Increased process "memory usage" after upgrading to SLES 12 SP2

This document (7021290) is provided subject to the disclaimer at the end of this document.

Environment

SUSE Linux Enterprise Server 12 Service Pack 2 (SLES 12 SP2)

Situation

A customer observed that the recorded process usage of some of their applications increased up to 200 % after upgrading from SLES 12 GA/SP0 to SLES 12 SP2.

Resolution

echo "4096" >  /sys/kernel/debug/fault_around_bytes

Cause

When using the code from a shared library, the code that gets executed will create a virtual-> physical mapping of the pages the code "touches".
A page in X86 is 4 kilobytes/4096. If only one small function in a library in SP0 [that does not have "do_fault_around"] is used, that is how much RSS that library will get charged.

For SP2, that same function call will map 64 kilobytes/65536, and this will be reflected in Resident Set Size (RSS) and Proportional Set Size (PSS).
The values can be found in /proc/<pid>/smaps.
Note though, that the physical pages are already present in the "page cache", so there is no difference in the actual memory usage here.

The file /sys/kernel/debug/fault_around_bytes does not exist on SLES 12 GA, but on SP2 it exists with a default value of 65536.

In the end the customer was able to restore the memory use characteristics/stats of SP0 on SP2 by
echo 4096 > /sys/kernel/debug/fault_around_bytes

Disclaimer

This Support Knowledgebase provides a valuable tool for SUSE customers and parties interested in our products and solutions to acquire information, ideas and learn from one another. Materials are provided for informational, personal or non-commercial use within your organization and are presented "AS IS" WITHOUT WARRANTY OF ANY KIND.

  • Document ID:7021290
  • Creation Date: 05-Sep-2017
  • Modified Date:03-Mar-2020
    • SUSE Linux Enterprise Server

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