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Jemalloc memory allocation profiling

Implemented in Percona Server for MySQL 8.0.25-15, Percona Server for MySQL can take advantage of the memory-profiling ability of the jemalloc allocator. This ability provides a method to investigate memory-related issues.

Requirements

This memory-profiling requires jemalloc_detected. This read-only variable returns true if jemalloc with the profiling-enabled option is being used by Percona Server for MySQL.

As root, customize jemalloc with the following flags:

Option Description
–enable-stats Enables statistics-gathering ability
–enable-prof Enables heap profiling and the ability to detect leaks.

Using LD_PRELOAD. Build the library, configure the malloc configuration with the prof:true string, and then use LD_PRELOAD to preload the libjemalloc.so library. The libprocess MemoryProfiler class detects the library automatically and enables the profiling support.

The following is an example of the required commands:

./configure --enable-stats --enable-prof && make && make install
MALLOC_CONF=prof:true
LD_PRELOAD=/usr/lib/libjemalloc.so

Use Percona Server for MySQL with jemalloc with profiling enabled

To detect if jemalloc is set, run the following command:

SELECT @@jemalloc_detected;

To enable jemalloc profiling in a MySQL client, run the following command:

set global jemalloc_profiling=on;

The malloc_stats_totals table returns the statistics, in bytes, of the memory usage. The command takes no parameters and returns the results as a table.

The following example commands display this result:

use performance_schema;

SELECT * FROM malloc_stats_totals;
Expected output 
+----+------------+------------+------------+-------------+------------+
| id | ALLOCATION | MAPPED     | RESIDENT   | RETAINED    | METADATA   |
+----+------------+------------+------------+-------------+------------+
|  1 | 390977528  | 405291008  | 520167424  | 436813824   | 9933744    |
+----+------------+------------+------------+-------------+------------+
1 row in set (0.00 sec)

The malloc_stats table returns the cumulative totals, in bytes, of several statistics per type of arena. The command takes no parameters and returns the results as a table.

The following example commands display this result:

use performance_schema;

mysql> SELECT * FROM malloc_stats ORDER BY TYPE DESC LIMIT 3;
Expected output 
+--------+-------------+-------------+-------------+-------------+
| TYPE   | ALLOCATED   | NMALLOC     | NDALLOC     | NRESQUESTS  |
+--------+-------------+-------------+-------------+-------------+
| small  | 23578872    | 586156      | 0           | 2649417     |
| large  | 367382528   | 2218        | 0           | 6355        |
| huge   | 0           | 0           | 0           | |
+--------+-------------+-------------+-------------+-------------+
3 rows in set (0.00 sec)

Dumping the profile

The profiling samples the malloc() calls and stores the sampled stack traces in a separate location in memory. These samples can be dumped into the filesystem. A dump returns a detailed view of the state of the memory.

The process is global; therefore, only a single concurrent run is available and only the most recent runs are stored on disk.

Use the following command to create a profile dump file:

flush memory profile;

The generated memory profile dumps are written to the /tmp directory.

You can analyze the dump files with jeprof program, which must be installed on the host system in the appropriate path. This program is a perl script that post-processes the dump files in their raw format. The program has no connection to the jemalloc library and the version numbers are not required to match.

To verify the dump, run the following command:

ls /tmp/jeprof_mysqld*
/tmp/jeprof_mysqld.1.0.170013202213
jeprof --show_bytes /tmp/jeprof_mysqld.1.0.170013202213 jeprof.*.heap

You can also access the memory profile to plot a graph of the memory use. This ability requires that jeprof and dot are in the /tmp path. For the graph to display useful information, the binary file must contain symbol information.

Run the following command:

jeprof --dot /usr/sbin/mysqld /tmp/jeprof_mysqld.1.0.170013202213 > /tmp/jeprof1.dot
dot --Tpng /tmp/jeprof1.dot > /tmp/jeprof1.png

Note

An example of allocation graph.

PERFORMANCE_SCHEMA tables

In 8.0.25.14, the following tables are implemented to retrieve memory allocation statistics for a running instance or return the cumulative number of allocations requested or allocations returned for a running instance.

More information about the stats that are returned can be found in jemalloc.

malloc_stats_totals

The current stats for allocations. All measurements are in bytes.

Column Name Description
ALLOCATED The total amount the application allocated
ACTIVE The total amount allocated by the application of active pages. A multiple of the page size and this value is greater than or equal to the stats.allocated value. The sum does not include allocator metadata pages and stats.arenas.<i>.pdirty or stats.arenas.<i>.pmuzzy.
MAPPED The total amount in chunks that are mapped by the allocator in active extents. This value does not include inactive chunks. The value is at least as large as the stats.active and is a multiple of the chunk size.
RESIDENT A maximum number the allocator has mapped in physically resident data pages. All allocator metadata pages and unused dirty pages are included in this value. Pages may not be physically resident if they correspond to demand-zeroed virtual memory that has not yet been touched. This value is a maximum rather than a precise value and is a multiple of the page size. The value is greater than the stats.active.
RETAINED The amount retained by the virtual memory mappings of the operating system. This value does not include any returned mappings. This type of memory, usually de-committed, untouched, or purged. The value is associated with physical memory and is excluded from mapped memory statistics.
METADATA The total amount dedicated to metadata. This value contains the base allocations which are used for bootstrap-sensitive allocator metadata structures. Transparent huge pages usage is not included.

malloc_stats

The cumulative number of allocations requested or allocations returned for a running instance.

Column Name Description
Type The type of object: small, large, and huge
ALLOCATED The number of bytes that are currently allocated to the application.
NMALLOC A cumulative number of times an allocation was requested from the arena’s bins. The number includes times when the allocation satisfied an allocation request or filled a relevant tcache if opt.tcache is enabled.
NDALLOC A cumulative number of times an allocation was returned to the arena’s bins. The number includes times when the allocation was deallocated or flushed the relevant tcache if opt.tcache is enabled.
NREQUESTS The cumulative number of allocation requests satisfied.

System variables

The following variables have been added:

jemalloc_detected

Description: This read-only variable returns true if jemalloc with profiling enabled is detected. The following options are required:

  • Jemalloc is installed and compiled with profiling enabled

  • Percona Server for MySQL is configured to use jemalloc by using the environment variable LD_PRELOAD.

  • The environment variable MALLOC_CONF is set to prof:true.

The following options are:

  • Scope: Global

  • Variable Type: Boolean

  • Default Value: false

jemalloc_profiling

Description: Enables jemalloc profiling. The variable requires jemalloc_detected.

  • Command Line: –jemalloc_profiling[=(OFF|ON)]

  • Config File: Yes

  • Scope: Global

  • Dynamic: Yes

  • Variable Type: Boolean

  • Default Value: OFF

Disable profiling

To disable jemalloc profiling, in a MySQL client, run the following command:

set global jemalloc_profiling=off;