Is there a way to determine size of the old region of the heap in runtime

Question

Size of the old region is controlled using -XX:NewRatio parameter, how would I determine its value in runtime? Thanks in advance!

Update I haven't been attentive enough, the question is basically a copy of How to get vm arguments from inside of java application?. I would delete it but there is an interesting discussion below so I will do it a little bit later when it is finished

I want to know how much memory I have for an int array. As far as I know big arrays are allocated directly in the old region — Ruslan Sverchkov, Jul 13 '13 at 18:04
And what if you don't have the required size? What is supposed to happen in this case? How large is this potential array so that you need this information? — fge, Jul 13 '13 at 18:07
I'm trying to implement external sorting for ints file, heap size is restricted. I argee that this is not a good job for java but that is what they want from me) This potential array should be as large as possible in order to sort as much as possible ints at once — Ruslan Sverchkov, Jul 13 '13 at 18:11
OK, let me restate the problem; you have a large file full of int values; are they binary representation of ints? If yes, what endianness? Or is it something else? — fge, Jul 13 '13 at 18:23
I dont fully understand, probably because I'm not good in IO. By mapped file you mean MappedByteBuffer, is that correct? — Ruslan Sverchkov, Jul 13 '13 at 18:24
It's a few GB large and heap size is a few MB, ints are signed — Ruslan Sverchkov, Jul 13 '13 at 18:34
Uhm, wait, there are limitations to `FileChannel.map()` which you should be aware of — fge, Jul 13 '13 at 18:49

fge · Accepted Answer · 2013-07-13T19:34:57.460

OK, so, if I understand correctly, you have a very large file, several GB large, full of ints, and these ints are stored in big endian.

My proposal is the following: create a class implementing List<Integer> over such a large file, and use Collections.sort().

That is:

you take as an argument the file you need sorted;
you map that file in memory;
you implement the List interface;
you let Collections.sort() do the job.

Once Collections.sort() is done, you just .force() the channel from which you obtained the MappedByteBuffer. If no I/O error, you're good to go, replace the original file.

Yeah, that is quite some work, but it is doable. Demo code on demand.

NOTE: FileChannel.map() is limited; you may have to create several mappings. I'd recommend splitting into 1 GB mappings. This will quite complicate the implementation of .subList(), admittedly. Also, be aware that you'll only be able to address up to Integer.MAX_VALUE ints; which means your file cannot be larger than ((1 << 31 - 1) << 2). 1 << 33 is 8 GB.

You'll have to benchmark it though. This largely requires OS support for mapping. Also, as I said, beware of `.subList()`, this may require quite some work! But it is doable nevertheless. Now, maybe there is a better solution, but out of my head I don't know of any. — fge, Jul 13 '13 at 19:29

Is there a way to determine size of the old region of the heap in runtime

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