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I have a Fedora 30 [5.6.13-100.fc30.x86_64] machine with 32GB of RAM onboard. Right after boot both free -h and htop indicate that 21-22GB is used:

[ andriim@dev-r-vrt-007 ~/work ] $ free -h
              total        used        free      shared  buff/cache   available
Mem:           31Gi        22Gi       6.1Gi       2.0Mi       3.2Gi       8.8Gi
Swap:         2.0Gi       344Mi       1.6Gi

At the same time, all of the processes I can find in htop combined don't use nearly that much RAM. If I sort by resident memory usage, top 10 processes use around 300MB combined.

I tried finding the sum of RAM actually used by all processes and it also turned out to be nowhere close to 21GB:

[ root@dev-r-vrt-007 /tmp ] # ps aux | awk '{sum=sum+$6}; END {print sum/1024 " MB"}'
437.898 MB

Then I thought, maybe the issue is related to Kernel SLAB cache. That, however, doesn't seem to be the case either:

[ root@dev-r-vrt-007 ~ ] # slabtop -s c -o | head -n 12
 Active / Total Objects (% used)    : 3742847 / 3777379 (99.1%)
 Active / Total Slabs (% used)      : 75950 / 75950 (100.0%)
 Active / Total Caches (% used)     : 105 / 150 (70.0%)
 Active / Total Size (% used)       : 625537.34K / 637690.48K (98.1%)
 Minimum / Average / Maximum Object : 0.01K / 0.17K / 17.31K

  OBJS ACTIVE  USE OBJ SIZE  SLABS OBJ/SLAB CACHE SIZE NAME
 47552  47531  99%    2.00K   2972       16     95104K kmalloc-2k
1267328 1267252  99%    0.06K  19802       64     79208K vmap_area
593544 593484  99%    0.09K  14132       42     56528K kmalloc-96
 90376  87689  97%    0.59K   3476       26     55616K inode_cache
 95168  94892  99%    0.50K   2974       32     47584K kmalloc-512

Well, then, maybe HugePages are reserving the missing RAM? It seems they don't:

[ root@dev-r-vrt-007 ~ ] # cat /proc/meminfo | grep Huge
AnonHugePages:         0 kB
ShmemHugePages:        0 kB
FileHugePages:         0 kB
HugePages_Total:      16
HugePages_Free:       16
HugePages_Rsvd:        0
HugePages_Surp:        0
Hugepagesize:    1048576 kB
Hugetlb:        16777216 kB

I'm out of ideas. Any suggestions where else to look for the lost RAM?

Andy Mac
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  • Andy, would this question perhaps be better suited to SuperUser, our general computing sister site? – music2myear Oct 13 '20 at 17:40
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    What lost RAM? Where is the output from `free` or `htop`? – Michael Hampton Oct 13 '20 at 19:21
  • @music2myear I may have misplaced this question because this machine is a virtualization host. But thank you. – Andy Mac Oct 15 '20 at 07:19
  • @MichaelHampton I updated the post. However, I also solved the issue of missing RAM by disabling hugepages (which KVM machines couldn't use for some reason): ```echo 0 > /proc/sys/vm/nr_hugepages``` – Andy Mac Oct 15 '20 at 07:50

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