Public bug reported:

Mysql gradually consumes memory . The memory is never released back to OS . The 
easiest way to reproduce the issue is run few mysqldumps and observe memory 
usage. 
I used :
mysqldump DB1 > /dev/null &

About 5-6 times.
After dumps are completed mysql do not release memory and continue to consume 
even more memory. 
Mysqld process uses all system memory, then starts to use swap memory up to the 
point where the database is killed by OOM errors.  

here is memory usage after few mysqldumps commands :

ps -aux | grep mysql
mysql      607  0.7 49.0 1769788 1005184 ?     Ssl  Aug10  10:50 
/usr/sbin/mysqld

1GB on a system with 128MB innodb buffer pool. Performance schema does
not report any issues and states that myslq is taking only around 400MB
.

The issue seems to be related to the default memory allocation library malloc. 
Installing jemmalloc and forcing jemmalloc in mysql conf seems to resolve this 
issue .
I used the same method as described before and memory usage looks stable :

ps uax | grep mysql
mysql     1540  5.9 23.4 957244 481036 ?       Ssl  12:59   4:38 
/usr/sbin/mysqld


Same issue and workaround was reported on mysql bug forum by Machiel Richards .

lsb_release -rd
Description:    Ubuntu 16.04.2 LTS
Release:        16.04

mysql-server:
  Installed: 5.7.19-0ubuntu0.16.04.1
  Candidate: 5.7.19-0ubuntu0.16.04.1
  Version table:
 *** 5.7.19-0ubuntu0.16.04.1 500
        500 http://de.archive.ubuntu.com/ubuntu xenial-updates/main amd64 
Packages
        500 http://de.archive.ubuntu.com/ubuntu xenial-security/main amd64 
Packages
        100 /var/lib/dpkg/status
     5.7.11-0ubuntu6 500
        500 http://de.archive.ubuntu.com/ubuntu xenial/main amd64 Packages

** Affects: mysql-5.7 (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1710146

Title:
  mysql never realase memory

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