RFR: 8350852: Implement JMH benchmark for sparse CodeCache

[Dean Long]

On Thu, 27 Feb 2025 22:23:23 GMT, Evgeny Astigeevich <eastigeevich at openjdk.org> wrote:

> This benchmark is used to check performance impact of the code cache being sparse.
> 
> We use C2 compiler to compile the same Java method multiple times to produce as many code as needed. The Java method is not trivial. It adds two 40 digit positive integers. These compiled methods represent the active methods in the code cache. We split active methods into groups. We put a group into a fixed size code region. We make a code region aligned by its size. CodeCache becomes sparse when code regions are not fully filled. We measure the time taken to call all active methods.
> 
> Results: code region size 2M (2097152) bytes
> - Intel Xeon Platinum 8259CL
> 
> |activeMethodCount	|groupCount	|Methods/Group	|Score	|Error	|Units	|Diff	|
> |---	|---	|---	|---	|---	|---	|---	|
> |128	|1	|128	|19.577	|0.619	|us/op	|	|
> |128	|32	|4	|22.968	|0.314	|us/op	|17.30%	|
> |128	|48	|3	|22.245	|0.388	|us/op	|13.60%	|
> |128	|64	|2	|23.874	|0.84	|us/op	|21.90%	|
> |128	|80	|2	|23.786	|0.231	|us/op	|21.50%	|
> |128	|96	|1	|26.224	|1.16	|us/op	|34%	|
> |128	|112	|1	|27.028	|0.461	|us/op	|38.10%	|
> |256	|1	|256	|47.43	|1.146	|us/op	|	|
> |256	|32	|8	|63.962	|1.671	|us/op	|34.90%	|
> |256	|48	|5	|63.396	|0.247	|us/op	|33.70%	|
> |256	|64	|4	|66.604	|2.286	|us/op	|40.40%	|
> |256	|80	|3	|59.746	|1.273	|us/op	|26%	|
> |256	|96	|3	|63.836	|1.034	|us/op	|34.60%	|
> |256	|112	|2	|63.538	|1.814	|us/op	|34%	|
> |512	|1	|512	|172.731	|4.409	|us/op	|	|
> |512	|32	|16	|206.772	|6.229	|us/op	|19.70%	|
> |512	|48	|11	|215.275	|2.228	|us/op	|24.60%	|
> |512	|64	|8	|212.962	|2.028	|us/op	|23.30%	|
> |512	|80	|6	|201.335	|12.519	|us/op	|16.60%	|
> |512	|96	|5	|198.133	|6.502	|us/op	|14.70%	|
> |512	|112	|5	|193.739	|3.812	|us/op	|12.20%	|
> |768	|1	|768	|325.154	|5.048	|us/op	|	|
> |768	|32	|24	|346.298	|20.196	|us/op	|6.50%	|
> |768	|48	|16	|350.746	|2.931	|us/op	|7.90%	|
> |768	|64	|12	|339.445	|7.927	|us/op	|4.40%	|
> |768	|80	|10	|347.408	|7.355	|us/op	|6.80%	|
> |768	|96	|8	|340.983	|3.578	|us/op	|4.90%	|
> |768	|112	|7	|353.949	|2.98	|us/op	|8.90%	|
> |1024	|1	|1024	|368.352	|5.961	|us/op	|	|
> |1024	|32	|32	|463.822	|6.274	|us/op	|25.90%	|
> |1024	|48	|21	|457.674	|15.144	|us/op	|24.20%	|
> |1024	|64	|16	|477.694	|0.986	|us/op	|29.70%	|
> |1024	|80	|13	|484.901	|32.601	|us/op	|31.60%	|
> |1024	|96	|11	|480.8	|27.088	|us/op	|30.50%	|
> |1024	|112	|9	|474.416	|10.053	|us/op	|28.80%	|
> 
> - AArch64 Neoverse N1
> 
> |activeMethodCount	|groupCount	|Methods/Group	|Score	|Error	|Units	|Diff	|
> |---	|---	|---	|---	|---	|---	|---	|
> |128	|1	|128	|25.297	|0.792	|us/op	|	|
> |128	|32	|4	|31.451...

Interesting.  So there may be a "density" performance penalty even for short branch ranges if the source and destination are not in the same region.  So for example a branch at 0x1ffff0 with a target of 0x200000 are not in the same region, even though the distance is only 0x10.

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PR Comment: https://git.openjdk.org/jdk/pull/23831#issuecomment-2712078132