Re: Memory utilization

gwoodhouse@gmail.com wrote:


     Might be high, but doesn't seem ridiculous. A String
instance has its object-ness plus three ints and a char[]
reference. The char[] will have its own object-ness plus
a length. If object-ness takes eight bytes and a reference
takes four (32-bit JVM), we're up to at least 36 bytes. If
there's an eight-byte alignment requirement, 40 bytes is
right on the money.


     He's trying to create 2G pairs of String objects (72-80
bytes of overhead per pair) plus 2G HashMap.Entry objects
(~24 bytes each, under the same set of assumptions), hence
about 192-208GB -- and that's not counting the "payload" of
the char values, which will add another 80GB or thereabouts.
272GB is well beyond what a 32-bit JVM can manage, so he'll
need to go to a 64-bit version -- and this will expand the
String overhead to at least 40 bytes, and the HashMap.Entry
size to at least 36-40, raising the total to about 312-320GB.
And that's just for this one data structure; the rest of the
Java environment needs some heap, too. I'd guess that a
machine with half a terabyte of RAM could just about manage
it, with luck and a following wind.

     ... but in truth he's not really trying to populate this
enormous and pointlessly wasteful data structure; he's trying
to understand how the NetBeans profiler reports the memory
used by the program, and I'm afraid I can't help much with
that. After 4M pairs he sees


.... which works out to 6 bytes per string, 6 bytes per char[],
and <5 bytes per HashMap.Entry -- clearly, the profiler reports
some other notion of "memory used" than we've been talking
about in the last few messages. All I can suggest is RTFM.

--
Eric.Sosman@sun.com