Re: Dynamically loading readline on demand (was Re: rc futures)

[Paul Haahr <haahr@jivetech.com>]

Tim Goodwin wrote, replying to me, replying to him:
> > > I took an rc script that does nothing (makes no system calls) except
> > > fork() and wait() 10000 times.
> > 
> > Are you sure it doesn't exec at all?
> 
> You tell me.
> 
>     for (i in 0 1 2 3 4 5 6 7 8 9)
>     ...
>     for (m in 0 1 2 3 4 5 6 7 8 9)
>     @{ ~ 0 0 }
> 
> According to strace on my Linux box, each loop calls fork(), close(),
> rt_sigaction() twice, _exit(), and wait().

Fascinating.  My guess was definitely wrong.

> > > ([...]  Why does dynamic linking increase the user time?
> > The short answer is ``because dynamic
> > linking is done with user-space code.'')
> 
> Yeah: in crt0.  But (I'm sure) that isn't involved here.  I'd expect
> fork() to take more *system* time (since there are more MAP_SHARED page
> table entries to fiddle with), but I don't understand the increase in
> user time.

The only thing I can guess is that using the PIC version of the code in
the shared libraries and the shared library calling sequences is hurting
much more than I would have expected.  (Chapter 8 of Levine's book goes
into the issues here.)  My guess would have been at most a 10% hit, not
the 30% you're reporting.  Detailed profiling might shed more light.

--p