[foreign-preview] RFR: 8276648: Downcall stubs are never freed

[Maurizio Cimadamore]

On Tue, 22 Feb 2022 14:06:28 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

> Hi,
> 
> The current code adds native invokers (downcall stubs) to a CocurrentHashMap cache, and they stay there until the application exits.
> 
> This patch replaces that cache with a SoftReferenceCache (brought to the top level from AbstractLinker), which allows at least the values to be evicted when they become unreachable.
> 
> Those value, which are NativeEntryPoint instances, are also registered with a cleaner, whose cleanup action will free the stub from the code cache.
> 
> Thanks,
> Jorn

Looks good

src/java.base/share/classes/jdk/internal/foreign/abi/NativeEntryPoint.java line 68:

> 66: 
> 67:         CacheKey key = new CacheKey(methodType, abi, Arrays.asList(argMoves), Arrays.asList(returnMoves), needsReturnBuffer);
> 68:         return INVOKER_CACHE.get(key, k -> {

This uses a combination of soft references (cache) and phantom references (cleaner). So, cache entries will be evicted when the NEP is softly reachable - meaning that the GC will clear the entries when under pressure. But the cleaner won't be able to run until the entry is evicted (because the entry has a soft reference to the NEP, and to be phantom-reachable you need NOT to be soft-reachable). Assuming that's what you want, then it looks ok. E.g. the cleaner here will "just" make sure that some action is executed when the entry is evicted.

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Marked as reviewed by mcimadamore (Committer).

PR: https://git.openjdk.java.net/panama-foreign/pull/651