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Debian GNU/Linux Reference Guide
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8.6.2 Free memory

free and top give good information on memory resources. Do not worry about the size of "used" in the "Mem:" line, but read the one under it (38792 in the example below).

     $ free -k # for 256MB machine
                  total       used       free     shared    buffers cached
     Mem:        257136     230456      26680      45736     116136 75528
     -/+ buffers/cache:      38792     218344
     Swap:       264996          0     264996

The exact amount of physical memory can be confirmed by grep '^Memory' /var/log/dmesg, which in this case gives "Memory: 256984k/262144k available (1652k kernel code, 412k reserved, 2944k data, 152k init)".

     Total         = 262144k = 256M (1k=1024, 1M=1024k)
     Free to dmesg = 256984k = Total - kernel - reserved - data - init
     Free to shell = 257136k = Total - kernel - reserved - data

About 5MB is not usable by the system because the kernel uses it.


Debian GNU/Linux Reference Guide
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