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I am booting a disk image via PXE, it is a full system on an initrd ramdisk. I would like to measure the actual free memory of the system (not including buffers and file system caching). I was told this is done by adding the MemFree, Buffers and Cached values from /proc/meminfo. For a regular system this gives accurate results.

However with my ramdisk setup it does not give accurate results. I assume that the memory that is used by the ramdisk is added to the "Cached" value:

[root@node1 ~]# cat /proc/meminfo
MemTotal:        8173700 kB
MemFree:         7443696 kB
Buffers:               0 kB
Cached:           650236 kB
SwapCached:            0 kB
MemCommitted:          0 kB
VirtualSwap:           0 kB
Active:            27156 kB
Inactive:         632204 kB
Active(anon):       4636 kB
Inactive(anon):     4616 kB
Active(file):      22520 kB
Inactive(file):   627588 kB
Unevictable:           0 kB
Mlocked:               0 kB
SwapTotal:             0 kB
SwapFree:              0 kB
Dirty:                 0 kB
Writeback:             0 kB
AnonPages:          9156 kB
Mapped:             8712 kB
Shmem:               128 kB
Slab:              38548 kB
SReclaimable:      29420 kB
SUnreclaim:         9128 kB
KernelStack:         536 kB
PageTables:         1268 kB
NFS_Unstable:          0 kB
Bounce:                0 kB
WritebackTmp:          0 kB
CommitLimit:     4086848 kB
Committed_AS:      51216 kB
VmallocTotal:   34359738367 kB
VmallocUsed:      288032 kB
VmallocChunk:   34359448036 kB
HardwareCorrupted:     0 kB
AnonHugePages:         0 kB
HugePages_Total:       0
HugePages_Free:        0
HugePages_Rsvd:        0
HugePages_Surp:        0
Hugepagesize:       2048 kB
DirectMap4k:       10240 kB 
DirectMap2M:     8378368 kB

Now this unfortunately gives me incorrect values for two metrics I would like to calculate: %MemUsed and AmountMemFreeActual. With a Ramdisk the "Cached" value is at least in part unreclaimable, and therefore cannot be used to calculate the actual Free memory.

To calculate both correctly, I would need to know the memory usage of the ramdisk, and i do not think I can simply take the "Cached" value, since this might also include other Cached Filesystem data.

Preferably I would like to take the ramdisk memory use out of the picture for both metrics, since it is unreclaimable memory AFAIK. Any ideas where to look?

Edit: "free" (@answer1) does provide the same wrong values. The "free" value from the "free" output is misleading anyways. "Free" memory is usually "Free"+"Buffers"+"Cached". My problem is that the ramdisk as non reclaimable memory (unlike usually cached filesystem data) cannot be reclaimed.

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dfens123
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  • you might have to manually subtract the usage of any tmpfs - `df -t tmpfs` (e.g. if you are using an initramfs) - and any old-style ramdisk devices (e.g. initrd). At least when running an old-style initrd, I expect you can just look at the size of the ramdisk: `blockdev --getsize64 /dev/ram0`. – sourcejedi Oct 25 '18 at 15:09

1 Answers1

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Is there a reason you can't use the free command to determine how much RAM is available?

# free output in MBs
$ free -m
             total       used       free     shared    buffers     cached
Mem:          7800       6724       1075          0        397       1952
-/+ buffers/cache:       4374       3425
Swap:         5823         27       5796

The above shows that without the buffers and cache there's ~3.4GB free.

slm
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    thats correct. The Problem with that 3.4 GB `free` calculation is the assumption that `cached` and `buffered` is reclaimable memory. With initramfs that assumption is wrong concerning `cached`, thus the `free` calculation `+/- buffers/cache` is incorrect/meaningless. – dfens123 Jun 05 '13 at 13:44