent the optimization by adding an exit trap, etc.


> Shouldn't this behavior be changed to make zsh
> consistent with the other shells?
>

I would not count on this ever happening.

Re: child time accounting is different from other shells

[Vincent Lefevre]

On 2014-09-10 14:02:28 -0700, Bart Schaefer wrote:
> You can also prevent the optimization by adding an exit trap, etc.

Actually any trap prevents the optimization.

-- 
Vincent Lefèvre <vincent@vinc17.net> - Web: <https://www.vinc17.net/>
100% accessible validated (X)HTML - Blog: <https://www.vinc17.net/blog/>
Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)