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I came across script that prints some memory informations and here is my output:

SYSTEM MEMORY INFORMATION:
mem_wire:        4133535744 (   3942MB) [ 12%] Wired: disabled for paging out
mem_active:  +     37191680 (     35MB) [  0%] Active: recently referenced
mem_inactive:+  19774816256 (  18858MB) [ 59%] Inactive: recently not referenced
mem_cache:   +    904966144 (    863MB) [  2%] Cached: almost avail. for allocation
mem_free:    +    216158208 (    206MB) [  0%] Free: fully available for allocation
mem_gap_vm:  +   8224043008 (   7843MB) [ 24%] Memory gap: UNKNOWN
-------------- ------------ ----------- ------
mem_all:     =  33290711040 (  31748MB) [100%] Total real memory managed
mem_gap_sys: +   1040580608 (    992MB)        Memory gap: Kernel?!
-------------- ------------ -----------
mem_phys:    =  34331291648 (  32740MB)        Total real memory available
mem_gap_hw:  +     28446720 (     27MB)        Memory gap: Segment Mappings?!
-------------- ------------ -----------
mem_hw:      =  34359738368 (  32768MB)        Total real memory installed

SYSTEM MEMORY SUMMARY:
mem_used:       13463797760 (  12840MB) [ 39%] Logically used memory
mem_avail:   +  20895940608 (  19927MB) [ 60%] Logically available memory
-------------- ------------ ----------- ------
mem_total:   =  34359738368 (  32768MB) [100%] Logically total memory

Can anyone tell me what mem_gap_vm is? And why is so big? Strange thing is mem_active too. It's too small, there is Postgres instance running with 8GB shared_buffers.

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What's the value of kern.sysv.shmall? –  David Schwartz Apr 11 '12 at 7:32
    
kern.ipc.shmall = 8804999168 –  JohnHawkins Apr 11 '12 at 7:37
    
I bet that's what it is. Your mention of Postgres shared buffers fits in perfectly with that. –  David Schwartz Apr 11 '12 at 7:57
    
I did reboot, and now active is ~4GB and gap_vm is ~300 KB. So dunno. Weird. –  JohnHawkins Apr 11 '12 at 8:24
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