How does CDM calculate memory usage on Linux

Document ID : KB000118534
Last Modified Date : 21/11/2018
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Question:
What formula does the cdm probe use to calculate memory usage?
Answer:
Cdm reads the proc/meminfo file to obtain the memory usage qos data.

for example
# cat /proc/meminfo
MemTotal:        8175444 kB
MemFree:          154404 kB
Buffers:               4 kB
Cached:          1978176 kB
SwapTotal:       8257532 kB
SwapFree:        2962720 kB
 

The probe uses the following formulas for the different qos metrics

 
QOS_MEMORY_TOTAL (Total Memory MB) = (MemTotal + SwapTotal) / 1024
QOS_PHYSICAL_MEMORY_TOTAL (Physical Memory MB) = MemTotal / 1024
QOS_MEMORY_SWAP (SwapUsed MB) = (SwapTotal - SwapFree) / 1024
QOS_MEMORY_USAGE (Total Memory Used MB) = (MemTotal - MemFree - Buffers - Cached + SwapUsed) /1024
QOS_MEMORY_PHYSICAL (Physical Memory Used MB) = (MemTotal - MemFree - Buffers - Cached) /1024
QOS_MEMORY_PERC_USAGE (Total Memory Used %) = (Total Memory Used/Total Memory) * 100
QOS_MEMORY_PHYSICAL_PERC (Physical Memory Used %) = (Physical Memory Used/Physical Memory) * 100

Since cdm v5.40 the QOS_MEMORY_PHYSICAL metric, by default, excludes the buffer and cache. (see KB000047141)

If you follow the resolution in KB000047141 to include the buffer and cache, the calculation for the metric is
QOS_MEMORY_PHYSICAL (Physical Memory Used MB) = (MemTotal - MemFree) /1024

 
Additional Information:
  • The meminfo file reports the memory in KB and the probes reports the QOS Metric in MB hence we divide by 1024 in the above formulas.
  • You will very likley see a small difference in the values in meminfo and the QOS metric. Memory usage is a constantly shifting value and the probe will have collected at a slightly different time to when you viewed the meminfo file.