webmin memory usage seems wrong

Question

I'll be looking through the list of Running Processes in webmin, and it seems like the memory usage for my applications is wrong.

For instance,

mysql 253956 kB /usr/sbin/mysqld

Which would equal some 256+megabytes.

The problem with this, is it also reports that the ram on my server isn't used up, and there is only 256megabytes available.

From what I understand a capitol B = Bytes, and a lowercase b=bits.

I even wrote a program that runs on here that says it's using almost 200 MB of space, when there's no way that's possible.

Any idea why the numbers are way off? Or is it possible they mean kilobits, instead of kilobytes?

I'm running Ubuntu 11.04 server 64bit on RackSpace if that helps.

Thanks.

Answer 1

It depends what reading it is using to pull that memory count from. Memory management in a modern OS is not nearly as simple as you might first expect it to be!

For instance if you take memory readings from top VIRT is almost always noticeably bigger than RES (see articles like this (search for top vert res shr) for more detailed discussion). It is most likely that webmin is reporting the virt value which includes memory shared with other processes (library code that is loaded into real memory only once but mapped into the virtual address spaces of multiple processes) and memory that is allocated but not yet used (it is available in the processes virtual address space, but no real RAM or swap pages have been allocated for it yet).

It may also be that some of the memory allocated to currently swapped out to disk, does the output of free indicate that any swap space in use?

From what I understand a capitol B = Bytes, and a lowercase b=bits.

Unfortunately while many people make that distinction when writing it isn't a "real" standard so can not be relied upon. Unless bits/bytes is explicitly stated you unfortunately need to use context to try conclude which is being intended. When reporting memory from the OS's point of view and formatted drive space B or b almost always refer to bytes not bits.