memory usage of byte ?
Ulf.Zibis at CoSoCo.de
Sun Jul 6 08:04:37 PDT 2008
Peter, thanks for your extensive explanation. This is what I wanted to know.
Am 05.07.2008 22:26, Peter B. Kessler schrieb:
> Ulf Zibis wrote:
>> Hi all,
>> can you tell me something about memory consuming in the current Sun JVM?
>> How much bytes do byte arrays consume per byte?
>> ... or in other words: is it less memory-consuming using byte arrays
>> than int arrays, if these are large?
>> Same question about short and char arrays?
>> Thanks for a short answer.
>> -- please CC to my email.
> Every object in the HotSpot JVM has a 2-word header, where the
> word size is 32-bits in the 32-bit JVM and 64-bits in the 64-bit
> JVM (duh). An array then has a word that holds the length of
> the array. Following that comes the data, in whatever size is
> appropriate: boolean and byte elements take 1 byte each, chars
> and shorts take 2 bytes, ints and floats take 4 bytes, and longs
> and doubles take 8 bytes. References to other objects take either
> 4 or 8 bytes depending on whether you are in a 32-bit JVM or a
> 64-bit one (with a twist with compressed oops). If the data needs
> to be aligned (e.g., doubles), then padding is inserted between
> the array length and the data if needed. After the data there's
> enough bytes of padding to get us to an 8-byte boundary (so that
> all objects start on an 8-byte boundary).
> So in answer to your direct questions: byte arrays are less
> memory-consuming than int arrays, especially if the arrays are
> large. Short and char arrays are the same size, though there
> are published schemes to squeeze char arrays if you aren't using
> the high-order bytes. We don't use any of those, yet. (But you
> are welcome to contribute to that implementation if you think
> it's important. The trick is not slowing down the people that
> aren't using it.)
> You can use -XX:+PrintClassHistgram on the java command line and
> "kill -QUIT" (from another shell) to see how big things are.
> A more user-friendly way of browsing the heap of a Java program
> is "jmap -dump" followed by "jhat". See the manual pages for
> those tools to see how to use them.
> ... peter
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