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% ps aux
This command gives you a listing of all the processes running on the machine. Find the process with the largest value in the
%MEM . Divide the value in the
RSS field by the percentage of memory used to get a rough figure for how much memory your machine has:
memory = RSS/(%MEM/100)
For instance, if the largest process shows 5% memory usage and a resident set size (RSS) of 840 KB, your machine has 840000/(5/100) = 16 MB of memory. You could also reboot the machine! It will tell you how much memory is available when it comes up. If the answer from the size command shows a total that is anywhere near the amount of memory you have, you stand a good chance of paging when you run — especially if you are doing other things on the machine at the same time.
Your system’s performance monitoring tools tell you if programs are paging. Some paging is OK; page faults and “page-ins” occur naturally as programs run. Also, be careful if you are competing for system resources along with other users. The pic- ture you get won’t be the same as when you have the computer to yourself.
To check for paging activity on a Berkeley UNIX derivative, use the vmstat command. Commonly people invoke it with a time increment so that it reports paging at regular intervals:
% vmstat 5
This command produces output every five seconds.
procs memory page disk faults cpu
r b w avm fre re at pi po fr de sr s0 d1 d2 d3 in sy cs us sy id0 0 0 824 21568 0 0 0 0 0 0 0 0 0 0 0 20 37 13 0 1 98
0 0 0 840 21508 0 0 0 0 0 0 0 1 0 0 0 251 186 156 0 10 900 0 0 846 21460 0 0 0 0 0 0 0 2 0 0 0 248 149 152 1 9 89
0 0 0 918 21444 0 0 0 0 0 0 0 4 0 0 0 258 143 152 2 10 89
Lots of valuable information is produced. For our purposes, the important fields are
avm or
active virtual memory , the
fre or
free real memory , and the
pi and
po numbers showing paging activity. When the
fre figure drops to near zero, and the
po field shows a lot of activity, it’s an indication that the memory system is overworked.
On a SysV machine, paging activity can be seen with the sar command:
% sar -r 5 5
This command shows you the amount of free memory and swap space presently available. If the free memory figure is low, you can assume that your program is paging:
Sat Apr 18 20:42:19
[r]freemem freeswap
4032 82144
As we mentioned earlier, if you must run a job larger than the size of the memory on your machine, the same sort of advice that applied to conserving cache activity applies to paging activity.
By the way, are you getting the message “Out of memory?” If you are running
csh , try typing
unlimit to see if the message goes away. Otherwise, it may mean that you don’t have enough swap space available to run the job. Try to minimize the stride in your code, and where you can’t, blocking memory references helps a whole lot.
A note on memory performance monitoring tools: you should check with your workstation vendor to see what they have available beyond vmstat or sar . There may be much more sophisticated (and often graphical) tools that can help you understand how your program is using memory.
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