runtime: cleanup and add config options (#1807)

* runtime: cleanup and add config options

This patch finishes the cleanup as part of the transition to Tokio 0.2.
A number of changes were made to take advantage of having all Tokio
types in a single crate. Also, fixes using Tokio types from
`spawn_blocking`.

* Many threads, one resource driver

Previously, in the threaded scheduler, a resource driver (mio::Poll /
timer combo) was created per thread. This was more or less fine, except
it required balancing across the available drivers. When using a
resource driver from **outside** of the thread pool, balancing is
tricky. The change was original done to avoid having a dedicated driver
thread.

Now, instead of creating many resource drivers, a single resource driver
is used. Each scheduler thread will attempt to "lock" the resource
driver before parking on it. If the resource driver is already locked,
the thread uses a condition variable to park. Contention should remain
low as, under load, the scheduler avoids using the drivers.

* Add configuration options to enable I/O / time

New configuration options are added to `runtime::Builder` to allow
enabling I/O and time drivers on a runtime instance basis. This is
useful when wanting to create lightweight runtime instances to execute
compute only tasks.

* Bug fixes

The condition variable parker is updated to the same algorithm used in
`std`. This is motivated by some potential deadlock cases discovered by
`loom`.

The basic scheduler is fixed to fairly schedule tasks. `push_front` was
accidentally used instead of `push_back`.

I/O, time, and spawning now work from within `spawn_blocking` closures.

* Misc cleanup

The threaded scheduler is no longer generic over `P :Park`. Instead, it
is hard coded to a specific parker. Tests, including loom tests, are
updated to use `Runtime` directly. This provides greater coverage.

The `blocking` module is moved back into `runtime` as all usage is
within `runtime` itself.

1 files changed, 18 insertions, 0 deletions

diff --git a/tokio/src/runtime/blocking/schedule.rs b/tokio/src/runtime/blocking/schedule.rs
new file mode 100644
index 00000000..461b12c3
--- /dev/null
+++ b/tokio/src/runtime/blocking/schedule.rs
@@ -0,0 +1,18 @@
+use crate::task::{Schedule, Task};
+
+/// `task::Schedule` implementation that does nothing. This is unique to the
+/// blocking scheduler as tasks scheduled are not really futures but blocking
+/// operations.
+pub(super) struct NoopSchedule;
+
+impl Schedule for NoopSchedule {
+    fn bind(&self, _task: &Task<Self>) {}
+
+    fn release(&self, _task: Task<Self>) {}
+
+    fn release_local(&self, _task: &Task<Self>) {}
+
+    fn schedule(&self, _task: Task<Self>) {
+        unreachable!();
+    }
+}