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I've made a program that does like "tail -f" on a number of log files on a machine, using Apache Tailer from commons IO. Basically it runs in a thread, opens the file as a RandomAccessFile, checks its length, seeks to the end etc. It sends all log lines collected to a client.

The somewhat uncomfortable thing about it, is that on Linux it can show an enormous amount of VIRT memory. Right now it says 16.1g VIRT (!!) and 203m RES.

I have read up a little on virtual memory and understood that it's often "nothing to worry about".. But still, 16 GB? Is it really healthy?

When I look at the process with pmap, none of the log file names are shown so I guess they are not memory mapped.. And I read (man pmap) that "[ anon ]" in the "Mapping" column of pmap output means "allocated memory". Now what does that mean? :)

However, pmap -x shows:

Address           Kbytes     RSS   Dirty Mode   Mapping
...
----------------  ------  ------  ------
total kB        16928328  208824  197096

..so I suppose it's not residing in RAM, after all.. But how does it work memory-wise when opening a file like this, seeking to the end of it, etc?

Should I worry about all those GB of VIRT memory ? It "watches" 84 different log files right now, and the total size of these on disk are 31414239 bytes.

EDIT: I just tested it on another, less "production-like", Linux machine and did not get the same numbers. VIRT got to ~2,5 GB at most there. I found that some of the default JVM settings were different (checked with "java -XX:+PrintFlagsFinal -version"):

Value              Small machine    Big machine
InitialHeapSize    62690688         2114573120
MaxHeapSize        1004535808       32038191104
ParallelGCThreads  2                13

..So, uhm.. I guess it grabs more heap on the big machine, since the max limit is (way) higher? And I also guess it's a good idea to always specify those values explicitly..

boffman
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  • Ok, you have 31 MB of files but 16 GB of (reserved) virtual memory? Now that's a little odd - what did you give your VM as Heap size? This would be totally normal if you gave your VM a minimal heap size of 16 GB, for example. – kutschkem Mar 14 '13 at 16:31
  • Hmm I actually run it without any JVM args at all right now. Is that good or bad? :-) – boffman Mar 14 '13 at 18:43

1 Answers1

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A couple of things:

  • Each Tailer instance will have its own thread. And each thread has a stack. By default (on a 64bit JVM) thread stacks are 1Mb each, so you will be using 84Mb for stacks. You might consider reducing that using the -Xss option at launch time.

  • A large virt size is not necessarily bad. But if it translates into a demand on physical memory ... and you don't have that much ... then that really is bad.

Hmm I actually run it without any JVM args at all right now. Is that good or bad? :-)

I understand now. Yes it is bad. The JVM's default heap sizing on a large 64bit machine are far more than you really need.

Assuming that your application is only doing simple processing of the log lines, I recommend that you set the max heap size to a relatively small size (e.g. 64Mb). That way if you do get a leak it won't impact on the rest of your system by gobbling up lots of real memory.

Stephen C
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  • Well, but what exactly is "far more"? I would expect that it's more like 512Mb or 1GB, but not 16 GB. Btw. what exactly does it mean if the program has 16 GB of virtual memory, but only 200 MB reside in memory? Does it mean the rest is on disk? Or does it "just" mean the OS tells the program it can use this much memory if it wants to? – kutschkem Mar 15 '13 at 15:17