From 6a9443d955ce742112c6c05ed68fc8c3d5b6c3e6 Mon Sep 17 00:00:00 2001
From: Wessel Dankers <wsl@fruit.je>
Date: Sun, 30 Oct 2022 18:06:51 +0100
Subject: [PATCH] Only use available CPUs

Not all online CPUs may be available for the current process,
especially when CPU affinity is involved. In such cases too many
threads will be created, which will unnecessarily compete for CPU
time.

Use sched_getaffinity() (if available) to determine the correct
number of threads to create.
---
 configure.ac |  2 +-
 src/cpu.c    | 23 +++++++++++++++++++++++
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/configure.ac b/configure.ac
index 9773993..2678f64 100644
--- a/configure.ac
+++ b/configure.ac
@@ -68,7 +68,7 @@ AC_SYS_LARGEFILE
 AC_FUNC_MALLOC
 AC_FUNC_REALLOC
 AC_FUNC_STRTOD
-AC_CHECK_FUNCS([memchr memmove memset strerror strtol])
+AC_CHECK_FUNCS([memchr memmove memset strerror strtol sched_getaffinity])
 AC_CHECK_HEADER([sys/endian.h],
                [
                  AC_CHECK_DECLS([htole64, le64toh], [], [], [
diff --git a/src/cpu.c b/src/cpu.c
index c0155ef..3d2e813 100644
--- a/src/cpu.c
+++ b/src/cpu.c
@@ -1,5 +1,28 @@
+#define _GNU_SOURCE
+
 #include <unistd.h>
 
+#include "config.h"
+
+#ifdef HAVE_SCHED_GETAFFINITY
+
+#include <sched.h>
+#include <stdio.h>
+
+size_t num_threads(void) {
+    cpu_set_t cpu_set;
+    CPU_ZERO(&cpu_set);
+
+    if (sched_getaffinity(0, sizeof cpu_set, &cpu_set) == -1)
+        return sysconf(_SC_NPROCESSORS_ONLN);
+    else
+        return CPU_COUNT(&cpu_set);
+}
+
+#else
+
 size_t num_threads(void) {
     return sysconf(_SC_NPROCESSORS_ONLN);
 }
+
+#endif