On Wed, 18 Nov 2020 10:08:20 GMT, Maurizio Cimadamore <mcimadam...@openjdk.org> wrote:
>> The current memory segment implementation defines a hierarchy with 3 >> concrete classes: one for heap segments, one for native segments and one for >> mapped segments. >> >> Since there can be many kinds of heap segments (e.g. created from a byte[] >> or from a float[]) the current implementation is prone to type profile >> pollution problems: if enough heap segments are created (of different >> kinds), the JIT compiler will give up on speculating on the heap segment >> kind, which will then result in poor performances. >> >> This issue can be reproduced in one of the existing benchmark, by adding >> some initialization code which is enough to pollute the types profiles. When >> that happens, performance numbers look like the following: >> >> Benchmark (polluteProfile) Mode Cnt Score >> Error Units >> LoopOverNonConstantHeap.segment_loop false avgt 10 0.285 ± >> 0.003 ms/op >> LoopOverNonConstantHeap.segment_loop true avgt 10 5.540 ± >> 0.143 ms/op >> >> (Thanks to Vlad for coming up for the exact incantation which leads to >> profile pollution :-) ) >> >> The solution is to create a sharp subclass for each heap segment case. With >> this, C2 has always a sharp Unsafe *base* to work with, and performances are >> stable regardless of profile pollution attempts. >> >> This patch also tweaks the benchmark for heap segments so that it checks it >> with and without profile pollution. > > Maurizio Cimadamore has updated the pull request incrementally with one > additional commit since the last revision: > > Fix typo Marked as reviewed by sundar (Reviewer). ------------- PR: https://git.openjdk.java.net/jdk/pull/1259
