[lttng-dev] [PATCH lttng-ust v2] Use MAP_POPULATE to reduce pagefault when available

[Mathieu Desnoyers]

----- On Jul 29, 2019, at 2:49 PM, Jonathan Rajotte jonathan.rajotte-julien at efficios.com wrote:

> 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.
> 
> Links: https://lists.lttng.org/pipermail/lttng-dev/2019-July/029116.html
>       https://lists.lttng.org/pipermail/lttng-dev/2019-July/029122.html

Next time please do:

Link: https://lists.lttng.org/pipermail/lttng-dev/2019-July/029116.html
Link: https://lists.lttng.org/pipermail/lttng-dev/2019-July/029122.html

I'll edit it on my side,

Thanks,

Mathieu

> 
> Tested-by: Yiteng Guo <guoyiteng at gmail.com>
> 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

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com