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[PATCH v2 3/6] util/oslib-posix: Don't create too many threads with smal


From: David Hildenbrand
Subject: [PATCH v2 3/6] util/oslib-posix: Don't create too many threads with small memory or little pages
Date: Thu, 22 Jul 2021 14:36:32 +0200

Let's limit the number of threads to something sane, especially that
- We don't have more threads than the number of pages we have
- We don't have threads that initialize small (< 64 MiB) memory

Signed-off-by: David Hildenbrand <david@redhat.com>
---
 util/oslib-posix.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/util/oslib-posix.c b/util/oslib-posix.c
index 2ae6c3aaab..a1d309d495 100644
--- a/util/oslib-posix.c
+++ b/util/oslib-posix.c
@@ -40,6 +40,7 @@
 #include <libgen.h>
 #include "qemu/cutils.h"
 #include "qemu/compiler.h"
+#include "qemu/units.h"
 
 #ifdef CONFIG_LINUX
 #include <sys/syscall.h>
@@ -529,7 +530,8 @@ static void *do_madv_populate_write_pages(void *arg)
     return NULL;
 }
 
-static inline int get_memset_num_threads(int smp_cpus)
+static inline int get_memset_num_threads(size_t hpagesize, size_t numpages,
+                                         int smp_cpus)
 {
     long host_procs = sysconf(_SC_NPROCESSORS_ONLN);
     int ret = 1;
@@ -537,6 +539,12 @@ static inline int get_memset_num_threads(int smp_cpus)
     if (host_procs > 0) {
         ret = MIN(MIN(host_procs, MAX_MEM_PREALLOC_THREAD_COUNT), smp_cpus);
     }
+
+    /* Especially with gigantic pages, don't create more threads than pages. */
+    ret = MIN(ret, numpages);
+    /* Don't start threads to prealloc comparatively little memory. */
+    ret = MIN(ret, MAX(1, hpagesize * numpages / (64 * MiB)));
+
     /* In case sysconf() fails, we fall back to single threaded */
     return ret;
 }
@@ -546,7 +554,7 @@ static bool touch_all_pages(char *area, size_t hpagesize, 
size_t numpages,
 {
     static gsize initialized = 0;
     MemsetContext context = {
-        .num_threads = get_memset_num_threads(smp_cpus),
+        .num_threads = get_memset_num_threads(hpagesize, numpages, smp_cpus),
     };
     size_t numpages_per_thread, leftover;
     void *(*touch_fn)(void *);
-- 
2.31.1




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