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executor: rewrite the work-stealing thread pool (#1657)

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This patch is a ground up rewrite of the existing work-stealing thread
pool. The goal is to reduce overhead while simplifying code when
possible.

At a high level, the following architectural changes were made:

- The local run queues were switched for bounded circle buffer queues.
- Reduce cross-thread synchronization.
- Refactor task constructs to use a single allocation and always include
  a join handle (#887).
- Simplify logic around putting workers to sleep and waking them up.

**Local run queues**

Move away from crossbeam's implementation of the Chase-Lev deque. This
implementation included unnecessary overhead as it supported
capabilities that are not needed for the work-stealing thread pool.
Instead, a fixed size circle buffer is used for the local queue. When
the local queue is full, half of the tasks contained in it are moved to
the global run queue.

**Reduce cross-thread synchronization**

This is done via many small improvements. Primarily, an upper bound is
placed on the number of concurrent stealers. Limiting the number of
stealers results in lower contention. Secondly, the rate at which
workers are notified and woken up is throttled. This also reduces
contention by preventing many threads from racing to steal work.

**Refactor task structure**

Now that Tokio is able to target a rust version that supports
`std::alloc` as well as `std::task`, the pool is able to optimize how
the task structure is laid out. Now, a single allocation per task is
required and a join handle is always provided enabling the spawner to
retrieve the result of the task (#887).

**Simplifying logic**

When possible, complexity is reduced in the implementation. This is done
by using locks and other simpler constructs in cold paths. The set of
sleeping workers is now represented as a `Mutex<VecDeque<usize>>`.
Instead of optimizing access to this structure, we reduce the amount the
pool must access this structure.

Secondly, we have (temporarily) removed `threadpool::blocking`. This
capability will come back later, but the original implementation was way
more complicated than necessary.

**Results**

The thread pool benchmarks have improved significantly:

Old thread pool:

```
test chained_spawn ... bench:   2,019,796 ns/iter (+/- 302,168)
test ping_pong     ... bench:   1,279,948 ns/iter (+/- 154,365)
test spawn_many    ... bench:  10,283,608 ns/iter (+/- 1,284,275)
test yield_many    ... bench:  21,450,748 ns/iter (+/- 1,201,337)
```

New thread pool:

```
test chained_spawn ... bench:     147,943 ns/iter (+/- 6,673)
test ping_pong     ... bench:     537,744 ns/iter (+/- 20,928)
test spawn_many    ... bench:   7,454,898 ns/iter (+/- 283,449)
test yield_many    ... bench:  16,771,113 ns/iter (+/- 733,424)
```

Real-world benchmarks improve significantly as well. This is testing the hyper hello
world server using: `wrk -t1 -c50 -d10`:

Old scheduler:

```
Running 10s test @ http://127.0.0.1:3000
  1 threads and 50 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency   371.53us   99.05us   1.97ms   60.53%
    Req/Sec   114.61k     8.45k  133.85k    67.00%
  1139307 requests in 10.00s, 95.61MB read
Requests/sec: 113923.19
Transfer/sec:      9.56MB
```

