RFR: 8237842: Separate definitions for default cache line and padding sizes

[Aleksey Shipilev]

On Wed, 6 Dec 2023 09:58:42 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

> Sure. It think it is a good patch. However, given that you added this commment:
> 
> ```
> // Hardware prefetchers on current implementations may pull 2 cache lines
> // on access, therefore we pessimistically assume twice the cache line size
> // for padding.
> ```
> 
> Do you have anything that backs up the claim that this is the case for "current implementations"? 

I was merely trying to explain why `*2` is even there. The common explanation is referring to a "common wisdom" about adjacent cache line prefetchers. Granted, that might have been true only a decade ago, and it might not hold true anymore. I can change "current" to "some" or "some old" if you think that is more neutral.

Again, I don't think this PR should be discussing `*2` thing. It should be a separate deep dive and clear few-liner change later. That reminds me we would probably change the default for `ContendedPaddingWidth`, probably by tying it to `DEFAULT_PADDING_SIZE` once this lands. We should not be doing it here, though, because it would have footprint implications on platforms with large cache lines, like S390X.

-------------

PR Comment: https://git.openjdk.org/jdk/pull/16973#issuecomment-1842566889