29

I run free -m on a debian VM running on Hyper-V:

             total       used       free     shared    buffers     cached
Mem:         10017       9475        541        147         34        909
-/+ buffers/cache:       8531       1485
Swap:         1905          0       1905

So out of my 10GB of memory, 8.5GB is in use and only 1500MB is free (excluding cache).

But I struggle to find what is using the memory. The output of ps aux | awk '{sum+=$6} END {print sum / 1024}', which is supposed to add up the RSS utilisation is:

1005.2

In other words, my processes only use 1GB of memory but the system as a whole (excluding cache) uses 8.5GB.

What could be using the other 7.5GB?

ps: I have another server with a similar configuration that shows used mem of 1200 (free mem = 8.8GB) and the sum of RSS usage in ps is 900 which is closer to what I would expect...

EDIT

cat /proc/meminfo on machine 1 (low memory):

MemTotal:       10257656 kB
MemFree:          395840 kB
MemAvailable:    1428508 kB
Buffers:          162640 kB
Cached:          1173040 kB
SwapCached:          176 kB
Active:          1810200 kB
Inactive:         476668 kB
Active(anon):     942816 kB
Inactive(anon):   176184 kB
Active(file):     867384 kB
Inactive(file):   300484 kB
Unevictable:           0 kB
Mlocked:               0 kB
SwapTotal:       1951740 kB
SwapFree:        1951528 kB
Dirty:                16 kB
Writeback:             0 kB
AnonPages:        951016 kB
Mapped:           224388 kB
Shmem:            167820 kB
Slab:              86464 kB
SReclaimable:      67488 kB
SUnreclaim:        18976 kB
KernelStack:        6736 kB
PageTables:        13728 kB
NFS_Unstable:          0 kB
Bounce:                0 kB
WritebackTmp:          0 kB
CommitLimit:     7080568 kB
Committed_AS:    1893156 kB
VmallocTotal:   34359738367 kB
VmallocUsed:       62284 kB
VmallocChunk:   34359672552 kB
HardwareCorrupted:     0 kB
AnonHugePages:         0 kB
HugePages_Total:       0
HugePages_Free:        0
HugePages_Rsvd:        0
HugePages_Surp:        0
Hugepagesize:       2048 kB
DirectMap4k:       67520 kB
DirectMap2M:    10418176 kB

cat /proc/meminfo on machine 2 (normal memory usage):

MemTotal:       12326128 kB
MemFree:         8895188 kB
MemAvailable:   10947592 kB
Buffers:          191548 kB
Cached:          2188088 kB
SwapCached:            0 kB
Active:          2890128 kB
Inactive:         350360 kB
Active(anon):    1018116 kB
Inactive(anon):    33320 kB
Active(file):    1872012 kB
Inactive(file):   317040 kB
Unevictable:           0 kB
Mlocked:               0 kB
SwapTotal:       3442684 kB
SwapFree:        3442684 kB
Dirty:                44 kB
Writeback:             0 kB
AnonPages:        860880 kB
Mapped:           204680 kB
Shmem:            190588 kB
Slab:              86812 kB
SReclaimable:      64556 kB
SUnreclaim:        22256 kB
KernelStack:       10576 kB
PageTables:        11924 kB
NFS_Unstable:          0 kB
Bounce:                0 kB
WritebackTmp:          0 kB
CommitLimit:     9605748 kB
Committed_AS:    1753476 kB
VmallocTotal:   34359738367 kB
VmallocUsed:       62708 kB
VmallocChunk:   34359671804 kB
HardwareCorrupted:     0 kB
AnonHugePages:         0 kB
HugePages_Total:       0
HugePages_Free:        0
HugePages_Rsvd:        0
HugePages_Surp:        0
Hugepagesize:       2048 kB
DirectMap4k:       63424 kB
DirectMap2M:    12519424 kB
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2 Answers 2

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23
+50

I understand you're using Hyper-V, but the concepts are similar. Maybe this will set you on the right track.

Your issue is likely due to virtual memory ballooning, a technique the hypervisor uses to optimize memory. See this link for a description

I observed your exact same symptoms with my VMs in vSphere. A 4G machine with nothing running on it would report 30M used by cache, but over 3G "used" in the "-/+ buffers" line.

Here's sample output from VMWare's statistics command. This shows how close to 3G is being tacked on to my "used" amount:

vmware-toolbox-cmd stat balloon
3264 MB

In my case, somewhat obviously, my balloon driver was using ~3G

I'm not sure what the similar command in Hyper-V is to get your balloon stats, but I'm sure you'll get similar results

Improve this answer
5
  • Thanks - you are definitely onto something. lsmod | grep hv_ shows hv_balloon on the low memory machine but not on the other - so the balloon module is loaded on one and not the other. And the behaviour looks very much like this description.
    – assylias
    Commented Feb 11, 2016 at 5:06
  • Not sure what the equivalent to vmware-toolbox-cmd is on Hyper V though.
    – assylias
    Commented Feb 11, 2016 at 5:06
  • @assylias I know, sorry. I looked myself while writing this answer and came up empty. However, if you write a program that quickly allocates a lot of memory, that may convince the hypervisor that your VM needs the resources. Similar to disk cache eviction test case, but different root cause.
    – Matt
    Commented Feb 11, 2016 at 13:20
  • You can unset the dynamic ram feature in Hyper-V to solve this issue.
    – Ashish Negi
    Commented Aug 23, 2017 at 11:06
  • 3
    I don't really see the solution here I'm afraid.
    – Jamie Hutber
    Commented Nov 2, 2018 at 11:51
1

https://serverfault.com/questions/85470/meaning-of-the-buffers-cache-line-in-the-output-of-free

Short answer: the kernel uses the buffers/cache memory for various tasks, such as caching files. This memory is available to applications if it is needed, so you are correct in saying you have 944 MB used.

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6
  • According to that link, 944MB is the amount of cache
    – assylias
    Commented Feb 3, 2016 at 19:15
  • 2
    No, 944MB is the amount of RAM actually in use by applications and not available to other applications. Reread that post: "Linux (like most modern OS) will always try to use free RAM for caching stuff, so Mem: free will almost always be very low. Therefore the line -/+ buffers/cache: is shown, because it shows how much memory is free when ignoring caches; caches will be freed automatically if memory gets scarce, so they do not really matter."
    – vik
    Commented Feb 4, 2016 at 19:33
  • yes and the -/+ buffers/cache shows 1.5GB free mem...
    – assylias
    Commented Feb 4, 2016 at 19:58
  • Please understand the 1485 free in the -/+ buffers cache is NOT the amount of memory available for applications on the system. The amount of memory actually available to your applications is: (10017 - (9475 - 8531)) = 9073. Does this make sense?
    – vik
    Commented Feb 4, 2016 at 20:13
  • I think you are wrong: the second line excludes the cache & buffers and is the actual memory utilisation of the applications (and that's what your link says...).
    – assylias
    Commented Feb 4, 2016 at 20:39

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