RFR: 8339771: RISC-V: Reduce icache flushes [v2]

[Robbin Ehn]

On Tue, 10 Sep 2024 12:53:18 GMT, Robbin Ehn <rehn at openjdk.org> wrote:

>> Hey, please consider,
>> 
>> All code which is offline (behind a barrier) do not need global icache flushes.
>> As we can instead in slow path locally (thread and hart) emit fence.i.
>> But if we were to be context switch do a hart which have not had fence.i emitted we can still fetch stale instructions.
>> To handle this case new now have kernel support:
>> https://docs.kernel.org/arch/riscv/cmodx.html
>> 
>> It's not perfect as we will be emitting fence.i on any context switch for any thread with this patch, even if that thread do not execute on code heap (non attached native thread), and even if there was no changes to code heap.
>> But this is in many cases much faster as the icache flush global IPI is very intrusive.
>> Particular cases are running a concurrent gc with small head room.
>> In such scenario I measured 15% increased throughput on VF2.
>> A large CPU or less head room (faster GC cycles) will yield even more performance boost.
>> 
>> Note that this requires 6.10 kernel.
>> 
>> I'm running VF2 with 6.11-rc3 kernel and this passed tier1-3. (With setting on)
>> 
>> Later we probably want this default on, but as it's hard to test I'll leave default off.
>
> Robbin Ehn has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Comment, moved init after feature enabling

default	-XX:-UseCtxFencei   -XX:+UseCtxFencei	
fop	                2830	2731        3.63%
h2	              25361    25403	-0.17%
jython	      32482    32006	1.49%
luindex	        4231	4369	-3.16%
lusearch	        6142	5867	4.69%
lusearch-fix	6337	6243	1.51%
pmd	        9171	8970	2.24%
                                                        1.46%
zgc xmx512m			
fop	2583	2376	8.71%
h2	34520	33518	2.99%
jython	36494	35243	3.55%
luindex	4615	4497	2.62%
lusearch	6705	6732	-0.40%
lusearch-fix	6827	6485	5.27%
pmd	8385	8128	3.16%
			3.70%
zgc xmx128m			
fop	3336	2718	22.74%
jython	44385	35957	23.44%
luindex	4552	4460	2.06%
lusearch	9604	7272	32.07%
lusearch-fix	10095	7489	34.80%
pmd	11238	9169	22.57%
			22.95%

-------------

PR Comment: https://git.openjdk.org/jdk/pull/20913#issuecomment-2354669265