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KB16829 - What is Swap Memory or Virtual Memory?

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Last Modified Date1/10/2017 12:32 AM
Synopsis
This articles provides details about what is swap and virtual memory.
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What is swap memory?

Swap Memory or Virtual Memory is the area on the PCS / PPS hard disk that stores the process memory image temporarily. When the physical memory demand is low, the process memory images are brought back to the physical memory from the swap memory on the hard disk. This mechanism allows the total number of processes running to exceed the physical memory.


Why is swap memory used?

Swap space is used only when the amount of physical memory (RAM) is full.  When swap or virtual memory begins to be heavily utilized, we will notice performance issues because the memory stored on hard disk have a slower access time as memory stored in RAM.  This must be transferred in and out of RAM to be processed.


When swap memory should be a concern?

  • High utilization of swap memory that never decreases may indicate memory leaks or other critical issues
  • Short period of swap memory does indicate heavy load scenarios, but does not necessary indicate a problem.
  • Once swap memory is utilized, a small amount of swap memory will remain consumed by certain processes and the system kernel.  To clear all memory cached, a system reboot may be required.
  • A slight usage of swap memory can be seen on devices running 8.2RX and above and is considered normal.


How to monitor swap memory?

Under PCS/PPS Admin GUI > Log/ Monitoring > SNMP, there is the option to set the SNMP trap threshold for "Swap Memory (Virtual Memory)". If this value is set to 0% (default value), then traps will be generated even for minor utilization of Swap Memory, so it is best to set it to higher values. Our recommendation is to set the value to 20% or 30%, based on the suitability.

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