Speedup max direct memory estimation via Unsafe

[franz1981]

Motivation:

PlatformDependent::estimateMaxDirectMemory reflectevly allocates MX Beans to detect JVM's arg MaxDirectMemory (often failing to find any, because unconfigured).

Modifications:

Allow unsafe to find Bits's max memory static field and use it as a fast path to detect the same direct memory limits used by the JVM runtime

Result:

Faster and less memory hungry (in term of mallocs) startup

[franz1981]

FYI this is the malloc flamegraph from a Quarkus's startup, for the Netty's PlatformDependent static init:

about half of the malloc events are due to allocating the MX beans (for no reasons, most of the time!).

[normanmaurer]

@franz1981 honestly I think the bigger advantage is that this will work even if Reflection is not possible

[franz1981]

@normanmaurer

Yep, that's indeed the other "good" thing, although i don't feel well using Unsafe again

[geoand]

Thanks for this @franz1981!

[franz1981]

@normanmaurer
i see that

private static final Pattern MAX_DIRECT_MEMORY_SIZE_ARG_PATTERN = Pattern.compile(
            "\\s*-XX:MaxDirectMemorySize\\s*=\\s*([0-9]+)\\s*([kKmMgG]?)\\s*$"); 

is not really needed anymore and I can just skip allocating it, in case we have our fast-path satisfied, saving both time and some useless allocation/memory footprint, wdyt?

[normanmaurer]

@franz1981 can you please port to main as well ?