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

Aleksey Shipilev aleksey.shipilev at oracle.com
Fri May 1 14:09:06 UTC 2015



On 04/24/2015 09:05 PM, Aleksey Shipilev wrote:
> 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.

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