Finding and Trimming Linux Bloat - page 2
Analyzing Linux Memory
If you examine your system's memory use with a program like ps or top, you'll typically see two numbers. ps calls them RSS and VSZ, while top calls them RES and VIRT.
For instance, ps might display something like this:
$ ps uw --sort=-rss USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND akkana 6813 0.4 5.1 315160 158708 tty1 Sl 09:19 3:21 firefox-bin akkana 16883 10.6 2.4 104464 75824 tty1 Sl 22:31 0:02 /usr/bin/gimp akkana 32549 0.0 0.6 33832 18832 tty1 S 18:52 0:05 xchat akkana 6678 0.1 0.5 25552 16372 tty1 S 20:11 0:01 emacs akkana 4688 0.0 0.4 29272 14312 tty1 Sl 08:56 0:02 python twit akkana 4039 0.0 0.2 12408 7344 tty1 S 08:55 0:02 openbox akkana 511 0.0 0.1 9460 5680 pts/0 S+ 18:54 0:00 mutt akkana 7063 0.0 0.1 8796 4932 tty1 S 09:21 0:00 xterm akkana 4033 0.0 0.1 9232 3832 tty1 S 08:55 0:00 xterm akkana 4035 0.0 0.0 5524 2844 tty1 S 08:55 0:00 python -u mybeepd crickets.wav akkana 7064 0.0 0.0 4452 2568 pts/1 Ss 09:21 0:00 -tcsh akkana 3849 0.0 0.0 4188 2092 tty1 S 08:55 0:00 -tcsh akkana 4041 0.0 0.0 4004 2084 pts/0 Ss 08:56 0:00 -tcsh akkana 3944 0.0 0.0 3504 1548 tty1 S+ 08:55 0:00 /bin/bash /usr/bin/startx akkana 7952 0.0 0.0 2700 940 pts/1 R+ 20:30 0:00 ps uw --sort=-rss
So, what are those numbers? And is Firefox really using 315 megabytes?
RSS stands for Resident Set Size. It's theoretically the amount of physical RAM the program is actually taking in your system right now, not counting anything that might be swapped to disk. In practice, it's a little more complicated than that, as you'll see in a moment.
VSZ stands for Virtual SiZe -- the total memory of the process, all its code, data and libraries, whether it's currently loaded in memory or swapped to disk.
So in other words, VSZ is RSS plus whatever is swapped to disk. Right?
Well, no. You can find out what's swapped with free:
$ free total used free shared buffers cached Mem: 3103496 1274712 1828784 0 137016 777512 -/+ buffers/cache: 360184 2743312 Swap: 2249060 0 2249060
The second line shows how much swap is being used: none, on my system right now. Yet Firefox is using 315M of VSZ but only (only!) 159M of RSS. How can that be?
What the ps and top man pages don't tell you is that there are other types of memory included in VSZ. For instance, when a program makes a large large memory allocation request, the kernel may not allocate all of it right away -- it might just reserve it in case the program ever actually tries to access that memory. So VSZ represents RSS plus swapped memory plus memory that the program thinks it might need to use some day but actually may never touch.
Okay, so that VSZ number is bogus. Firefox isn't really using 315M. Whew! What about that 159M of RSS?
Well, there are mitigating factors there too. The biggest is shared libraries.
I'm sure you've encountered them before. A shared library (also called a dynamically loaded library) is a file like libX11.so.6 or /usr/lib/libgtk-x11-2.0.so.0, where so stands for shared object. The point of a shared library is that you can have common code -- like the routines that talk to the X server and create windows, read JPG files, or handle fonts -- that are shared among lots of apps. So Firefox, GIMP, gedit and abiword could all be using the same X11, gtk+, pango and cairo libraries, and you'd only need those libraries loaded once each in memory instead of four times each.
But Linux memory applications aren't so good at figuring out what parts of memory are shared. When ps tells you that Firefox is taking up 159M of RSS and GIMP is taking up 76M, it's counting some shared libraries twice.
How much memory does that represent? That's not an easy question to answer using programs like ps. In the next installment I'll talk about some ways of examining how much memory your processes are really using.
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