Jon Masamitsu Jon.Masamitsu at Sun.COM
Tue Jul 10 22:23:50 UTC 2007

We've heard from users in the past that it would be nice if
there was no maximum heap size limit.  Users say
they don't know what the limit for their application is and
have to experiment to find an adequate value.

I did the following refworkload experiments.

Ran "reference_client" and "startup2" (sets of client
application like benchmarks) with the -client
(uses the serial GC which does not have GC ergonomics) and
a 1g heap.   The point of this experiment was to see if the
size of the committed heap increased because of the 1g heap
size limit.  There was no discernible difference in the
committed size of the heap nor in the amount of GC work.

I ran "reference_server" (set of server application like
benchmarks)  with -server and the parallel GC (does
has GC ergonomics) with -XX:DefaultMaxRAM=1g (the default)
and with -XX:DefaultMaxRAM=1700m (the amount of physical
memory on the platform minus some for
 the OS) and -XX:DefaultMaxRAMFraction=1. 
The point of this experiment was to see if the
GC ergonomics drove the  heap to sizes larger that the 1g
default limit.  As one would expect the heaps for some
benchmarks grew considerably (up to the higher limits
in some cases) and for some the heaps did not change.

I think that these results are not unexpected.  The heap sizing
policy used by the serial collector depends on the amount of
live data and the live data for the client like applications
fit nicely into the 64m default heap size (i.e., the larger
1g heap was not needed).  On the larger server like benchmarks run
with GC ergonomics, the high default throughput goal of GC
ergonomics means that some benchmarks will just use as much
heap as they can get in trying to reach the throughput goal.

I'd like to propose an additional policy under a new command line
flag.  Let me use MaximizeHeapSize for the name for now.  If
MaximizeHeapSize is set, the VM will calculate
the amount of physical memory on the platform and
try to use that as the upper limit on the heap size.   As
with the current GC ergonomics the upper limit will
be ~4g on a 32bit system.   If the VM cannot actually
reserve that amount of space at initialization, it will
try reserving some smaller amounts until it succeeds.  There
will be some minimum size (probably 64m) that it will not
go below.

The user would have to turn MaximizeHeapSize on
explicitly.  It will be off by default until we get some
experience with it.  Users who turn it on and also use
GC  ergonomics will manage the heap size via
a smaller or larger throughput goal (i.e., the larger
the throughput goal the more heap space the VM will
try to get to meet the throughput goal).
For the non ergonomics collectors it should only matter
if the application would otherwise have bumped into a
heap limit (i.e., the application would have thrown
an out-of-memory or just spent too much time doing
garbage collections because the heap limit set by
default or on the command line was too small).  In that
case this change would allow the application to
get the additional space it needs.  I expect there will
be some corner cases or bugs where the heap will grow  to
the limit when it shouldn't. 

Comments on this proposal?


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