[lttng-ust] Use MAP_POPULATE to reduce pagefault when available
Commit Message
Any ring buffer configuration bigger than PAGE_SIZE would result
in increase latency for the first tracepoint hit (1200ns) lending 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 exists 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 indicate 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.
[1] https://lists.lttng.org/pipermail/lttng-dev/2019-July/029122.html
Tested-by: Yiteng Guo <guoyiteng at gmail.com> [1]
Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien at efficios.com>
---
libringbuffer/mmap.h | 30 ++++++++++++++++++++++++++++++
libringbuffer/shm.c | 5 +++--
2 files changed, 33 insertions(+), 2 deletions(-)
create mode 100644 libringbuffer/mmap.h
Comments
----- On Jul 29, 2019, at 2:23 PM, Jonathan Rajotte jonathan.rajotte-julien at efficios.com wrote:
> Any ring buffer configuration bigger than PAGE_SIZE would result
> in increase latency for the first tracepoint hit (1200ns) lending on a
landing
> 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 exists on freebsd (MAP_PREFAULT_READ) but I do
exists -> exit
> not have access to a system to test it and ensure it does indeed results
> in the same effect. It mostly indicate that it prefaults for the
indicate -> indicates
> read case so I doubt it is the case.
>
> Default to using MAP_POPULATE on linux only for now. Support of
linux -> Linux
> prefaulting on other platforms will be added as needed.
>
> [1] https://lists.lttng.org/pipermail/lttng-dev/2019-July/029122.html
>
> Tested-by: Yiteng Guo <guoyiteng at gmail.com> [1]
I think you might want to add a # before [1] so automated tools don't
get confused here. But even better, just describe it has been tested
above in the commit message.
The rest of the patch looks good, can you re-submit with the commit message
changes ?
Thanks,
Mathieu
> Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien at efficios.com>
> ---
> libringbuffer/mmap.h | 30 ++++++++++++++++++++++++++++++
> libringbuffer/shm.c | 5 +++--
> 2 files changed, 33 insertions(+), 2 deletions(-)
> create mode 100644 libringbuffer/mmap.h
>
> diff --git a/libringbuffer/mmap.h b/libringbuffer/mmap.h
> new file mode 100644
> index 00000000..f195c152
> --- /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 at 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 10b3bcef..909991ed 100644
> --- 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;
> --
> 2.17.1
nits:
On 2019-07-29 2:32 p.m., Mathieu Desnoyers wrote:
>> A similar flag seems to exists on freebsd (MAP_PREFAULT_READ) but I do
>
> exists -> exit
exist ?
>> [1] https://lists.lttng.org/pipermail/lttng-dev/2019-July/029122.html
>>
>> Tested-by: Yiteng Guo <guoyiteng at gmail.com> [1]
>
> I think you might want to add a # before [1] so automated tools don't
> get confused here. But even better, just describe it has been tested
> above in the commit message.
The "Link:" tag can be used for that.
On 2019-07-29 2:23 p.m., Jonathan Rajotte wrote:
> Any ring buffer configuration bigger than PAGE_SIZE would result
> in increase latency for the first tracepoint hit (1200ns) lending on a
in an increased latency
----- On Jul 29, 2019, at 2:41 PM, Sebastien Boisvert sboisvert at gydle.com wrote:
> nits:
>
> On 2019-07-29 2:32 p.m., Mathieu Desnoyers wrote:
>>> A similar flag seems to exists on freebsd (MAP_PREFAULT_READ) but I do
>>
>> exists -> exit
>
> exist ?
Indeed :)
>
>
>>> [1] https://lists.lttng.org/pipermail/lttng-dev/2019-July/029122.html
>>>
>>> Tested-by: Yiteng Guo <guoyiteng at gmail.com> [1]
>>
>> I think you might want to add a # before [1] so automated tools don't
>> get confused here. But even better, just describe it has been tested
>> above in the commit message.
>
> The "Link:" tag can be used for that.
Yes, good point!
Thanks,
Mathieu
new file mode 100644
@@ -0,0 +1,30 @@
+#ifndef _LTTNG_MMAP_H
+#define _LTTNG_MMAP_H
+
+/*
+ * Copyright (c) 2019 - Jonathan Rajotte <jonathan.rajotte-julien at 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 */
@@ -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;