Java Memory Limit -Xmx suffix: upper vs lower case m/M and g/G

Question

It is commonly known that it is possible to limit the Java heap size with -Xmx<amount><unit>, where unit is the data amount unit like Gigabyte, Megabyte, etc. I know that -Xmx128M means 128 Mebibytes (= 128 * 1024 * 1024 bytes).

But is it true, that it is also possible to use decimal units like megabytes using -Xmx100m (which would be 100 * 1000 * 1000 bytes)?

So is it possible to use this decimal units by using lower-case unit suffixes like k, m, g instead of K, M, G?

BTW The case generally matters, it's just that javac doesn't care. Note: there is no need to wonder whether g/G means 100^3 or 1024^3 because it won't be exactly either. ;) On my system running `System.out.println(Runtime.getRuntime().maxMemory());` with `-Xmx1g` prints `954728448` — Peter Lawrey, May 16 '14 at 17:11

score 41 · Accepted Answer · edited Nov 21 '17 at 18:03

41

There is no difference between k and K both means kibibyte and so does m/M = mebibyte & g/G = gibibyte. you can set 100m as the value and it will be 100 * 1024 * 1024 bytes. generally it is advised to use this value in powers of 2.

Hope it helps.

edited Nov 21 '17 at 18:03

Cody A. Ray

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answered May 16 '14 at 16:33

Sanjeev

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*why* is it advisable to use in powers of two ? I would question that. If you want to add 1g to an 8g server, that should be no issue. – Brian Agnew Apr 07 '20 at 13:52

score 5 · Answer 2 · answered May 16 '14 at 16:27

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Why don't you just try -Xmx100m and -Xmx100M and check if there is any difference.
k, m, g work exactly like K, M, G - they all mean binary units.

answered May 16 '14 at 16:27

apangin

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score 2 · Answer 3 · edited May 23 '17 at 12:10

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All memory sizes used by the JVM command line arguments are specified in binary units (Kibibyte, Mebibyte, etc.) regardless of unit capitalization; where a 'kilobyte' is 1024 bytes, and the others are increasing powers of 1024.

This answer goes into a more complete technical answer to the question.

edited May 23 '17 at 12:10

Community

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answered Apr 11 '17 at 21:52

TerekC

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Java Memory Limit -Xmx suffix: upper vs lower case m/M and g/G

3 Answers3