How does gnome system monitor calculates the amount of used memory?

Question

How does gnome system monitor calculates the amount of used memory? I tend to trust the values displayed by the system monitor because are consistent and do agree with the amount of ram used when running a test an application. However when I compare those values with the numbers displayed in /proc/meminfo I see no connection at all. Currently on my laptop I have 3.5 GB of ram and the amount of used memory is shown to be 814 MB. On the other hand here is what /proc/meminfo shows:

MemTotal:        3715348 kB
MemFree:          923216 kB
Buffers:          154144 kB
Cached:          1804380 kB
SwapCached:            0 kB
Active:          1190432 kB
Inactive:        1331876 kB
Active(anon):     609064 kB
Inactive(anon):    83176 kB
Active(file):     581368 kB
Inactive(file):  1248700 kB
Unevictable:          32 kB
Mlocked:              32 kB
SwapTotal:       2104476 kB
SwapFree:        2104476 kB
Dirty:               132 kB
Writeback:             0 kB
AnonPages:        563820 kB
Mapped:           137208 kB
Shmem:            128456 kB
Slab:             108932 kB
SReclaimable:      77224 kB
SUnreclaim:        31708 kB
KernelStack:        3000 kB
PageTables:        27924 kB
NFS_Unstable:          0 kB
Bounce:                0 kB
WritebackTmp:          0 kB
CommitLimit:     3962148 kB
Committed_AS:    1870184 kB
VmallocTotal:   34359738367 kB
VmallocUsed:      379280 kB
VmallocChunk:   34359259364 kB
HardwareCorrupted:     0 kB
HugePages_Total:       0
HugePages_Free:        0
HugePages_Rsvd:        0
HugePages_Surp:        0
Hugepagesize:       2048 kB
DirectMap4k:        8688 kB
DirectMap2M:     3840000 kB

Answer 1

Compare what System Monitor shows to $ free -m

The amount shown as "in use" by System Monitor is the amount in the second line of output from free. AFAIK, this is the amount of RAM actually used, excluding what is cached. free -m often shows much more as "used" (first line of output) because it includes that used by the cache. This is not necessarily a bad thing, as empty/unused RAM is wasted RAM; rather have it "in use" as cache than sitting empty.