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Memory usage calculation for a host depicted in corresponding Grid Available Memory graph

Question & Answer


Question

Why is the available memory reported in graph " - Grid Available Memory" different from the 'free' command output in linux?

Answer

The available memory reported by LSF is the sum of free, buffers and cached from 'free' command.

Some of the memory in the system is used for disk cache, i.e. memory temporarily used by operating system to speed up data access by keeping data in memory and therefore avoiding reads from disk every time. That memory will be made available to an application if the reported free memory runs out. That is why memory reserved by disk cache is also counted for free memory by LSF.



For example,


# free -m
              total       used       free     shared    buffers     cached
Mem:          3961       3719        241          0        163        256
-/+ buffers/cache:       3300        661
Swap:         2047          1       2046

The actual free memory available to a system = (total memory - inuse memory), where inuse memory = (memory reported as used - (buffers + cached)). i.e. 3961 - (3716 - (163+256)) = 661M

Similar value is reported by lsload command of LSF.
# lsload
HOST_NAME    status  r15s   r1m  r15m   ut    pg  ls    it   tmp   swp   mem
hostname-1.   ok      0.4   0.3   0.6   5%   0.0   1     2   12G  1.9G  644M

Note: Above values reported by lsload, free, and "Cur:" in graph are similar, but not identical because it depends on the time of collection.

Also whether a system starts swapping even before free memory runs out depends on vm.swappiness. If it is not 0, a system may start swapping even before free memory runs out.
# sysctl vm.swappiness # (to display current value)
# sysctl -w vm.swappiness=0 # only use swap when we're actually out of available RAM
[{"Product":{"code":"SSZT2D","label":"IBM Spectrum LSF RTM"},"Business Unit":{"code":"BU059","label":"IBM Software w\/o TPS"},"Component":"Not Applicable","Platform":[{"code":"PF025","label":"Platform Independent"}],"Version":"10.1","Edition":"","Line of Business":{"code":"LOB10","label":"Data and AI"}},{"Product":{"code":"SSVMSD","label":"Platform RTM"},"Business Unit":{"code":"BU054","label":"Systems w\/TPS"},"Component":"Graphs","Platform":[{"code":"PF025","label":"Platform Independent"}],"Version":"Version Independent","Edition":"","Line of Business":{"code":"","label":""}}]

Document Information

More support for:
IBM Spectrum LSF RTM

Software version:
10.1

Document number:
630607

Modified date:
30 August 2019

UID

isg3T1024865

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