[Toybox] toybox: xargs: add actual support for -P max-proc flag

[enh]

On Mon, Aug 3, 2020 at 3:52 AM Rob Landley <rob at landley.net> wrote:
>
> On 7/31/20 4:19 PM, Mark Salyzyn via Toybox wrote:
> > On 7/31/20 12:30 PM, Mark Salyzyn wrote:
> >> -P flag was fake just to provide compatibility.  Add support for -P
> >> maximum process count to actually create parallelism.  Does not
> >> support SIGUSR1 or SIGUSR2 increment/decrement signals as publicly
> >> documented for other variants.  If max-proc is 0, run as many
> >> processes as possible simultaneously.
> >>
> > The Bug: referenced were scrubbed, and regardless hides some history. When using
> > toybox xargs for a kernel build, it added 5 minutes to the total build time when
> > it ignored the -P8 flag and exec'd only one at a time.
> >
> > Sincerely -- Mark Salyzyn
>
> I did a slightly smaller implementation (I'd looked at implementing -P before,
> but waited for somebody to complain about its absence), which supports SIGUSR1
> and SIGUSR2.
>
> Does what I checked in work for you?

it passes the tests on Android (i synced AOSP this morning), but i'll
let salyzyn report specifically on the kernel build times.

(i've attached a patch to fix the -P help text alignment.)

> Thanks,
>
> Rob
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