[Bug 1894453] Re: Building Ceph packages with RelWithDebInfo

[Tyler Stachecki]

Checked out ceph-15.2.3 source on a up-to-date focal VM.

When running dpkg-buildpackage, CMakeCache.txt gets generated as
expected.  However, the generated CMAKE_BUILD_TYPE is "None".

A bit odd, but going a step further, one can grep for CMAKE_C_FLAGS in that same file and see something to the effect of:
CMAKE_C_FLAGS_DEBUG:STRING=<sensible compiler settings for debug builds>
CMAKE_C_FLAGS_NONE:STRING=
...
...
CMAKE_C_FLAGS_RELWITHDEBINFO:STRING=<sensible compiler settings for release builds>

It seems, though, that since "None" is used, all of those are actually ignored and
CMAKE_C_FLAGS:STRING=<compiler flags from dpkg-buildpackage>
is used

That particular set of flags seems to be missing -DNDEBUG.  It's worth
noting that -DNDEBUG is defined within
CMAKE_C_FLAGS_RELWITHDEBINFO:STRING and including it may result in a
more optimized build.

This is not quite the Gentoo issue as Trent alluded.

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https://bugs.launchpad.net/bugs/1894453

Title:
  Building Ceph packages with RelWithDebInfo

Status in ceph package in Ubuntu:
  In Progress

Bug description:
  This is to consider/discuss the possibility of whether the Ceph
  packages (UCA) should be built with RelWithDebInfo build option
  (CMAKE_BUILD_TYPE [0]) which can be supplied to the build script
  do_cmake.sh [1].

  Currently, Upstream as well as UCA builds default to 'Debug' [0]
  builds. But is any reason why shouldn't go for RelWithDebInfo (i.e.
  "Release with Debug information")?

  
  There's one issue noted with RelWithDebInfo which has a workaround too [2].

  In general, gcc's -O2 [3] build should generate better code as well as
  there's a specific case for Ceph [4] that it improves performance.

  [0] https://cmake.org/cmake/help/latest/variable/CMAKE_BUILD_TYPE.html

  [1] https://github.com/ceph/ceph/blob/master/do_cmake.sh

  [2] https://docs.ceph.com/docs/master/dev/perf/#common-issues

  [3] https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html

  [4] https://bugs.gentoo.org/show_bug.cgi?id=733316

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