RFR: 8341334: CDS: Parallel pretouch and relocation [v6]

[Aleksey Shipilev]

On Mon, 4 Nov 2024 15:06:18 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> In Leyden performance investigations, we have figured that `ArchiveRelocationMode=0` is saving 5..7 ms on HelloWorld startup. Mainline defaults to `ARM=1`, _losing_ as much. `ARM=0` was switched to `ARM=1` with [JDK-8294323](https://github.com/openjdk/jdk/commit/be70bc1c58eaec876aa1ab36eacba90b901ac9b8), which was delivered to JDK 17+ in in Apr 2023.
>> 
>> Profiling shows we spend time mem-faulting the memory loading the RO/RW regions, about 15 MB total. 15 MB in 5ms amounts to >4GB/sec, close to the single-threaded limits. I suspect the impact is larger if we relocate larger Metaspace, e.g. after dumping a CDS archive from a large application. There is little we can do to make the actual relocation part faster: the overwhelming majority of samples is on kernel side.
>> 
>> This PR implements the infrastructure for fast archive workers, and leverages it to do two things: 
>> 1. Parallel pretouch of mmap-ed regions. This causes the faults we would otherwise process in a single thread to be processed in multiple threads. The key thing is to eat the page faults in multiple threads; pretouching with a single thread would not help. This improvement gives the biggest bang in both `ArchiveRelocationMode`-s.
>> 2. Parallel core regions relocation. The RW/RO regions this code traverses is large, and we can squeeze more performance by parallelizing it. Without pretouch from (1), this step serves as one for RW/RO regions. 
>> 
>> (I'll put some performance data in the comments to show how these interact)
>> 
>> We can, in principle, only do (1), and have only a few hundred microseconds of unrealized gain without (2). I think we can do parallelism for heap region relocations as well, but so far I see it is a very small fraction of time spent in loading, so I left it for future work. I think (1) covers a lot of ground for heap region relocations already.
>> 
>> Additional testing:
>>  - [x] Linux x86_64 server fastdebug, `runtime/cds`
>>  - [x] Linux AArch64 server fastdebug, `all`
>
> Aleksey Shipilev has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains eight additional commits since the last revision:
> 
>  - Merge branch 'master' into JDK-8341334-cds-parallel-relocation
>  - Make sure we gracefully shutdown whatever happens, refix shutdown race
>  - Simpler bitmap distribution
>  - Capitalize constants
>  - Do not create worker threads too early: Mac/Windows are not yet ready to use Semaphores
>  - Don't change the patching order in -ArchiveParallelIteration case
>  - Flags
>  - Work

Looking for more reviews. @iklam, @cl4es, maybe?

We discussed it briefly during Leyden meetings, and I think we want to have this CDS-only part in JDK 24, and then consider moving this to a shared runtime code, so that we would be able to do concurrent pretouch for any memory, and have worker pool available for runtime tasks.

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PR Comment: https://git.openjdk.org/jdk/pull/21302#issuecomment-2457087764