[llvm-dev] System hangs during last stages of LLVM build | Tips on speeding it up ?

[Tim Northover via llvm-dev]

Hi Sanjay,

On 23 February 2017 at 09:21, SANJAY SRIVALLABH SINGAPURAM via
llvm-dev <llvm-dev at lists.llvm.org> wrote:
> My system hangs every time during last stages of building LLVM ( starting at
> 95% in a CMake build ) using CMake or Ninja, sometime close to the linking
> of llvm-dysmutil. Could you please suggest tips that could speed up the
> compilation ?

This is probably because multiple linker processes are using all your
memory and bogging down in swap. There are various things you can do
to improve performance:

  * There's an LLVM_PARALLEL_LINK_JOBS option which you can use to
reduce the number of ld instances running at once (you'll need to be
using Ninja rather than Make for this to work, but that's faster
anyway). This is a slightly more fine-grained approach than Tobias's
"-j 1"; systems can usually support more compile jobs than link.
 * If on Linux, you can switch to ld.gold (or lld if you're feeling
brave!), which is much faster than the default ld.bfd
 * Also on ELF platforms, there's LLVM_USE_SPLIT_DWARF which separates
the debug info into a different file. This really speeds things up,
but you need a recent(ish) gdb to be able to debug the result.

Those are the obvious ways to reduce memory. You might also consider
only building the targets you need, which will speed things up but not
get around swapping usually.

Cheers.

Tim.