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This implements a hash set which is wait-free for lookup and lock-free for insert (unless resizing) to use for fstring de-duplication. As highlighted in https://bugs.ruby-lang.org/issues/19288, heavy use of fstrings (frozen interned strings) can significantly reduce the parallelism of Ractors. I tried a few other approaches first: using an RWLock, striping a series of RWlocks (partitioning the hash N-ways to reduce lock contention), and putting a cache in front of it. All of these improved the situation, but were unsatisfying as all still required locks for writes (and granular locks are awkward, since we run the risk of needing to reach a vm barrier) and this table is somewhat write-heavy. My main reference for this was Cliff Click's talk on a lock free hash-table for java https://www.youtube.com/watch?v=HJ-719EGIts. It turns out this lock-free hash set is made easier to implement by a few properties: * We only need a hash set rather than a hash table (we only need keys, not values), and so the full entry can be written as a single VALUE * As a set we only need lookup/insert/delete, no update * Delete is only run inside GC so does not need to be atomic (It could be made concurrent) * I use rb_vm_barrier for the (rare) table rebuilds (It could be made concurrent) We VM lock (but don't require other threads to stop) for table rebuilds, as those are rare * The conservative garbage collector makes deferred replication easy, using a T_DATA object Another benefits of having a table specific to fstrings is that we compare by value on lookup/insert, but by identity on delete, as we only want to remove the exact string which is being freed. This is faster and provides a second way to avoid the race condition in https://bugs.ruby-lang.org/issues/21172. This is a pretty standard open-addressing hash table with quadratic probing. Similar to our existing st_table or id_table. Deletes (which happen on GC) replace existing keys with a tombstone, which is the only type of update which can occur. Tombstones are only cleared out on resize. Unlike st_table, the VALUEs are stored in the hash table itself (st_table's bins) rather than as a compact index. This avoids an extra pointer dereference and is possible because we don't need to preserve insertion order. The table targets a load factor of 2 (it is enlarged once it is half full). |
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| .. | ||
| pending.rb | ||
| runner.rb | ||
| test_attr.rb | ||
| test_autoload.rb | ||
| test_block.rb | ||
| test_class.rb | ||
| test_constant_cache.rb | ||
| test_env.rb | ||
| test_eval.rb | ||
| test_exception.rb | ||
| test_fiber.rb | ||
| test_finalizer.rb | ||
| test_flip.rb | ||
| test_flow.rb | ||
| test_fork.rb | ||
| test_gc.rb | ||
| test_insns.rb | ||
| test_io.rb | ||
| test_jump.rb | ||
| test_literal.rb | ||
| test_literal_suffix.rb | ||
| test_load.rb | ||
| test_marshal.rb | ||
| test_massign.rb | ||
| test_method.rb | ||
| test_objectspace.rb | ||
| test_proc.rb | ||
| test_ractor.rb | ||
| test_string.rb | ||
| test_struct.rb | ||
| test_syntax.rb | ||
| test_thread.rb | ||
| test_yjit.rb | ||
| test_yjit_30k_ifelse.rb | ||
| test_yjit_30k_methods.rb | ||
| test_yjit_rust_port.rb | ||