[CMake] Build with Debug Fission on by default on Debug builds

• Component: WebKitGTK
• Status: RESOLVED FIXED

[Carlos Alberto Lopez Perez]

We are having some issues on the GTK Debug buildbots. The bots are running out of RAM sometimes.

On bug 131177 support for building with Debug Fission was added.

Now I suggest to enable this by default when building the Debug build.

I have measured the memory usage of building with and without this option for the Debug build:

* Without Debug fission:
  Max memory usage by the linker = 10.2 GB


* With Debug fission:
  Max memory usage by the linker = 7.8 GB

Not it only reduces the amount of RAM required to build WebKit in 2.4GB, but it also builds faster.


Concerns about Debug Fission builds breaking ccache were sorted out with ccache 3.2.3  https://ccache.samba.org/releasenotes.html#_ccache_3_2_3

[Carlos Alberto Lopez Perez]

Some more stats:

* Rebuilding the Debug build from ccache without Debug fission:
  - Time to build = 06m:42s
  - Size of WebKitBuild/Debug = 16GB


* Rebuilding the Debug build from ccache with Debug fission:
  - Time to build = 04m:05s
  - Size of WebKitBuild/Debug = 8.9G


The time to build is mostly linking time, since I'm rebuilding from previously generated ccache DB. And this is on a system with an SSD. On a system with a rotational media it will probably improve build times further.

Its also nice that the size of the build output has been reduced (almost to half). This will help to save disk space also.

[Carlos Alberto Lopez Perez]

Created attachment 260302 [details]
Patch

[Martin Robinson]

Can you comment on compiler requirements? Does our minimum supported compiler also support debug fission?

[Carlos Alberto Lopez Perez]

(In reply to comment #3)
> Can you comment on compiler requirements? Does our minimum supported
> compiler also support debug fission?

According to https://gcc.gnu.org/wiki/DebugFission it is implemented on GCC 4.7

We already require GCC 4.9, so this shouldn't be a problem.
I also tested this with Clang (version 3.6) and it worked without problems.

[Michael Catanzaro]

(In reply to comment #3)
> Can you comment on compiler requirements? Does our minimum supported
> compiler also support debug fission?

Clang has supported debug fission for a while now. GCC has supported it since 4.7 [1] and nowadays we require 4.8 or 4.9, depending on who you ask :), so we're good. It makes debug builds dramatically faster, especially incremental builds, so I agree it should be on by default.

[1] http://gcc.gnu.org/wiki/DebugFission

[Carlos Alberto Lopez Perez]

Comment on attachment 260302 [details]
Patch

Clearing flags on attachment: 260302

Committed r189179: <http://trac.webkit.org/changeset/189179>

[Carlos Alberto Lopez Perez]

All reviewed patches have been landed.  Closing bug.