Re: [TEST] make -j patch [take 2]

Hi,

On Fri, Nov 12, 2004 at 10:35:00AM +0100, Poul-Henning Kamp wrote:
> In message <20041112090905.GD41844_at_ip.net.ua>, Ruslan Ermilov writes:
> 
> >But you don't give an opportunity to control this on a sub-make
> >level (that's what I ask for). 
> 
> Why would that be of any use ?  If you run "make universe" the task
> at hand is to get "make universe" to complete.  You should not care
> which partcular submake starts how many jobs when, you should only
> care that it works as efficient as possible.
> 
Please read below.

> >level (that's what I ask for).  If you implement what I suggest,
> >"make -j 12 universe" will still do as you saw it last night,
> >while still allowing "make -j4 universe JFLAG=3D-j3" to build four
> >worlds in parallel, each with three jobs.  Isn't it nice?  ;)
> 
> You're wrong about how this works.
> 
I LIKE THIS FEATURE BUT IT NEEDS TO BE IMPROVED.  :-)

> Try plotting your load-avg over time and you will see that you have
> neither enforced the 4 nor the 3 jobs.
> 
Of course not.  And I didn't say I wanted this.  What I want is
to build 4 worlds in parallel, each with 3 available jobs, so
that the total number of jobs will not exceed 12.

> Or look at the one I just posted:
> 
> 	http://phk.freebsd.dk/misc/make.png
> 
> Then explain to me how you think it is beneficial that "-j 12"
> gives make(1) permission to start 65 jobs.
> 
God damn it, I never said I don't like your patch and idea.
I like it very much.  I just said (and not only me actually)
that there should be an option to start afresh, i.e., that
a sub-make can ignore the global limit set by its parent,
and start on its own.  I'm working on a patch now, it's tiny.

> When I say "make -j 12" I mean "12 jobs, damnit!", not "up to
> 12^(number of levels of subdirectories)".
> 
The global limit is fine.  What is not fine, is that it's made
global permanently.  Now, when you say "make -j 12 buildworld"
you won't necessarily be given 12 jobs to build this world,
because your parent (e.g., "make universe") may have already
eaten some of them, or set this limit to a lower value.

And when I say "make -j4 buildworld", I want it to use four
jobs for this buildworld, whether it's typed on a command
line, or invoked as part of "make universe", still with
global limit of four jobs, but only for _this_ buildworld.
To picture it down:

	make -j12 universe
		should build all worlds using 12 jobs

	make -j2 universe JFLAG=-j6
		should build two worlds in parallel,
		each world using 6 jobs

In both cases, the maximum number of makes simultaneously
running in the system should not exceed 12.

The reason I prefer the latter is that it's more practical
to build each world with more jobs available than to build
more worlds simultaneously with less jobs available for
individual world.  If I want to test my change with "make
universe", I'm interested in finding if it broke something
quickly, right?  If it broke one architecture, it's not
acceptable for committing.  Hence, I want to complete with
one world, then switch to another.  I'd typically run
"make universe JFLAG=-jX" when testing, i.e., build worlds
sequentially, and only each world in parallel, but YMMV.

> The submakes will not respect
> an explicit "-j N" argument if they find the environment
> variable.  You can of course eliminate that if you want to
> override this behaviour.
> 
Yeah, that's the ONLY thing I want to be improved, and I
will do it.  Would you like to see a patch before it gets
committed?

> >Or perhaps you could explain how your patch helps parallelism
> >or improves something, I fail to see it.  All I see is that
> >it causes N shells to be used instead of one, but this is
> >unmeasureable in buildworld times.
> 
> I don't know the reason, but the result was about 5% speedup on
> parallel buildworld.
> 
Actually, I see a huge slowdown, and I *know* the reason.
With the attached makefile:

$ time -h make new
        31,90s real             10,24s user             21,28s sys
$ time -h make old
        0,88s real              0,56s user              0,30s sys
$ time -h make -j4 new
        8,67s real              7,64s user              0,91s sys
$ time -h make -j4 old
        0,88s real              0,61s user              0,25s sys

Add -l to see the number of context switches.  Note how "new"
is worse in every case, and note that -jX doesn't really matter
for "old".

"old" corresponds to the old version of bsd.subdir.mk, and
"new" corresponds to your proposed change to bsd.subdir.mk.

In the old version, only a single shell is used to print all
10000 numbers, and the shell is passed one line of stdin.

In your version, non-parallel make will start 10000 shells
to print numbers.  With -j4, only a single shell will be
used, but make(1) will have to parse 10000 lines, and pass
30000 lines over a pipe to a shell.  (You can see it
yourself through "make -j4 -dj").  The "4" doesn't really
matter, -j1 will give the same time (~8 vs ~31).

Sorry, but I just don't trust your 5% speedup claim.  ;)

> I'll leave it to you if anything needs done, You're the
> makefile wizard.
> 
Thanks.

> I've done the bit I set out to do: prevent
> boundless creation of jobs.
> 
That was much appreciated, thank you!


Cheers,
-- 
Ruslan Ermilov
ru_at_FreeBSD.org
FreeBSD committer