ThinLTO: limit link / codegen parallelism to 16.

build/config/compiler/BUILD.gn

Index: build/config/compiler/BUILD.gn
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index 3c71d12c9bbbc65f1efeaef2206b364cec1505e3..70d6bed84c092ccb666f73399fe2c4ac1ebc1621 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -435,6 +435,12 @@ config("compiler") {
     if (use_thin_lto) {
       cflags += [ "-flto=thin" ]
       ldflags += [ "-flto=thin" ]
+
+      if (is_linux && !use_lld) {
+        # Limit the parallelism to the sweet spot on most of the machines.
+        # As of now, ThinLTO does scale beyond 16 cores anyway.

[dpranke, 2016/09/15 03:35:20]

I think you mean "does not scale", right?

Does this flag impose a maximum number of jobs, or does it impose a fixed
number, i.e., would we still try to use 16 jobs on a 4-core machine?

Also, is the if (...) check even needed? At this time won't this always be true?

[krasin1, 2016/09/15 03:43:42]

right. Fixed.
Jobs is not really a good name here. It's actually a number of threads that the
linker will start to generate native code from the bitcode. It will still be a
single process (ld.gold with LLVMgold.so loaded) with 16 threads.

It's perfectly fine to run 16 threads on 4-core machine. While the performance
might be 10%-15% lower, I have verified locally that -jobs=48 and one core
(limited with schedtool -a 1) works reasonably fast.
with lld (which we aim to support), the flag will likely be -Wl,--job=16, but
it's not clear yet, as it's only to be implemented. On Mac, the flag will be
different as well.

So, this check is to avoid a time bomb: even though we only test ThinLTO with
Linux+Gold plugin now, I would like ThinLTO to still work for other options too,
may be not as fast as it could be with the proper limit.

+        ldflags += [ "-Wl,-plugin-opt,jobs=16" ]
+      }
     } else {
       # Note: ThinLTO does not currently have this feature implemented
       # For Full LTO, it provides a measurable runtime speedup of Chrome.