RFR: 8264783: G1 BOT verification should not verify beyond allocation threshold

[Thomas Schatzl]

On Wed, 7 Apr 2021 01:16:05 GMT, Hamlin Li <mli at openjdk.org> wrote:

>> The G1 BOT contains an allocation threshold which basically acts as a "last known valid entry" index for the per-region BOT (which are views on the global BOT table).
>> 
>> Currently G1 BOT verification actually "verifies" the BOT within a region past that BOT index.
>> 
>> This causes issues with young regions; actually there is already code that prevents their BOT verification. This is perfectly fine, allocations in young regions do not update the BOTs.
>> 
>> With JDK-8262068/PR #2760 this existing filtering of young regions (such that their BOT is not verified) does not work because there may be young regions that are not compacted (so with a BOT that have that last known valid entry at the start of the region) are still labelled as old.
>> 
>> This change proposes to not try to verify the BOT beyond the last known valid index (which is arguably not worth doing), which also covers the existing young filtering.
>> 
>> There are alternatives that may work in particular for JDK-8262068:
>> a) always compact young regions (which recreates the BOT)
>> b) create a "dummy" BOT that spans the entire part of the region containing live objects
>> 
>> However they were rejected by me because
>> option a) takes time and directly counters that optimization for no reason
>> option b) makes finding the start of an object within these regions slow (they are not refined when there is at least *some* bot)
>> 
>> and finally I do not think there is much point in trying to be clever about areas in the BOT that are known to not contain useful values. 
>> 
>> Testing: tier1-3
>
> The change looks good to me.
> Thanks Thomas!

Thanks @Hamlin-Li @albertnetymk for your revies.

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PR: https://git.openjdk.java.net/jdk/pull/3356