bees: soft-limit computed thread counts to 8

https://github.com/Zygo/bees/issues/91 describes problems encountered
when running bees on systems with many CPU cores.

Limit the computed number of threads (using --thread-factor or the
default) to a maximum of 8 (i.e. the number of logical cores in a modern
laptop).  Users can override the limit by using --thread-count.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

src/bees.cc

@@ -786,6 +786,11 @@ bees_main(int argc, char *argv[])
 			thread_factor = BEES_DEFAULT_THREAD_FACTOR;
 		}
 		thread_count = max(1U, static_cast<unsigned>(ceil(thread::hardware_concurrency() * thread_factor)));
+		if (thread_count > BEES_DEFAULT_THREAD_LIMIT) {
+			BEESLOGNOTICE("Limiting computed thread count to " << BEES_DEFAULT_THREAD_LIMIT);
+			BEESLOGNOTICE("Use --thread-count to override this limit");
+			thread_count = BEES_DEFAULT_THREAD_LIMIT;
+		}
 	}
 
 	if (load_target != 0) {

src/bees.h

@@ -85,6 +85,9 @@ const size_t BEES_OPEN_FILE_LIMIT = (BEES_FILE_FD_CACHE_SIZE + BEES_ROOT_FD_CACH
 // Worker thread factor (multiplied by detected number of CPU cores)
 const double BEES_DEFAULT_THREAD_FACTOR = 1.0;
 
+// Don't use more than this number of threads unless explicitly configured
+const size_t BEES_DEFAULT_THREAD_LIMIT = 8;
+
 // Log warnings when an operation takes too long
 const double BEES_TOO_LONG = 5.0;