[ELF] Cap parallel::strategy to 16 threads when --threads= is unspeci…

…fied

When --threads= is unspecified, we set it to
`parallel::strategy.compute_thread_count()`, which uses
sched_getaffinity (Linux)/cpuset_getaffinity (FreeBSD)/std::thread::hardware_concurrency (others).
With extensive testing on many machines (many configurations from
{aarch64,x86-64} x {Linux,FreeBSD,Windows} x allocators(native,mimalloc,rpmalloc) combinations)
with varying workloads, we discovered that when the concurrency is larger than
16, the linking process is slower than using --threads=16 due to parallelism
overhead outweighs optimizations. This is particularly harmful for machines with
many cores or when the link job competes with other jobs.

Cap parallel::strategy when --threads= is unspecified.
For some workloads changing the concurrency from 8 to 16 has nearly no improvement.

--thinlto-jobs= is unchanged since ThinLTO backend compiles are embarrassingly
parallel.

Link: https://discourse.llvm.org/t/avoidable-overhead-from-threading-by-default/69160

Reviewed By: peter.smith, andrewng

Differential Revision: https://reviews.llvm.org/D147493

lld/ELF/Driver.cpp

@@ -1421,7 +1421,9 @@ static void readConfigs(opt::InputArgList &args) {
   }
 
   // --threads= takes a positive integer and provides the default value for
-  // --thinlto-jobs=.
+  // --thinlto-jobs=. If unspecified, cap the number of threads since
+  // overhead outweighs optimization for used parallel algorithms for the
+  // non-LTO parts.
   if (auto *arg = args.getLastArg(OPT_threads)) {
     StringRef v(arg->getValue());
     unsigned threads = 0;
@@ -1430,6 +1432,9 @@ static void readConfigs(opt::InputArgList &args) {
             arg->getValue() + "'");
     parallel::strategy = hardware_concurrency(threads);
     config->thinLTOJobs = v;
+  } else if (parallel::strategy.compute_thread_count() > 16) {
+    log("set maximum concurrency to 16, specify --threads= to change");
+    parallel::strategy = hardware_concurrency(16);
   }
   if (auto *arg = args.getLastArg(OPT_thinlto_jobs_eq))
     config->thinLTOJobs = arg->getValue();