GNU make bug with -j

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2 total messages Started by Jim Winters Tue, 08 Aug 1989 20:32

#355

Author: Jim Winters
Date: Tue, 08 Aug 1989 20:32

28 lines
1248 bytes

Hi.

I think there's a bug in GNU make version 3.27.

I tried to use GNU make because I wanted to use the "-j" option, but
the problem was that it didn't run the jobs concurrently.

I looked into this in the source a little.  Make is picking up the
"-j" option fine, but it later resets the value to "1".

It looks like make is creating a MAKEFLAGS variable in
construct_makeflags, and is then reading that variable as though it
were originally in the environment.  construct_makeflags always sets
the "-j" option to 1 (I assume that's to avoid an explosion of
processes).

I didn't understand enough of what was going on in the code to know
why make processes the variables this way.

I don't really think it makes any difference what makefile you use,
but here's a teeny one that demonstrates the problem:
-----
a:
	sleep 5;echo toot
	sleep 5;echo toot
-----

Thanks for your attention to this.

Re: GNU make bug with -j

#356

Author: Skip Montanaro
Date: Fri, 11 Aug 1989 05:09

33 lines
1738 bytes

In article <8908082032.AA03081@SSD.HARRIS.COM> jimbo@ssd.harris.com (Jim Winters) writes:

   I tried to use GNU make because I wanted to use the "-j" option, but
   the problem was that it didn't run the jobs concurrently.
   ...
   I don't really think it makes any difference what makefile you use,
   but here's a teeny one that demonstrates the problem:

   a:
	   sleep 5;echo toot
	   sleep 5;echo toot

There is no bug that I know of with the -j flag. Your example Makefile has
only a single target, so there are no multiple targets to possibly build in
parallel. Make can't infer that the two commands necessary to update target
'a' are independent. Try this example instead:

a : a.o b.o c.o
	cc -o a a.o b.o c.o
a.o : a.c
	sleep 5; cc -c a.c
b.o : b.c
	sleep 5; cc -c b.c
c.o : c.c
	sleep 5; cc -c c.c

"gmake -jn" (n >= 3) correctly compiles all three C source files in
parallel. It can tell there are no side effects because of the target/source
dependencies. We've put the -j flag to good use on our Convex and our
Stellar, even though we've only been using GNU make for a short time.

--
Skip Montanaro (montanaro@sprite.crd.ge.com)

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