RFR (XS) 8076759: AbstractStringBuilder.append(...) should ensure enough capacity for the upcoming "trivial" append calls

[Aleksey Shipilev]

Hi,

This seems to be a simple one-liner fix, but the background is more
complicated. See the bug:
  https://bugs.openjdk.java.net/browse/JDK-8076759

The bottom line is that our current resizing policy in ASB is hostile
for long appends. There is a heuristics that extends the capacity to
match the *exact* length of append if doubling the array would not help.

This heuristics has a nasty corner case: if there is an upcoming append
after a large one, then we are guaranteed to re-size again. If an
upcoming append is large in itself, the resizing is inevitable even
under the doubling-the-storage strategy; but if we only do a small
append, then we can be smarter.

After trying a few options to fix this (see below), I have settled on
just adding a simple static "pad", to absorb the trivial appends after a
large append:
  http://cr.openjdk.java.net/~shade/8076759/webrev.00/

The choice of "32" as magic number is deliberate: arraycopy likes large
power-of-two strides (and it does not like to make catch up loops for
small residuals). "16" is too small to fit the decimal representation of
Long.MIN_VALUE, therefore, we pick "32".

There are other approaches, briefly mentioned here:
  http://cr.openjdk.java.net/~shade/8076759/patches.txt

There is a direct correlation between the allocation pressure, and test
performance:
  http://cr.openjdk.java.net/~shade/8076759/data-perf.png
  http://cr.openjdk.java.net/~shade/8076759/data-foot.png

Naively, one could expect doubling the storage ("mult2") until we reach
$minimalCapacity solves the problem, but it wastes too much memory, and
only reaches the "plus32" on power-of-two sizes. That is also the
Achilles' Heel of the heuristics, because appending the
power-of-two-plus-one-sized string will set us up for the original
problem. This effect can be alleviated by doing the padding as well
("mult2-plus32"). Exactly the same trouble manifests on smaller strings
that go through the usual double-the-storage route, and this is why a
proposed patch makes the pad on common path.

I do believe the current heuristics is smart about large appends, and
mult2* strategies undo it. Therefore, I would think keeping the
minimumCapacity cap is a good thing, and just adding the pad is a good
solution. Thus, it is in the webrev.

Thanks,
-Aleksey.