[Wien] Fragmented RAM slowdown of lapw1_mpi

[Laurence Marks]

I believe that I've finally managed to trace a generic slowdown of mainly
lapw1_mpi. It's been around since 2013 and is probably related to how files
are used in WIEN2k & NFS. Maybe someone knows a friendly OS person who can
provide some information.

The symptom is that lapw1 -p with mpi over a period of some days slows
down, with the Wall time increasing although the CPU time also increases a
little. It appears that this is related to fragmentation of the RAM, so
large matrices are starting to occupy disjointed locations in memory and
are therefore slower. A cure appears to be to run the builtin RAM
defragger, "echo 1 >/proc/sys/vm/compact_memory" [1]. This probably does
not need to be done more than every few days or perhaps only once a week.

Anyone come across anything similar? It may well have to do with how NFS &
Read/Write I/O is done.

[1] WARNING: do not attempt this unless you are quite linux experienced. An
alternative to check is to do "cat /proc/buddyinfo", looking up what that
information means (e.g.
https://www.uninformativ.de/blog/postings/2017-12-23/0/POSTING-en.html ).

-- 
Professor Laurence Marks
Department of Materials Science and Engineering
Northwestern University
www.numis.northwestern.edu
Corrosion in 4D: www.numis.northwestern.edu/MURI
Co-Editor, Acta Cryst A
"Research is to see what everybody else has seen, and to think what nobody
else has thought"
Albert Szent-Gyorgi
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