[Gc] The difference when setting initial-heap-size and not?

Boehm, Hans hans.boehm at hp.com
Thu Feb 2 13:56:32 PST 2006

The number of blocks in use is the number of heap chunks (usually pages)
on which any live objects reside.  The reported number of bytes is the
number of bytes in those blocks.  If you only had small objects, this
will be the number of blocks times the block size (probably 4K).  Large
objects reside in larger "blocks", so the number of bytes will typically
be a bit larger than that.

The collector versions used in gcj (i.e. versions < 7) don't calculate
the actual amount of live memory unless you recompile the collector
without the -DSILENT flag.  In that case, additional counting code is
compiled in.  (You do not want to do this for production code.  It will
slow down the marker, especially if you enable parallel marking, which
admittedly isn't likely here.)  But it would be interesting to see the
output here for debugging purposes.

My guess is that your results are normal.  With a larger heap size,
small objects are distributed over more pages in the heap.


> -----Original Message-----
> From: gc-bounces at napali.hpl.hp.com 
> [mailto:gc-bounces at napali.hpl.hp.com] On Behalf Of Martin 
> Egholm Nielsen
> Sent: Thursday, February 02, 2006 6:20 AM
> To: gc at napali.hpl.hp.com
> Subject: [Gc] The difference when setting initial-heap-size and not?
> Hi,
> I know I'm jumping all around the scene now, but in the attempt do 
> produce GC-dumps for my other news thread I discovered 
> something I found 
> peculiar.
> When starting my application without an initial heap size, but with a 
> maximum heap size, I end up in a steady state with:
> ***Blocks in use:
> --- 8< ---
> blocks = 887, bytes = 3801088
> However, if I specify an initial heap size of 14megs, the resulting 
> steady state says:
> ***Blocks in use:
> --- 8< ---
> blocks = 1499, bytes = 6307840
> There is no difference between the resulting steady states over 
> different runs, and it's reproducable.
> Maybe I just don't understand the concept "Blocks in use", 
> but as I read 
> it, it indicates the amount of memory actually being referred 
> to by my 
> application.
> Am I all wrong?
> BR,
>   Martin

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