8346011: [Lilliput] Compact Full-GC Forwarding #191
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The current forwarding scheme that is used during full-GC overrides much of the mark-word. This is a problem for current compact headers when the heap is larger than 8TB (in which case we currently disable compact headers altogether) and becomes a much worse issue with 4-byte-compact-headers, because it would override the compressed class-pointer in the upper header bits.
This implementation uses a side-table (currently 1/512th of the heap size) to store part of the target addresses, and 9 bits in the header for the rest of it. For the full description of the algorithm, see top of fullGCForwarding.hpp.
Some performance testing results: https://gist.github.com/rkennke/5a53d21337fc6e696041062d6b972dd6
Interpretation: The new full-GC forwarding is slightly slower across the board. The different is almost completely a constant ~10ms. This corresponds to the setup costs of allocating and clearing the side-table. That cost could be reduced by allocating and clearing the table up-front, when the GC gets initialized, and then only ever clearing the table when full-GC is finished. That brings down the numbers to almost exactly baseline level, but I think we wouldn't want to hold on to that memory all the time, especially not in G1 and Shenandoah, where full-GC is an exceptional mode that is not intended to run frequently.
Testing:
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git
Checkout this PR locally:
$ git fetch https://git.openjdk.org/lilliput.git pull/191/head:pull/191
$ git checkout pull/191
Update a local copy of the PR:
$ git checkout pull/191
$ git pull https://git.openjdk.org/lilliput.git pull/191/head
Using Skara CLI tools
Checkout this PR locally:
$ git pr checkout 191
View PR using the GUI difftool:
$ git pr show -t 191
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Download this PR as a diff file:
https://git.openjdk.org/lilliput/pull/191.diff
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