i have an CentOS 5.x server with OpenVZ virtualization and i see free ram is like 500Mb out of 24 GB

but im sure this do not means that these 23GB are fully used and not free for reuse...

So please how to discover RAM that can be used?

There is /proc/meminfo
MemTotal:     24624888 kB
MemFree:        687028 kB
Buffers:       1169556 kB
Cached:       11854044 kB
SwapCached:       1040 kB
Active:       11516564 kB
Inactive:      6498712 kB
HighTotal:           0 kB
HighFree:            0 kB
LowTotal:     24624888 kB
LowFree:        687028 kB
SwapTotal:    26738680 kB
SwapFree:     26731940 kB
Dirty:           14036 kB
Writeback:           0 kB
AnonPages:     4981876 kB
Mapped:         382912 kB
Slab:          5584068 kB
PageTables:     140284 kB
NFS_Unstable:        0 kB
Bounce:              0 kB
CommitLimit:  39051124 kB
Committed_AS: 18446744073546496164 kB
VmallocTotal: 34359738364 kB
VmallocUsed:    376292 kB
VmallocChunk: 34359354412 kB
HugePages_Total:     0
HugePages_Free:      0
HugePages_Rsvd:      0
Hugepagesize:     2048 kB

- 6GB right that amount of RAM that can be reused at anytime?

Here is mentioned that the line to look at when want to see real available memory is the one highlighted:

free -m
         total       used       free     shared    buffers     cached
Mem:         24047      20870       3177          0        893       9329
-/+ buffers/cache:      10647      13400
Swap:        26111          6      26105
So available for re-use is 13,4Gb ?

PS: to watch memory usage in real time, one can use "watch" like: watch -n 0.5 free -m

Third thing to see memory ussage on linux is "htop"

htop appears to support free -m because it shows like 10Gb in use and rest (24-10=13Gb) in cache (yellow) or free which i hope means that this can be re-used at anytime?