RFR: 8285447: StackWalker minimal batch size should be optimized for getCallerClass

[Volker Simonis]

On Fri, 8 Sep 2023 16:57:14 GMT, Mandy Chung <mchung at openjdk.org> wrote:

> Typically it will find the caller class at the second stack frame from the frame of executing `StackWalker::getCallerClass`.   The initial size of the buffer can be changed from 8 to 4 (the top 2 elements are reserved for implementation use).   If it fetches another batch, `getCallerClass` may be invoked via core reflection, so subsequent batches can be increased to a larger size.   This PR also moves the benchmark for `getCallerClass` in its own file because it does not need to test with different depth and can be separated from StackWalkBench.

In general looks good. But once you on this, I suggest to add the following additional optimizations:
- `FrameBuffer.START_POS` is `2` but as far as I can see, currently only the 0th element is reserved for passing a "magic" object passed between the JVM and Java code. So this should be set to `1`.
- `MIN_BATCH_SIZE` should be defined as in terms of `FrameBuffer.START_POS` (i.e. `FrameBuffer.START_POS + 1`)
- `StackFrameTraverser.batchSize()` should be changed to **really** honor the `estimateDepth` of `StackWalker.getInstance(.., estimateDepth)` (currently it is always `SMALL_BATCH` == `8` even if the caller specifies a smaller number):

-                int initialBatchSize = Math.max(walker.estimateDepth(), SMALL_BATCH);
+                int initialBatchSize = Math.max(walker.estimateDepth() + FrameBuffer.START_POS, MIN_BATCH_SIZE);

- In `CallerClassFinder.initFrameBuffer()` you can then use `this.frameBuffer = new ClassFrameBuffer(walker, FrameBuffer.START_POS + 2)`.

With these changes you would:
- save one more frame for the `getCallerClass()` case 
- save more frames for any user supplied `estimateDepth` values smaller than `8`
- don't implicitly expose `FrameBuffer.START_POS` to user space. Currently the user supplied `estimateDepth` value will be implicitly subtracted by `FrameBuffer.START_POS`
- make all the internal size computations depend explicitly on `FrameBuffer.START_POS` for better maintainability.

src/java.base/share/classes/java/lang/StackStreamFactory.java line 758:

> 756:          * So start the initial batch size with the minimum size.
> 757:          * If it fetches the second batch, getCallerClass may be invoked via
> 758:          * core reflection, can increase the next batch size.

This sentence reads strange to me. Maybe "If it fetches the second batch, getCallerClass may be invoked via core reflection, *so the next batch size will be increased.*"?

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Changes requested by simonis (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/15642#pullrequestreview-1619856398
PR Review Comment: https://git.openjdk.org/jdk/pull/15642#discussion_r1321463264