RFR: 8264223: CodeHeap::verify fails extra_hops assertion in fastdebug test [v3]

[Aleksey Shipilev]

On Fri, 26 Mar 2021 08:28:40 GMT, Hui Shi <hshi at openjdk.org> wrote:

>> When test with -XX:+VerifyCodeCache, many tests fail due to extra_hops assertion in CodeHeap::verify. See full dump in JBS.
>> 
>> # Internal Error (src/hotspot/share/memory/heap.cpp:838), pid=1525697, tid=1525715
>> # assert((count == 0) || (extra_hops < (16 + 2*count))) failed: CodeHeap: many extra hops due to optimization. blocks: 234, extra hops: 484.
>> Discussion in https://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2019-October/035508.html doesn't tell where assertion (extra_hops < (16 + 2*count) comes from.
>> 
>> In CodeHeap::mark_segmap_as_used wehn is_FreeBlock_join is true,  it creates extra hops in _segmap. _fragmentation_count in inced and trigger defrag_segmap when reach threshold. In my understanding, extra hop can not guarantee under (16 + 2*count). 
>> 
>> In following extreme case, before HeapBlock free, segmap is all 0 (each blob use 1 smallest segment), suppose free action is applied from right to left. This increase 9 unnecessary hop for 1 continous HeapBlock.  assertion (extra_hops < (16 + 2*count)  is not safe.
>> |0|0|0|0|0|0|0|0|0|0|
>> after free, it will be
>> |0|1|1|1|1|1|1|1|1|1|
>> 
>> Proposed fix is assert extra hops not exceed _fragmentation_count. And if it exceeds (16 + 2 * count), give warning on two many extra hops.
>> 
>> fastdebug tier1, tier2 with VerifyCodeCache passed on X86_64 linux, no extra assertion found.
>
> Hui Shi has updated the pull request incrementally with one additional commit since the last revision:
> 
>   update copyright year

It looks fine. But I have a question: are we reasonably sure that `extra_hops <= _fragmentation_count` always. More specifically, when `_fragmentation_count` drops to `0`, are `extra_hops` guaranteed to be `0` as well?

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PR: https://git.openjdk.java.net/jdk/pull/3212