RFR: 8191786: Thread-SMR hash table size should be dynamic

[Daniel D.Daugherty]

On Tue, 25 May 2021 01:10:37 GMT, David Holmes <dholmes at openjdk.org> wrote:

> What is the motivation behind this?

Finishing code that was left incomplete by the Thread-SMR project. We either
get rid of the hash_table_size calculations (from two places) in the base code
or we switch to a Hashtable that supports specifying a size at runtime instead
of compile time.


> What do we see happen with the current HT if we have too many threads?

We haven't seen any problems with the current code. I'm just cleaning up this
long standing piece of work.


> What will happen with the new HT when it has to resize - will we see a
> pause while that happens?

These Thread-SMR hash tables don't resize. They are created and live for
a very short time. They contain either hazard ptrs or they contain JavaThread*
that are protected by hazard ptrs.

ThreadsSMRSupport::free_list() creates a ThreadScanHashtable of the
current hazard ptrs (there typically are not very many of these).

ThreadsSMRSupport::is_a_protected_JavaThread() creates a
ThreadScanHashtable of JavaThread* that are protected by the
current hazard ptr in use by the system.

Both of these ThreadScanHashtable objects only live as long as the
function that created them, i.e., not long.

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PR: https://git.openjdk.java.net/jdk/pull/4168