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, 6 insertions, 2 deletions

diff --git a/tokio/src/macros/cfg.rs b/tokio/src/macros/cfg.rs
index 5e84a3ac..a3146688 100644
--- a/tokio/src/macros/cfg.rs
+++ b/tokio/src/macros/cfg.rs
@@ -1,8 +1,12 @@
 #![allow(unused_macros)]
 
-macro_rules! cfg_atomic_waker {
+macro_rules! cfg_resource_drivers {
     ($($item:item)*) => {
-        $( #[cfg(any(feature = "io-driver", feature = "time"))] $item )*
+        $(
+            #[cfg(any(feature = "io-driver", feature = "time"))]
+            #[cfg(not(loom))]
+            $item
+        )*
     }
 }