RFR: 8312023: Parallel pretouch should shortcut when only 1 thread is needed [v2]

[Kim Barrett]

On Thu, 30 Nov 2023 20:53:05 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> > There may have been some concern that some task might care about the kind of thread it runs on?
> 
> Exactly, in generic case you don't know if caller thread is set up to do the task in the way the worker threads are. This pretouching code is one of the cases where we know running in caller is fine. Existing code even does it when no worker pool is available.

That's easily dealt with.  Provide a configuration mechanism (a boolean argument for run_task or a variant of
run_task with a different name) that controls whether running on the current thread is permitted and defaults
to no.  Then let clients opt in to that feature.  In my opinion that's a lot better than having each client that can
run on the current thread provide an ad hoc implementation for doing so.

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PR Comment: https://git.openjdk.org/jdk/pull/16882#issuecomment-1835156226