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Re: Restricting the memory usage of a Java program

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On Friday 12 Aug 2016 00:04 CEST, Arne Vajhøj wrote:

> On 8/11/2016 5:42 PM, Cecil Westerhof wrote:
>> I made a little program for cropping images. When just starting it
>> as a Java program the memory usage was just way to much. So I wrote
>> the following script: #!/usr/bin/env bash
>>
>> declare -r CLASS=CropImage
>> declare -r MIN_MEM=128M
>> declare -r MAX_MEM=256M
>>
>> # Make sure you are in the right directory. Only start the Java
>> # application when moving to the right directory was succesful. cd
>> # "${0%/*}" && java "-Xms${MIN_MEM}" "-Xmx${MAX_MEM}" "${CLASS}" &
>>
>> And now the memory consumption is much less and as far as I can see
>> without negative consequences for the program.
>>
>> The values I use (128 MB and 256 MB) are just values I kind of
>> randomly choose. Is there a way to find ‘optimal’ values?
>
> System.gc();
> Runtime rt = Runtime.getRuntime();
> long actual_value = rt.totalMemory() - rt.freeMemory();
> long good_value = SOME_MAGIC_FACTOR * actual_value;

It is funny: I was always told that calling gc does not have an effect
on memory and that it can have a negative impact on performance. But I
saw a real difference. Maybe it is a good idea to call it once an
hour?

I added the following to my constructor:
    ses.scheduleAtFixedRate(new Runnable() {
            @Override
            public void run() {
                long divideValue = 1024 * 1024;
                long freeMemory  = rt.freeMemory()  / divideValue;
                long totalMemory = rt.totalMemory() / divideValue;

                giveMessage(String.format("Maximum to use %d",
                                          rt.maxMemory() / divideValue));
                giveMessage(String.format("Before %d - %d = %d",
                                          totalMemory, freeMemory,
                                          totalMemory - freeMemory));
                System.gc();
                freeMemory  = rt.freeMemory()  / divideValue;
                totalMemory = rt.totalMemory() / divideValue;
                giveMessage(String.format("After  %d - %d = %d",
                                          totalMemory, freeMemory,
                                          totalMemory - freeMemory));
            }
        }, 15, 60, TimeUnit.MINUTES);

The function giveMessage prints the input string with a time stamp to
standard out.

The first time I ran it with:
        }, 0, 15, TimeUnit.MINUTES);

I ran it for little more as ten hours and with both I saw that the
free memory was the same after the gc. So it is really useful to get
an indication.


Maybe I should rewrite it as an utility function, so I could use it in
all my applications. And of-course I should log to a (SQLite) database
instead of standard out. (It should be called logMessage instead of
giveMessage.)


I also have an instance running without the gc. I should let it run
for at least a day to see what happens.

They start with:
- script version
09:43:54: Maximum to use 228
09:43:54: Before 123 - 64 = 59
09:43:54: After  123 - 69 = 54

- Java version
09:48:20: Maximum to use 3540
09:48:20: Before 240 - 172 = 68
09:48:20: After  240 - 186 = 54

- Java version without gc
09:49:23: Maximum to use 3540
09:49:23: Before 240 - 172 = 68
09:49:23: After  240 - 172 = 68

When it runs at least a day I will give an update.


It is clear that max memory default is quit large.
Without the script total memory is higher, but not that much. So maybe
I do not need the scripts to curtail memory usage. I save work and
when sometimes the application needs a lot more it is not curtailed.
Of-course 120 MB is a lot on an embedded device and when you would
have 20 Java applications it is going to count also, but at the moment
I am running on a desktop with enough memory and am not running scores
of Java applications.
I can always start limiting memory when it becomes tight and when I
also have some information about the usage …

Just to be sure: maxMemory cannot change during the run of a Java
application?


There is 28 MB difference between -Xmx and what maxMemory returns.
Where did it go?

-- 
Cecil Westerhof
Senior Software Engineer
LinkedIn: http://www.linkedin.com/in/cecilwesterhof

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Thread

Restricting the memory usage of a Java program Cecil Westerhof <Cecil@decebal.nl> - 2016-08-11 23:42 +0200
  Re: Restricting the memory usage of a Java program Arne Vajhøj <arne@vajhoej.dk> - 2016-08-11 18:04 -0400
    Re: Restricting the memory usage of a Java program Cecil Westerhof <Cecil@decebal.nl> - 2016-08-12 10:34 +0200
      Re: Restricting the memory usage of a Java program Martin Gregorie <martin@address-in-sig.invalid> - 2016-08-12 10:08 +0000
      Re: Restricting the memory usage of a Java program Arne Vajhøj <arne@vajhoej.dk> - 2016-08-12 09:23 -0400
        Re: Restricting the memory usage of a Java program Cecil Westerhof <Cecil@decebal.nl> - 2016-08-13 01:11 +0200
          Re: Restricting the memory usage of a Java program Cecil Westerhof <Cecil@decebal.nl> - 2016-08-13 02:52 +0200
          Re: Restricting the memory usage of a Java program Arne Vajhøj <arne@vajhoej.dk> - 2016-08-12 21:05 -0400
  Re: Restricting the memory usage of a Java program Arne Vajhøj <arne@vajhoej.dk> - 2016-08-11 18:24 -0400

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