New scheduler:

```
Running 10s test @ http://127.0.0.1:3000
  1 threads and 50 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency   275.05us   69.81us   1.09ms   73.57%
    Req/Sec   153.17k    10.68k  171.51k    71.00%
  1522671 requests in 10.00s, 127.79MB read
Requests/sec: 152258.70
Transfer/sec:     12.78MB
```
This commit is contained in:
Carl Lerche
2019-10-19 11:09:40 -07:00
committed by GitHub
parent 2a181320b7
commit ed5a94eb2d
100 changed files with 7409 additions and 6796 deletions
Download Patch File Download Diff File Expand all files Collapse all files
tokio-executor/src/global.rs
+69 -30
View File
@@ -1,4 +1,7 @@
use super::{Executor, SpawnError};
#[cfg(feature = "thread-pool")]
use crate::thread_pool::ThreadPool;
use crate::{Executor, SpawnError};
use std::cell::Cell;
use std::future::Future;
use std::pin::Pin;
@@ -37,17 +40,18 @@ impl DefaultExecutor {
#[inline]
fn with_current<F: FnOnce(&mut dyn Executor) -> R, R>(f: F) -> Option<R> {
EXECUTOR.with(
|current_executor| match current_executor.replace(State::Active) {
State::Ready(executor_ptr) => {
let executor = unsafe { &mut *executor_ptr };
let result = f(executor);
current_executor.set(State::Ready(executor_ptr));
Some(result)
}
State::Empty | State::Active => None,
},
)
EXECUTOR.with(|current_executor| match current_executor.get() {
State::Ready(executor_ptr) => {
let executor = unsafe { &mut *executor_ptr };
Some(f(executor))
}
#[cfg(feature = "thread-pool")]
State::ThreadPool(threadpool_ptr) => {
let mut thread_pool = unsafe { &*threadpool_ptr };
Some(f(&mut thread_pool))
}
State::Empty => None,
})
}
}
@@ -55,10 +59,13 @@ impl DefaultExecutor {
enum State {
// default executor not defined
Empty,
// default executor is defined and ready to be used
// default executor is a thread pool instance.
#[cfg(feature = "thread-pool")]
ThreadPool(*const ThreadPool),
// default executor is set to a custom executor.
Ready(*mut dyn Executor),
// default executor is currently active (used to detect recursive calls)
Active,
}
thread_local! {
@@ -132,7 +139,26 @@ pub fn spawn<T>(future: T)
where
T: Future<Output = ()> + Send + 'static,
{
DefaultExecutor::current().spawn(Box::pin(future)).unwrap()
EXECUTOR.with(|current_executor| match current_executor.get() {
State::Ready(executor_ptr) => {
let executor = unsafe { &mut *executor_ptr };
executor.spawn(Box::pin(future)).unwrap();
}
#[cfg(feature = "thread-pool")]
State::ThreadPool(threadpool_ptr) => {
let thread_pool = unsafe { &*threadpool_ptr };
thread_pool.spawn_background(future);
}
State::Empty => panic!("must be called from the context of Tokio runtime"),
})
}
#[cfg(feature = "thread-pool")]
pub(crate) fn with_threadpool<F, R>(thread_pool: &ThreadPool, f: F) -> R
where
F: FnOnce() -> R,
{
with_state(State::ThreadPool(thread_pool as *const ThreadPool), f)
}
/// Set the default executor for the duration of the closure
@@ -143,9 +169,24 @@ pub fn with_default<T, F, R>(executor: &mut T, f: F) -> R
where
T: Executor,
F: FnOnce() -> R,
{
// While scary, this is safe. The function takes a
// `&mut Executor`, which guarantees that the reference lives for the
// duration of `with_default`.
//
// Because we are always clearing the TLS value at the end of the
// function, we can cast the reference to 'static which thread-local
// cells require.
let executor = unsafe { hide_lt(executor as &mut _ as *mut _) };
with_state(State::Ready(executor), f)
}
fn with_state<F, R>(state: State, f: F) -> R
where
F: FnOnce() -> R,
{
EXECUTOR.with(|cell| {
let was = cell.get();
let was = cell.replace(State::Empty);
// Ensure that the executor is removed from the thread-local context
// when leaving the scope. This handles cases that involve panicking.
@@ -159,16 +200,15 @@ where
let _reset = Reset(cell, was);
// While scary, this is safe. The function takes a
// `&mut Executor`, which guarantees that the reference lives for the
// duration of `with_default`.
//
// Because we are always clearing the TLS value at the end of the
// function, we can cast the reference to 'static which thread-local
// cells require.
let executor = unsafe { hide_lt(executor as &mut _ as *mut _) };
if let State::Ready(executor) = state {
let executor = unsafe { &mut *executor };
cell.set(State::Ready(executor));
if executor.status().is_err() {
panic!("executor not active; is this because `with_default` is called with `DefaultExecutor`?");
}
}
cell.set(state);
f()
})
@@ -183,7 +223,7 @@ unsafe fn hide_lt<'a>(p: *mut (dyn Executor + 'a)) -> *mut (dyn Executor + 'stat
#[cfg(test)]
mod tests {
use super::{with_default, DefaultExecutor, Executor};
use super::{with_default, DefaultExecutor};
#[test]
fn default_executor_is_send_and_sync() {
@@ -193,12 +233,11 @@ mod tests {
}
#[test]
#[should_panic]
fn nested_default_executor_status() {
let _enter = super::super::enter().unwrap();
let mut executor = DefaultExecutor::current();
let result = with_default(&mut executor, || DefaultExecutor::current().status());
assert!(result.err().unwrap().is_shutdown())
let _result = with_default(&mut executor, || ());
}
}