free page cache and dentries

I am using a large instance of EC2 (2 Dual Core AMD Opteron(tm) Processor 270, 7.5GB RAM), which is up since June 29th and showing availability of only 500MB of memory, in `top'. I had known the mathematics behind the RAM usage, that shows up in `top', but could not recollect.

Then, I thought that, since it had something to do with memory, why not look up `/proc/' entries. That's when I looked up the man page for `proc'. On detailed reading, I remembered the mathematics.

The used memory constitute of the `kernel buffers' and the `page cache'. See the output of `free'. Now my EC2 instance was showing almost 7GB memory used, out of which 5.5 GB was cached.

Now, I ran a small command on the shell:

echo 3 > /proc/sys/vm/drop_caches

Now, writing `3' to `/proc/sys/vm/drop_caches', clears the pagecache and dentries. To learn more, read manual page for `proc'.

When the cache was cleared, I had 6.5 GB of memory available. Usually, the cache is cleared by the Kernel, only on demand. So, if I run a program, which simply tries to use huge amount of memory, the Kernel, will clear the cache and make it available for the program to use.

Comments

[Njan]

Thanks da..
that was informative