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

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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

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