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From "Nadav Har'El" <...@math.technion.ac.il>
Subject Re: Websphere and Dark Matter
Date Tue, 23 Jan 2007 13:03:17 GMT
On Mon, Jan 22, 2007, John Haxby wrote about "Re: Websphere and Dark Matter":
> Nadav Har'El wrote:
> Are you implying that the process memory shrinks, that memory is 
> returned to the kernel? I didn't read the page you referenced that way.
> I know that if I allocate memory by memory mapping anonymous regions 
> with Linux/Unix I can give it back, but is that the technique that JVMs use?

Indeed - I've seen this actually happen in Windows (I didn't check on Linux).
Once more than 70% of the heap is free (which is the default setting on Sun's
JVM 1.4) the heap is shrunk and the memory use of the process (according to
Windows "Task Manager") is actually reduced.

You could easily verify whether this happens on Linux as well. Just write
a trivial program that allocates a large, say, 100 MB, object, sleeps for
a few seconds (or runs "ps"), and then "forgets" about this object and runs
System.gc() (perhaps a few times), and then runs ps again and see that the
process shrunk. You might want to use the "-verbose:gc" JVM option to see
some debugging messages about what the GC's do, and how large Java thinks its
heap is. I did not check this myself, so if you have any conclusions, please
report it back to this list - I think it's interesting for many people here.

Anyway, theoretically, Java should have no problem to shrink its memory use
because of the fact that it can (and does) move objects around, so memory
fragmentation is not a problem like it is in many other languages. This is
also possible technically to do in Linux (there are several ways to do this,
including anonymous maps). If Java was Open-Source, I could find out how they
did it (if at all), but last time I checked, it wasn't (I heard this is
changing, though).

> It's not generally a problem though. Provided you have a compacting 
> garbage collector (and the Sun Java GC is one) then the unused memory 
> will just get paged out. It may be a different story on windows and it's 
> certainly a different story on an embedded platform, but releasing 
> memory to the kernel under Linux is not generally necessary or desirable.

Believe it or not, this is exactly what every GC expert told me when I
asked this question (how can I force the JVM to reduce its memory use when
it doesn't need all of it): "don't worry about it". This is "conventional
wisdom" second only to "never run System.gc()". But this answer ("virtual
memory means you don't care about the process not shrinking") ignores two
very important issues (not just on embedded platforms, like you suggested):

1. If the JVM heap itself doesn't shrink (it's not just that the memory isn't
   returned to the system), Java will continue to fill the entire huge heap,
   allowing itself to gather more garbage before a GC is necessary. Since Java
   goes over all this memory filling it with garbage all the time, none of
   this memory can ever be paged out and "forgotten".

   For example, with the default setting (of 70%), it is possible that you
   need just 100 MB of memory, but the JVM will keep a 300 MB heap, and
   continuously go over the entire 200 MB free heap filling it with soon-to-
   become garbage objects.

2. The thinking behind your statement (which, like I said, was shared by most
   GC experts I asked) is usually rooted on the idea that the machine is
   running one huge JVM. If that is the case, then indeed, nobody cares if
   when the JVM's memory needs fluctuates, whether the heep stays large,
   whether it gets swapped to disk, or the JVM gets smaller.
   But think about a machine running dozens of separate JVMs: this was *NOT*
   the intention of the designers of Java, but it is what happens in reality
   on modern desktops (and sometimes servers): you run several independent
   programs written in Java. In this case, if every one of these programs
   takes up the maximum size that it ever needed, even if this waste is just
   in the swap space, we can be in trouble. Swap space is "cheap", but not
   cheap enough that you don't mind that each Java program you run takes up
   hundreds of megabytes of it more than it actually needs.

The use-case that led me to these insights was a desktop search program
written in Java. This program had large peak memory needs (when parsing
large files, and when building the index), at the order of 150 MB. But then,
the JVM did not shrink, and continued to waste 150 MB of memory on each
desktop. This was unacceptable, even if (like you said), when you don't
perform a search the entire 150 MB gets swapped out. It was unacceptable for
several reasons: First, this is not the only program running on the machine,
so what if each one wasted 150 MB of swap space? Second, when you do use
the search, the entire 150 MB heap gets used and swapped in, and worse -
swaps out 150 MB of other useful memory (slowing your work in other
applications).

-- 
Nadav Har'El                        |      Tuesday, Jan 23 2007, 4 Shevat 5767
IBM Haifa Research Lab              |-----------------------------------------
                                    |"Do you want to restart Windows now or
http://nadav.harel.org.il           |wait for the next crash?"

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