RFR: JDK-8296764 NMT: reduce loads in os::malloc

[Andrew Dinn]

On Thu, 10 Nov 2022 06:11:28 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

> We read the NMT level at every ps::malloc/realloc/free etc, even if NMT is off. That is unavoidable. But we also read a second variable that shadows the NMT level `_nmt_was_initialized`. That state is synonymous with NMT level "unknown," so there is no need for this second load.
> 
> Furthermore, NMT level is volatile. That had been necessary in older times since we used to shut down NMT on resource exhaustion. But since [JDK-8256844](https://bugs.openjdk.org/browse/JDK-8256844), NMT level is fixed at initialization, so we can remove the volatile specifier.
> 
> Patch 
> - removes `_nmt_was_initialized` and redirects it to `MemTracker::is_initialized()`, which gets inlined to a load of NMT level. 
> - makes NMT level non-volatile
> - removes _is_nmt_env_valid, which had been dead code

Looks good.

src/hotspot/share/services/nmtPreInit.hpp line 33:

> 31: #include "runtime/atomic.hpp"
> 32: #endif
> 33: #include "services/memTracker.hpp" // for MemTracker::is_initialized()

Is this comment really needed?

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Marked as reviewed by adinn (Reviewer).

PR: https://git.openjdk.org/jdk/pull/11080