RFR: 8257074 Update the ByteBuffers micro benchmark

[Maurizio Cimadamore]

On Wed, 25 Nov 2020 12:41:40 GMT, Chris Hegarty <chegar at openjdk.org> wrote:

> The ByteBuffers micro benchmark seems to be a little dated. 
> 
> It should be a useful resource to leverage when analysing the performance impact of any potential implementation changes in the byte buffer classes. More specifically, the impact of such changes on the performance of sharp memory access operations. 
> 
> This issue proposes to update the benchmark in the following ways to meet the aforementioned use-case: 
> 
> 1. Remove allocation from the individual benchmarks - it just creates noise. 
> 2. Consolidate per-thread shared heap and direct buffers. 
> 3. All scenarios now use absolute memory access operations - so no state of the shared buffers is considered. 
> 4. Provide more reasonable default fork, warmup, etc, out-of-the-box. 
> 5. There seems to have been an existing bug in the test where the size parameter was not always considered - this is now fixed.

Looks like a good cleanup. I agree that mixing access and instantiation is not good from a benchmark perspective - although, given that direct buffers have a rather convoluted allocation process, I guess it would be useful, in the long term, to track performances of that too (this can be done by coupling direct buffer allocation with calls to Unsafe::invokeCleaner, so that the benchmark is not affected by the GC activity).

Also, given a carrier e.g. `float` there are at least two ways to get a float from a byte array:

* ByteBuffer::getFloat (both direct/heap)
* FloatBuffer::get() (both direct/heap)

The benchmark here focuses on the first bullet, but I think at some point we should also cover the remaining axis, so that, for each carrier, we really do at least 4 tests. Of course if you take into account swappiness for non-byte carriers, you get an extra variant for direct buffers as well, as you wanna test native endian vs. non-native endian.

So many possibilities :-)

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Marked as reviewed by mcimadamore (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/1430