Free ratio based heap shrinking in the parallel collector

[Hiroshi Yamauchi]

Jon,

Thanks for your comments. My new revision is here

  http://cr.openjdk.java.net/~hiroshi/webrevs/heapshrink/

My comments are inlined below:

> In parallelScavengeHeap.cpp
>
> In the UseParallelGC collector the initial size (init_size) can be
> significantly larger than the minimum size.   If you want to shrink down
> the heap to minimize footprint, do you really want to limit by the
> initial size instead of the minimum size?
>
> 1001     maximum_desired_capacity = MAX2(maximum_desired_capacity, init_size);
>
> Similarly here the amount of used data could be very small and
> setting the floor at the initial size may not be what you
> want.
>
> 1034     minimum_desired_capacity = MAX2(minimum_desired_capacity, init_size);

I changed the code so that it uses the min size instead of the init size.

>
> In psYoungGen.cpp you guard output with PrintGC only.  In similar
> cases this type of output is guarded with Verbose also.  Does this
> output as is get printed in the middle of the usual
> -verbosegc (also known as PrintGC) line?
>
> 1000   if (PrintGC) {
> 1001     gclog_or_tty->print_cr(" Resizing young gen. expand_bytes=%d,%d",
> 1002                            eden_expand_bytes, survivor_expand_bytes);
> 1003     gclog_or_tty->print("BEFORE: Young Gen: ");
> 1004     gclog_or_tty->print("eden capacity : " SIZE_FORMAT ", ",
> 1005                         eden_space()->capacity_in_bytes());
> 1006     gclog_or_tty->print("eden used : " SIZE_FORMAT ", " ,
> 1007                         eden_space()->used_in_bytes());
> 1008     gclog_or_tty->print("survivor capacity : " SIZE_FORMAT ", ",
> 1009                         from_space()->capacity_in_bytes());
> 1010     gclog_or_tty->print_cr("survivor used : " SIZE_FORMAT ", " ,
> 1011                            from_space()->used_in_bytes());
> 1012   }

I added Verbose to the guard. And yes, the output show up in the
middle of PrintGC line.

>
>
> In psOldGen.cpp you could try to expand up to max_gen_size
> instead of returning?
>
>  501 void PSOldGen::try_to_expand_by(size_t expand_bytes) {
>  502   if (expand_bytes < MinHeapDeltaBytes ||
>  503       capacity_in_bytes() + expand_bytes > max_gen_size()) {
>  504     return;
>
> Additionally PSOldGen::expand() chooses to use MinHeapDeltaBytes
> as the minimum expansion size (in the sense that sizes for expansion
> are round up to MinHeapDeltaBytes).   You would rather not expand
> for less than MinHeapDeltaBytes?   I'll admit that there maybe some
> inconsistencies in the way MinHeapDeltaBytes is used.  Just checking
> that this is what you want to do.
>
> Again, seems to me that you want to shrink down to the minimum
> generation size as opposed to the initial size.
>
>  523 void PSOldGen::try_to_shrink_by(size_t shrink_bytes) {
>  524   if (shrink_bytes < MinHeapDeltaBytes ||
>  525       capacity_in_bytes() - shrink_bytes < init_gen_size()) {
>  526     return;
>  527   }

Now I have:

 502   if (capacity_in_bytes() + expand_bytes > max_gen_size()) {
 503     expand_bytes = max_gen_size() - capacity_in_bytes();
 504   }

and likewise for try_to_shrink_by().

1. It allows the subsequent code (eg expand()) to round up the size <
MinHeapDeltaBytes.
2. It rounds down the size that goes past the max/min gen size so that
the heap is expanded/shrunk to the max/min.

>
> In globals.hpp.  We may want this feature implemented in other
> collectors at some point - G1 comes to mind.  I'd drop the leading
> PS on the flag so that it is ResizeByFreeRatioWithSystemGC.
>
> 3079   product(bool, PSResizeByFreeRatioWithSystemGC, false,                     \
> 3080           "Resize the heap by free ratio in System.gc() "                   \
> 3081           "under UseParallelGC")
>

Done.

Hiroshi