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

[Mathieu Desnoyers]

----- On Jul 29, 2019, at 2:55 PM, Mathieu Desnoyers mathieu.desnoyers at efficios.com wrote:

> ----- 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,


I'm also adding this missing bit for make dist:

--- 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

Thanks,

Mathieu

> 
> 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

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