Limit default number of eval threads to 32

Scalability bottlenecks currently cause diminishing returns above
this. It also prevents creating huge numbers of threads on machines
with lots of cores.

Author: edolstra

src/libexpr/parallel-eval.cc

@@ -17,8 +17,10 @@ thread_local bool Executor::amWorkerThread{false};
 
 unsigned int Executor::getEvalCores(const EvalSettings & evalSettings)
 {
+    /* Note: the default number of cores is currently limited to 32
+       due to scalability bottlenecks. */
     return evalSettings.evalProfilerMode != EvalProfilerMode::disabled ? 1
-           : evalSettings.evalCores == 0UL                             ? Settings::getDefaultCores()
+           : evalSettings.evalCores == 0UL                             ? std::min(32U, Settings::getDefaultCores())
                                                                        : evalSettings.evalCores;
 }