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[vSRX] Memory utilization calculation and description of different types of memory



Article ID: KB32247 KB Last Updated: 09 Nov 2017Version: 1.0

This article provides the calculations for the memory fields shown in "show chassis routing-engine" on vSRX devices, and it also provides an explanation of the types of memory shown in "show system memory".


Explanation of 'show chassis routing-engine' memory fields

vSRX> show chassis routing-engine
Routing Engine status:
    Temperature                 0 degrees C / 32 degrees F
    CPU temperature             0 degrees C / 32 degrees F
    Total memory               977 MB Max   459 MB used ( 47 percent)
    Memory utilization          38 percent


The calculations for the memory fields are listed below. The left side of the equation depicts the field in 'show chassis routing-engine', whilst the right side of the equation ​depicts parameters from 'show system memory' output.

  • ​Total Memory (977MB) = [Total Memory (960MB) + Reserved Memory (17MB) ]

  • Memory Used (459MB) = [Total Memory (960MB) + Reserved Memory (17MB) ] – [Cache memory (428MB) ] -  [Free ​memory (90MB) ]

  • Memory utilization (381MB)* = [Total memory (977MB)  - Free Memory (90 MB) - cache  mem (429 MB) - inactive memory (77MB) ] ​
    Note: This parameter is not depicted in any of the command output, but is used to calculate Memory utilization %.

  • Memory utilization % (38%) = [ Memory utilized (381MB) * 100] / Total Memory (977 MB) ]

Note: Starting with Junos OS Release 15.1x49-D70 and Junos OS Release 17.3R1, there was a change in the method for calculating the memory utilization by a Routing Engine. The inactive memory is now subtracted from the total available memory. There is thus, a decrease in the reported value for used memory; as the inactive memory is now considered as free.

Explanation of types of memory in "show system memory"

vSRX> show system memory
System memory usage distribution:
       Total memory:  983500 Kbytes (100%)
    Reserved memory:   17844 Kbytes (  1%)

       Wired memory:   60564 Kbytes (  6%)
      Active memory:  293732 Kbytes ( 29%)
    Inactive memory:   78912 Kbytes (  8%)
       Cache memory:  439028 Kbytes ( 44%)
        Free memory:   92632 Kbytes (  9%)

Memory disk resident memory:    6924 Kbytes
VM-Kbytes(  %  ) Resident(  %  ) Map-name
  1045504(99.99)   555108(53.09) kernel map
   321884(30.79)    21612(06.71) kmem map
     4288(00.41)     4288(99.99) exec map
    16092(01.54)      832(05.17) pipe map
    71680(06.86)    70976(99.02) buffer map
    32768(03.13)    32768(99.99) pager map


The descriptions of the fields are listed below.

Total memory Actual memory available to the system at the time of system initialization.
Reserved memory Amount of total memory reserved for the system at the time of system initialization, but not managed by virtual memory sub-system.
Wired memory Amount of total memory wired in the system that never paged out.
Active memory Amount of total memory actively in use.
Inactive Amount of total memory in use, but not referenced recently
Cache Amount of total memory almost available for allocation.
Free Amount of total memory available for allocation.
Memory disk resident memory Memory used by the memory disks, but not accounted for in the kernel map. For example, swap-based memory disk.
VM-Kbytes Virtual memory used as per all virtual mappings in the kernel or process map and percentage thereof based on maximum addressable virtual memory as per any kernel, system, architecture, or resource limits.
Resident: Real memory used as per actual resident pages of all mappings in the kernel or process map and percentage thereof based on the total real memory available in the system
Map-name Name of the map
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