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gc is freeing and claiming memory

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Because I saw certain applications took a lot of residential memory
and swap space I am looking a bit into memory management. I have the
following code:
    long freeMemory  = rt.freeMemory()  / divideValue;
    long totalMemory = rt.totalMemory() / divideValue;

    logMessage(String.format("Maximum to use %d",
                              rt.maxMemory() / divideValue));
    logMessage(String.format("Before %d - %d = %d",
                              totalMemory, freeMemory,
                              totalMemory - freeMemory));
    System.gc();
    freeMemory  = rt.freeMemory()  / divideValue;
    totalMemory = rt.totalMemory() / divideValue;
    logMessage(String.format("After  %d - %d = %d",
                              totalMemory, freeMemory,
                              totalMemory - freeMemory));

One of the outputs is:
    15:33:03: Maximum to use 228
    15:33:03: Before 142 - 80 = 62
    15:33:03: After  146 - 93 = 53

So while on the one hand there is 9 MB of memory freed, but at the
same time there is another 4 MB reserved. (I have seen a bigger
difference, there was on the one hand 4 MB freed and at the same time
32 MB extra reserved. But I did not save that.)

What is happening here?

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

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Thread

gc is freeing and claiming memory Cecil Westerhof <Cecil@decebal.nl> - 2016-08-12 15:56 +0200
  Re: gc is freeing and claiming memory Florian Weimer <fw@deneb.enyo.de> - 2016-08-13 16:35 +0200

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