On RedHat Linux 6.2 I'm running free -m
and it shows nearly all 8GB
used
total used free shared buffers cached Mem: 7989 7734 254 0 28 7128 -/+ buffers/cache: 578 7411 Swap: 4150 0 4150
But at the same time in top -M
I cannot see any processes using all this memory:
top - 16:03:34 up 4:10, 2 users, load average: 0.08, 0.04, 0.01 Tasks: 169 total, 1 running, 163 sleeping, 5 stopped, 0 zombie Cpu(s): 0.7%us, 0.3%sy, 0.0%ni, 98.6%id, 0.4%wa, 0.0%hi, 0.0%si, 0.0%st Mem: 7989.539M total, 7721.570M used, 267.969M free, 28.633M buffers Swap: 4150.992M total, 0.000k used, 4150.992M free, 7115.312M cached PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 1863 sroot 20 0 398m 24m 9.8m S 0.3 0.3 3:12.87 App1 1 sroot 20 0 2864 1392 1180 S 0.0 0.0 0:00.91 init 2 sroot 20 0 0 0 0 S 0.0 0.0 0:00.00 kthreadd 3 sroot RT 0 0 0 0 S 0.0 0.0 0:00.07 migration/0 4 sroot 20 0 0 0 0 S 0.0 0.0 0:00.00 ksoftirqd/0 5 sroot RT 0 0 0 0 S 0.0 0.0 0:00.00 migration/0 6 sroot RT 0 0 0 0 S 0.0 0.0 0:00.00 watchdog/0 7 sroot RT 0 0 0 0 S 0.0 0.0 0:00.08 migration/1 8 sroot RT 0 0 0 0 S 0.0 0.0 0:00.00 migration/1
I also tried this ps mem script but it onlt shows about 400MB memory being used.
You can also filter processes by memory usage in top. To do this, press SHIFT + m as shown: Top will filter the processes by memory usage in descending order. Doing this can help identify the process using the most memory, giving you a chance to take action.
The reason Linux uses so much memory for disk cache is because the RAM is wasted if it isn't used. Keeping the cache means that if something needs the same data again, there's a good chance it will still be in the cache in memory.
Don't look at the "Mem" line, look at the one below it.
The Linux kernel consumes as much memory as it can to provide the I/O cache (and other non-critical buffers, but the cache is going to be most of this usage). This memory is relinquished to processes when they request it. The "-/+ buffers/cache" line is showing you the adjusted values after the I/O cache is accounted for, that is, the amount of memory used by processes and the amount available to processes (in this case, 578MB used and 7411MB free).
The difference of used memory between the "Mem" and "-/+ buffers/cache" line shows you how much is in use by the kernel for the purposes of caching: 7734MB - 578MB = 7156MB in the I/O cache. If processes need this memory, the kernel will simply shrink the size of the I/O cache.
Also, as the first line shows total used free shared buffers cached Mem: 7989 7734 254 0 28 7128 -/+ buffers/cache: 578 7411
If we add (cached[7128] + buffers[28] + free[254]), we will get approximately the second line's free[7411] value 7128 + 28 + 254 = 7410
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