Lets us sever the dependency between stats & exec ctx (finally).
More work likely needs to go into the *mechanism* used here (I'm not a
fan of the per thread index), but that's also something we can address
later.
This PR implements a work-stealing thread pool for use inside
EventEngine implementations. Because of historical risks here, I've
guarded the new implementation behind an experiment flag:
`GRPC_EXPERIMENTS=work_stealing`. Current default behavior is the
original thread pool implementation.
Benchmarks look very promising:
```
bazel test \
--test_timeout=300 \
--config=opt -c opt \
--test_output=streamed \
--test_arg='--benchmark_format=csv' \
--test_arg='--benchmark_min_time=0.15' \
--test_arg='--benchmark_filter=_FanOut' \
--test_arg='--benchmark_repetitions=15' \
--test_arg='--benchmark_report_aggregates_only=true' \
test/cpp/microbenchmarks:bm_thread_pool
```
2023-05-04: `bm_thread_pool` benchmark results on my local machine (64
core ThreadRipper PRO 3995WX, 256GB memory), comparing this PR to
master:
2023-05-04: `bm_thread_pool` benchmark results in the Linux RBE
environment (unsure of machine configuration, likely small), comparing
this PR to master.
---------
Co-authored-by: drfloob <drfloob@users.noreply.github.com>
* ThreadPool benchmarks
These are nearly identical to the EventEngine benchmarks at the moment. We can consider removing the redundant tests from the EventEngine code and focusing on EventEngine-specific things (e.g., timer cancellation)
* rm unused header
* rm leak
* fix: moved dependencies
Craig Tiller
