diff --git a/drama/src/rev-mc.c b/drama/src/rev-mc.c
index 8ff89fb..400c10a 100644
--- a/drama/src/rev-mc.c
+++ b/drama/src/rev-mc.c
@@ -329,7 +329,7 @@ find_row_mask(std::vector<set_t> &sets,
 
     std::vector<std::thread> workers;
     int thread_num = cpu == 0 ? std::thread::hardware_concurrency() : cpu;
-    if (thread_num > 4) { thread_num = 4; }
+    if (cpu == 0 && thread_num > 4) { thread_num -= 4; }
     fprintf(stderr, "thread_num: %d\n", thread_num);
 
     uint64_t step = 1000;