How to detect a memory leak?

First, make sure to have a temp folder available which has enough free space. The following commands create dumps which can be several GBs in size.

You can create a new tmp folder using the following command. You may want to change /tmp to a different filesystem with enough space

TMPDIR=$(mktemp -d -t -p /tmp)

Steps to find Memory Leak

1. Find out the PID of the process which causing memory leak (you can also use e.g. htop if available) and store it in a variable called pid

   ps -aux
   

2. Given that the PID is available in the variable pid, you can capture the memory consumption using /proc/$pid/smaps and save into some file like beforeMemInc.txt.

   cat /proc/$pid/smaps > $TMPDIR/beforeMemInc.txt
   

3. Wait some time for memory consumption to increase.

4. Capture /proc/$pid/smaps again and save it as afterMemInc.txt

   cat /proc/$pid/smaps > $TMPDIR/afterMemInc.txt
   

5. Find the difference between first smaps and 2nd smaps, e. g. with

   diff -u $TMPDIR/beforeMemInc.txt $TMPDIR/afterMemInc.txt
   

6. Note down the address range where memory got increased, for example:

      beforeMemInc.txt            afterMemInc.txt
   ---------------------------------------------------
   2b3289290000-2b3289343000   2b3289290000-2b3289343000  #ADDRESS
   Shared_Clean:    0 kB       Shared_Clean:    0 kB          
   Shared_Dirty:    0 kB       Shared_Dirty:    0 kB
   Private_Clean:   0 kB       Private_Clean:   0 kB
   Private_Dirty:  28 kB       Private_Dirty:  36 kB  
   Referenced:     28 kB       Referenced:     36 kB
   Anonymous:      28 kB       Anonymous:      36 kB  #INCREASE MEM
   AnonHugePages:   0 kB       AnonHugePages:   0 kB
   Swap:            0 kB       Swap:            0 kB
   KernelPageSize:  4 kB       KernelPageSize:  4 kB
   MMUPageSize:     4 kB       MMUPageSize:     4 kB
   Locked:          0 kB       Locked:          0 kB
   VmFlags: rd wr mr mw me ac  VmFlags: rd wr mr mw me ac
   

7. Use GDB to dump memory on running process or get the coredump using

   gcore -o $TMPDIR/process $PID
   

8. I used gdb on the running process to dump the memory to some file.

   cd $TMPDIR
   gdb -p $pid
   dump memory memory.dump 0x2b3289290000 0x2b3289343000
   

9. Now, use strings command or hexdump -C to print the memory.dump

   strings memory.dump
   

   From this you get readable information which helps you locate those strings in your source code.

10. Analyse your source to find the leak.

The drop_cache trick will not free memory, it will reset the cache. Uses ps command if you want to identify which processes uses the more memory.

For instance to monitor the list of the top 15 of resident memory users.

$ watch "ps --sort -rss -eo pid,pmem,rss,vsz,comm | head -16"
  PID %MEM   RSS    VSZ COMMAND
 2590 13.4 136892 825000 firefox
 1743 10.7 109020 300780 Xorg
 2067  8.5 86764 1118140 unity-2d-shell
 3307  4.1 42560 627780 unity-2d-spread
 2068  2.9 29904 617644 unity-2d-panel
 2092  2.5 25524 1291204 nautilus
 2457  1.9 20292 530276 gnome-terminal
 2351  1.9 20016 821488 unity-scope-vid
 2161  1.9 19476 531968 unity-panel-ser
 2034  1.7 18256 759716 gnome-settings-
 2074  1.5 16176 518016 nm-applet
 2273  1.5 15452 580416 unity-lens-vide
 2051  1.4 15112 524260 metacity
 2395  1.2 12836 407336 update-notifi

You could check also the shared memory reservation but you will only know who is the owner of the segments.

Pmap allocation:

$ ls -l /run/shm
total 272
-r-------- 1 ed      ed      67108904 Nov 29 18:17 pulse-shm-1884617860
-r-------- 1 lightdm lightdm 67108904 Nov 29 18:11 pulse-shm-2352897759
-r-------- 1 ed      ed      67108904 Nov 29 18:12 pulse-shm-3444873503
-r-------- 1 ed      ed      67108904 Nov 29 18:12 pulse-shm-3485341848
-r-------- 1 lightdm lightdm 67108904 Nov 29 18:11 pulse-shm-535843976
-r-------- 1 ed      ed      67108904 Nov 29 19:12 pulse-shm-789046959
-r-------- 1 ed      ed      67108904 Nov 29 18:38 pulse-shm-863909656

$ df /run/shm 
Filesystem     1K-blocks  Used Available Use% Mounted on
none              509332   272    509060   1% /run/shm

note that reserved allocations are much higher than real the allocated pages (df 'used')

System V allocations :

$ ipcs -m 

------ Shared Memory Segments --------
key        shmid      owner      perms      bytes      nattch     status      
0x00000000 294912     ed         700        122880     2          dest         
0x00000000 327681     ed         700        4823040    2          dest         
0x00000000 491522     ed         600        393216     2          dest         
0x00000000 589827     ed         700        4578120    2          dest         
0x00000000 425988     ed         700        27852      2          dest         
0x00000000 458757     ed         600        393216     2          dest         

Edit: Need to pass --sort -rss to ps to get the processes with the most memory usage, otherwise the process list is sorted increasing numerical and gives the processes with least memory usage.

memprof is a tool for profiling memory usage and finding memory leaks. It can generate a profile how much memory was allocated by each function in your program. Also, it can scan memory and find blocks that you’ve allocated but are no longer referenced anywhere.

memprof works by pre-loading a library to override the C library’s memory allocation functions and does not require you to recompile your program.

memprof

Source: Ubuntu Manual