From c093f985e170e53b64a6239f459e4fd56a514da8 Mon Sep 17 00:00:00 2001
From: Holger Vogt <holger.vogt@uni-due.de>
Date: Wed, 11 Feb 2026 20:15:57 +0100
Subject: [PATCH] Move setting up OMP from CKTsetup() to inp_readall(). Add a
 check for abvailable logical CPU cores, and use half of them as default.

---
 src/frontend/inpcom.c            | 24 ++++++++++++++++++++++++
 src/spicelib/analysis/cktsetup.c | 19 +------------------
 2 files changed, 25 insertions(+), 18 deletions(-)

diff --git a/src/frontend/inpcom.c b/src/frontend/inpcom.c
index a0d30f249..75a3f3b07 100644
--- a/src/frontend/inpcom.c
+++ b/src/frontend/inpcom.c
@@ -58,6 +58,11 @@ Author: 1985 Wayne A. Christopher
 /* gtri - end - 12/12/90 */
 #endif
 
+#ifdef USE_OMP
+#include <omp.h>
+#include "ngspice/cpextern.h"
+#endif
+
 /* SJB - Uncomment this line for debug tracing */
 /*#define TRACE*/
 
@@ -73,6 +78,10 @@ Author: 1985 Wayne A. Christopher
 
 #define VALIDCHARS "!$%_#?@.[]&"
 
+#ifdef USE_OMP
+int nthreads = 4;
+#endif
+
 static struct library {
     char *realpath;
     char *habitat;
@@ -1088,6 +1097,21 @@ struct card *inp_readall(FILE *fp, const char *dir_name, const char* file_name,
         struct card *tmp_ptr1;
         struct names *subckt_w_params = new_names();
 
+        /* set the number of threads used by ngspice via OpenMP */
+#ifdef USE_OMP
+        if (!cp_getvar("num_threads", CP_NUM, &nthreads, 0)) {
+            nthreads = omp_get_num_procs();
+            fprintf(stdout, "\nNote: %d logical cores detected, ", nthreads);
+            nthreads = (int)(nthreads / 2);
+            fprintf(stdout, "max %d threads are used by ngspice.\n", nthreads);
+            //        fprintf(stdout, "    (May be overriden by 'set num_threads = xx in .spiceinit.)\n\n", nthreads);
+        }
+        else
+            fprintf(stdout, "\nNote: Number of threads used by ngspice set to %d.\n\n", nthreads);
+
+        omp_set_num_threads(nthreads);
+#endif
+
         /* skip title line */
         struct card *working = cc->nextcard;
 
diff --git a/src/spicelib/analysis/cktsetup.c b/src/spicelib/analysis/cktsetup.c
index 985739c64..b0fd47957 100644
--- a/src/spicelib/analysis/cktsetup.c
+++ b/src/spicelib/analysis/cktsetup.c
@@ -20,11 +20,6 @@ Author: 1985 Thomas L. Quarles
 #include "ngspice/enh.h"
 #endif
 
-#ifdef USE_OMP
-#include <omp.h>
-#include "ngspice/cpextern.h"
-#endif
-
 #define CKALLOC(var,size,type) \
     if(size && ((var = TMALLOC(type, size)) == NULL)){\
             return(E_NOMEM);\
@@ -35,9 +30,7 @@ CKTsetup(CKTcircuit *ckt)
 {
     int i;
     int error;
-#ifdef USE_OMP
-    int nthreads = 2;
-#endif
+
 #ifdef XSPICE
  /* gtri - begin - Setup for adding rshunt option resistors */
     CKTnode *node;
@@ -86,16 +79,6 @@ CKTsetup(CKTcircuit *ckt)
 
     matrix = ckt->CKTmatrix;
 
-#ifdef USE_OMP
-    if (!cp_getvar("num_threads", CP_NUM, &nthreads, 0))
-        nthreads = 2;
-
-    omp_set_num_threads(nthreads);
-/*    if (nthreads == 1)
-      printf("OpenMP: %d thread is requested in ngspice\n", nthreads);
-    else
-      printf("OpenMP: %d threads are requested in ngspice\n", nthreads);*/
-#endif
 
 #ifdef HAS_PROGREP
     SetAnalyse("Device Setup", 0);