[Openmp-commits] [openmp] [OpenMP][VE] Limit the number of threads to create (PR #66729)

[Kazushi Marukawa via Openmp-commits]

https://github.com/kaz7 created https://github.com/llvm/llvm-project/pull/66729

VE supports up to 64 threads per a VE process.  So, we limit the number of threads defined by KMP_MAX_NTH.  We also modify the __kmp_sys_max_nth initialization to use KMP_MAX_NTH as a limit.

>From 75a5561ad769628b13ae4c10313a090cdec92278 Mon Sep 17 00:00:00 2001
From: "Kazushi (Jam) Marukawa" <marukawa at nec.com>
Date: Sat, 2 Sep 2023 02:39:09 +0200
Subject: [PATCH] [OpenMP][VE] Limit the number of threads to create

VE supports up to 64 threads per a VE process.  So, we limit the number
of threads defined by KMP_MAX_NTH.  We also modify the __kmp_sys_max_nth
initialization to use KMP_MAX_NTH as a limit.
---
 openmp/runtime/src/kmp.h            | 7 +++++++
 openmp/runtime/src/z_Linux_util.cpp | 8 ++++++++
 2 files changed, 15 insertions(+)

diff --git a/openmp/runtime/src/kmp.h b/openmp/runtime/src/kmp.h
index b931b7ba66416ec..f26311f28516539 100644
--- a/openmp/runtime/src/kmp.h
+++ b/openmp/runtime/src/kmp.h
@@ -1153,8 +1153,15 @@ extern void __kmp_init_target_task();
 #if defined(PTHREAD_THREADS_MAX) && PTHREAD_THREADS_MAX < INT_MAX
 #define KMP_MAX_NTH PTHREAD_THREADS_MAX
 #else
+#ifdef __ve__
+// VE's pthread supports only up to 64 threads per a VE process.
+//   https://sxauroratsubasa.sakura.ne.jp/documents/veos/en/VEOS_high_level_design.pdf
+//   p. 14 Maximum number of threads per VE process is 64.
+#define KMP_MAX_NTH 64
+#else
 #define KMP_MAX_NTH INT_MAX
 #endif
+#endif
 #endif /* KMP_MAX_NTH */
 
 #ifdef PTHREAD_STACK_MIN
diff --git a/openmp/runtime/src/z_Linux_util.cpp b/openmp/runtime/src/z_Linux_util.cpp
index 42488eb6be5dc87..478c09baa1beefe 100644
--- a/openmp/runtime/src/z_Linux_util.cpp
+++ b/openmp/runtime/src/z_Linux_util.cpp
@@ -1894,6 +1894,13 @@ void __kmp_runtime_initialize(void) {
 
     /* Query the maximum number of threads */
     __kmp_type_convert(sysconf(_SC_THREAD_THREADS_MAX), &(__kmp_sys_max_nth));
+#ifdef __ve__
+    if (__kmp_sys_max_nth == -1) {
+      // VE's pthread supports only up to 64 threads per a VE process.
+      // So we use that KMP_MAX_NTH (predefined as 64) here.
+      __kmp_sys_max_nth = KMP_MAX_NTH;
+    }
+#else
     if (__kmp_sys_max_nth == -1) {
       /* Unlimited threads for NPTL */
       __kmp_sys_max_nth = INT_MAX;
@@ -1901,6 +1908,7 @@ void __kmp_runtime_initialize(void) {
       /* Can't tell, just use PTHREAD_THREADS_MAX */
       __kmp_sys_max_nth = KMP_MAX_NTH;
     }
+#endif
 
     /* Query the minimum stack size */
     __kmp_sys_min_stksize = sysconf(_SC_THREAD_STACK_MIN);