RFR: 8278917: Use Prev Bitmap for recording evac failed objects [v3]

[Hamlin Li]

On Mon, 20 Dec 2021 19:46:27 GMT, Stefan Johansson <sjohanss at openjdk.org> wrote:

>> src/hotspot/share/gc/g1/g1FullGCCompactTask.cpp line 95:
>> 
>>> 93:     // location, but also clears the bitmap for it. This is needed
>>> 94:     // for bitmap verification as well as for being able to use the
>>> 95:     // prev_bitmap for evacuation failures.
>> 
>> Instead of clearing the bit for each obj individually, I wonder if clearing the bitmap for the whole region will be faster. IOW, just call `collector()->mark_bitmap()->clear_region(hr);` unconditionally.
>
> I did this first, just removing the `G1VerifyBitmaps` condition. But this will cause a notable regression for regions with very few marked objects. I'm not certain that regression is to bad, but I did not see anything suggesting that clearing the whole bitmap afterwards would be quicker in any other scenario either. So went with the bit-by-bit approach.

I assume mem write is more expensive (or at least not more efficient) than mem read, so the performance changes make sense to me.

And a comment about the observation of the regression will be helpful in the future for others who think this might be a optimization point.

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PR: https://git.openjdk.java.net/jdk/pull/6867