Use MAP_POPULATE to reduce pagefault when available

Any ring buffer configuration bigger than PAGE_SIZE would result
in an increased latency for the first tracepoint hit (1200ns) landing on a
new PAGE_SIZE sized chunk of the mapped memory. This happens at least
for the first ring buffer traversal.

To alleviate this we can use MAP_POPULATE that will "prefault" the page
tables.

A similar flag seems to exist on freebsd (MAP_PREFAULT_READ) but I do
not have access to a system to test it and ensure it does indeed results
in the same effect. It mostly indicates that it prefaults for the
read case so I doubt it is the case.

Default to using MAP_POPULATE on Linux only for now. Support of
prefaulting on other platforms will be added as needed.

Link: https://lists.lttng.org/pipermail/lttng-dev/2019-July/029116.html
Link: https://lists.lttng.org/pipermail/lttng-dev/2019-July/029122.html
Tested-by: Yiteng Guo <guoyiteng@gmail.com>
Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

diff --git a/libringbuffer/Makefile.am b/libringbuffer/Makefile.am
index c153ea250415a7ac7a62e004a5c72362942c2b19..5c48dc0661d5f8cf48fb41f00fcd1ddbed6e2e41 100644 (file)
--- a/libringbuffer/Makefile.am
+++ b/libringbuffer/Makefile.am
@@ -8,7 +8,7 @@ libringbuffer_la_SOURCES = \
        shm.c shm.h shm_types.h shm_internal.h \
        ring_buffer_backend.c \
        ring_buffer_frontend.c \
-       api.h \
+       api.h mmap.h \
        backend.h backend_internal.h backend_types.h \
        frontend_api.h frontend.h frontend_internal.h frontend_types.h \
        nohz.h vatomic.h rb-init.h

diff --git a/libringbuffer/mmap.h b/libringbuffer/mmap.h
new file mode 100644 (file)
index 0000000..f195c15
--- /dev/null
+++ b/libringbuffer/mmap.h
@@ -0,0 +1,30 @@
+#ifndef _LTTNG_MMAP_H
+#define _LTTNG_MMAP_H
+
+/*
+ * Copyright (c) 2019 - Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; only
+ * version 2.1 of the License.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#include <sys/mman.h>
+
+#if defined(__linux__) && defined(MAP_POPULATE)
+# define LTTNG_MAP_POPULATE MAP_POPULATE
+#else
+# define LTTNG_MAP_POPULATE 0
+#endif /* __linux__ && MAP_POPULATE */
+
+#endif /* _LTTNG_MMAP_H */

diff --git a/libringbuffer/shm.c b/libringbuffer/shm.c
index 10b3bcef110a979ce2169ccf10417e470e122ba9..909991ed27c6f1ada4a04426e396d232d126df8e 100644 (file)
--- a/libringbuffer/shm.c
+++ b/libringbuffer/shm.c
@@ -40,6 +40,7 @@
 #endif
 #include <helper.h>
 #include <ust-fd.h>
+#include "mmap.h"
 
 /*
  * Ensure we have the required amount of space available by writing 0
@@ -154,7 +155,7 @@ struct shm_object *_shm_object_table_alloc_shm(struct shm_object_table *table,
 
        /* memory_map: mmap */
        memory_map = mmap(NULL, memory_map_size, PROT_READ | PROT_WRITE,
-                         MAP_SHARED, shmfd, 0);
+                         MAP_SHARED | LTTNG_MAP_POPULATE, shmfd, 0);
        if (memory_map == MAP_FAILED) {
                PERROR("mmap");
                goto error_mmap;
@@ -341,7 +342,7 @@ struct shm_object *shm_object_table_append_shm(struct shm_object_table *table,
 
        /* memory_map: mmap */
        memory_map = mmap(NULL, memory_map_size, PROT_READ | PROT_WRITE,
-                         MAP_SHARED, shm_fd, 0);
+                         MAP_SHARED | LTTNG_MAP_POPULATE, shm_fd, 0);
        if (memory_map == MAP_FAILED) {
                PERROR("mmap");
                goto error_mmap;