Writing a complex database server like PostgreSQL is not an easy task. Especially memory management is an important task, which needs special attention. Internally PostgreSQL makes use of so called “memory contexts”. The idea of a memory context is to organize memory in groups, which are organized hierarchically. The main advantage is that in case of an error, all relevant memory can be freed at once.

Understanding PostgreSQL memory contexts can be useful to solve a bunch of interesting support cases. Here is an example: Recently we have stumbled across a problem. A database server was constantly running out of memory and was finally killed by the OOM killer over and over again. Backend processes were constantly increasing memory consumption for non-obvious reasons. How can a problem like this be approached?

GDB comes to the rescue

GDB can come to the rescue and solve the riddle of memory consumption nicely. The basic procedure works as follows:

• Create a core dump of the process in question
• Come up with a GDB macro to debug memory
• Run the macro

The first part is actually quite simple. To extract a core dump of a running process we have to find out the process ID first:

[hs@jacqueline debug]$ ps ax | grep post
1987 pts/1 S 0:00 /usr/local/pg94/bin/postgres -D /tmp/db94
1989 ? Ss 0:00 postgres: checkpointer process
1990 ? Ss 0:00 postgres: writer process
1991 ? Ss 0:00 postgres: wal writer process
1992 ? Ss 0:00 postgres: autovacuum launcher process
1993 ? Ss 0:00 postgres: stats collector process
1999 ? Ss 0:00 postgres: hs test [local] idle
2004 pts/1 S+ 0:00 grep post

In this example the process ID of the process we want to inspect is 1999 (a simple, idle local backend).
Then it is time to create the core file. gcore can do exactly that for you:

[hs@jacqueline debug]$ gcore 1999
[Thread debugging using libthread_db enabled]
0x00000033e6ee98c2 in recv () from /lib64/libc.so.6
Saved corefile core.1999

The beauty here is that gcore is just a simple shell script calling some gdb magic internally. The result will be a core file we can then make use of:

[hs@jacqueline debug]$ ls -l
total 251220
-rw-rw-r-- 1 hs hs 257244232 Sep 2 09:23 core.1999

The harder part

Then comes the harder part: Writing the gdb macro to debug those memory contexts. gdb has a scripting language to handle that. Here is the code:

[hs@jacqueline debug]$ cat /tmp/debug/pg_gdb_marcos
define sum_context_blocks
set $context = $arg0
set $block = ((AllocSet) $context)->blocks
set $size = 0
while ($block)
set $size = $size + (((AllocBlock) $block)->endptr - ((char *) $block))
set $block = ((AllocBlock) $block)->next
printf "%s: %d\n",((MemoryContext)$context)->name, $size

define walk_contexts
set $parent_$arg0 = ($arg1)
set $indent_$arg0 = ($arg0)
set $i_$arg0 = $indent_$arg0
while ($i_$arg0)
printf " "
set $i_$arg0 = $i_$arg0 - 1
sum_context_blocks $parent_$arg0
set $child_$arg0 = ((MemoryContext) $parent_$arg0)->firstchild
set $indent_$arg0 = $indent_$arg0 + 1
while ($child_$arg0)
walk_contexts $indent_$arg0 $child_$arg0
set $child_$arg0 = ((MemoryContext) $child_$arg0)->nextchild

walk_contexts 0 TopMemoryContext

The last line in the file is the actual call executing the code just written. walk_contexts will go through those memory contexts starting at the TopMemoryContext.
To run the script the following line will be useful. The script can simply be piped into gdb. The result will list information about memory consumption:

[hs@jacqueline debug]$ gdb -c ./core.1999 /usr/local/pg94/bin/postgres < pg_gdb_marcos 
Loaded symbols for /lib64/libnss_files.so.2 
Core was generated by `postgres: hs test [local] idle '. 
#0 0x00000033e6ee98c2 in recv () from /lib64/libc.so.6 
Missing separate debuginfos, use: debuginfo-install glibc-2.12-1.132.el6_5.2.x86_64 
(gdb) >>>> > > >>>(gdb) (gdb) >>>> > > >>>>> > > >>(gdb) (gdb)
TopMemoryContext: 69936
MessageContext: 8192
Operator class cache: 8192
smgr relation table: 24576
TransactionAbortContext: 32768
Portal hash: 8192
PortalMemory: 0
Relcache by OID: 24576
CacheMemoryContext: 516096
pg_db_role_setting_databaseid_rol_index: 1024
pg_user_mapping_user_server_index: 1024
pg_user_mapping_oid_index: 1024


pg_class_oid_index: 1024
MdSmgr: 8192
ident parser context: 0
hba parser context: 3072
LOCALLOCK hash: 8192
Timezones: 83472
ErrorContext: 8192
(gdb) TopMemoryContext: 69936

The output is actually quite long so I decided to remove a couple of lines. What you see here is how memory contexts are organized and how much memory is in each memory context.
If you happen to see any context which uses insane amounts of memory, it will definitely bring you one step closer to finding the root cause of a memory related problem.