Add NVC_CONCURRENT_JOBS environment variable

NEWS.md

@@ -21,6 +21,9 @@
   (#900).
 - Fixed a crash when branch coverage is enabled and an if-statement
   contains a `return` (#903).
+- The `NVC_CONCURRENT_JOBS` environment variable can be used to scale
+  the number of worker threads NVC creates based on the number of
+  concurrently executing simulations.
 
 ## Version 1.12.2 - 2024-05-15
 - Fixed a crash when `'transaction` is used with a record type.

nvc.1

@@ -1162,7 +1162,15 @@ arguments.  Additionally foreign subprograms must not retain any
 pointers passed as arguments after the subprogram returns.  Violating
 these rules will result in unpredictable and hard to debug behaviour.
 .Sh ENVIRONMENT
-.Bl -tag -width "NVC_COLORS"
+.Bl -tag -width "NVC_CONCURRENT_JOBS"
+.It Ev NVC_CONCURRENT_JOBS
+Provides a hint for the number of concurrently executing simulations.
+This allows
+.Nm
+to scale its worker thread count to avoid overloading the system.
+This is set automatically by frameworks such as VUnit.
+See
+.Ev NVC_MAX_THREADS .
 .It Ev NVC_COLORS
 Controls whether
 .Nm
@@ -1175,6 +1183,12 @@ and
 which enables colour if stdout is connected to a terminal.
 The default is
 .Cm auto .
+.It Ev NVC_MAX_THREADS
+Limit the number of worker threads
+.Nm
+can create.
+The default is either eight or the number of available CPUs, whichever
+is smaller.
 .El
 .\" .Sh FILES
 .\" .Sh EXIT STATUS

src/thread.c

@@ -23,6 +23,7 @@
 
 #include <assert.h>
 #include <errno.h>
+#include <math.h>
 #include <signal.h>
 #include <stdlib.h>
 #include <string.h>
@@ -498,11 +499,22 @@ void thread_init(void)
 
    assert(my_thread->id == 0);
 
-   const char *env = getenv("NVC_MAX_THREADS");
-   if (env != NULL)
-      max_workers = MAX(1, MIN(atoi(env), MAX_THREADS));
+   const char *max_env = getenv("NVC_MAX_THREADS");
+   if (max_env != NULL)
+      max_workers = MAX(1, MIN(atoi(max_env), MAX_THREADS));
    else
-      max_workers = MIN(nvc_nprocs(), DEFAULT_THREADS);
+      max_workers = DEFAULT_THREADS;
+
+   const int num_cpus = nvc_nprocs();
+   max_workers = MIN(num_cpus, max_workers);
+
+   const char *jobs_env = getenv("NVC_CONCURRENT_JOBS");
+   if (jobs_env != NULL) {
+      const int num_jobs = MAX(1, atoi(jobs_env));
+      const int limit = (int)round((double)num_cpus / (double)num_jobs);
+      max_workers = MAX(1, MIN(max_workers, limit));
+   }
+
    assert(max_workers > 0);
 
 #ifdef DEBUG