[CMake] build times increase 1.5x from cmake 2.8.12 to 3

[Robert Maynard]

My initial thought is that the differences between -mmd and -md would
be pretty minimal. The original discussion on this change (
https://cmake.org/Bug/view.php?id=14914 ) has people agreeing with
that as this information is generated when the compiler builds each
file.

Have you used something like ninja browse / ninja graph (
https://ninja-build.org/manual.html ) to verify the build graph from
2.8.12 to 3.X are the same?

On Fri, Mar 23, 2018 at 3:41 PM, Miller Henry <MillerHenry at johndeere.com> wrote:
> I have a fairly large project that I’m trying to update from building with
> cmake 2.8.12 to 3.10.2. When I make the change my build times go up by half
> on our Jenkins ci build system.  The build machines are 32 core and my build
> time goes from 59m 38s to 1h 28m 33s. When I try this locally the time goes
> from 80 minutes to 84 minutes (a 6 core machine, but I’m running Debug while
> the ci system is running RelWithDebInfo so the times cannot be compared).
> My current effort is targeting 3.10.2, but I’ve had similar results with 3.2
> and 3.9.
>
> I compared command line of just one file and it looks like the only
> significant difference is 2.8.12 was passing -MMD to gcc, while 3.10.2 is
> using -MD. I’m suspecting that this is the significant difference. Is there
> a way to get cmake to use -MMD? Or better yet, since our CI machines never
> do an incremental build is there a way to turn off dependency generation
> that might speed my build up more?  We are using the ninja Generator.
>
> Of course I just sampled one of our 17000 build steps to decide the above
> was the problem. It is entirely possible that I’m off base and something
> else is the issue. If you have an idea of where I should look instead I’m
> open to that.
>
>
>
>
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