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util: Reduce memory barriers and integer divisions in the work queue.
Improves scaling on multicore ARM devices.
This commit is contained in:
parent
d87af8ac52
commit
a063fdde73
2 changed files with 50 additions and 31 deletions
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@ -45,8 +45,7 @@
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// NB: the "buffer pool" strategy is not done for speed, but rather for
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// correctness. For more info, see the comment on `swap_buffer`
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// FIXME: all atomic operations in this module use a SeqCst ordering. That is
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// probably overkill
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// FIXME: more than likely, more atomic operations than necessary use SeqCst
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pub use self::Stolen::{Empty, Abort, Data};
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@ -58,7 +57,7 @@ use std::ptr;
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use std::sync::Mutex;
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use std::sync::atomic::{AtomicIsize, AtomicPtr};
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use std::sync::atomic::Ordering::SeqCst;
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use std::sync::atomic::Ordering::{Relaxed, SeqCst};
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// Once the queue is less than 1/K full, then it will be downsized. Note that
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// the deque requires that this number be less than 2.
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@ -240,8 +239,8 @@ impl<T: Send + 'static> Deque<T> {
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}
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unsafe fn push(&self, data: T) {
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let mut b = self.bottom.load(SeqCst);
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let t = self.top.load(SeqCst);
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let mut b = self.bottom.load(Relaxed);
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let t = self.top.load(Relaxed);
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let mut a = self.array.load(SeqCst);
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let size = b - t;
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if size >= (*a).size() - 1 {
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@ -256,11 +255,11 @@ impl<T: Send + 'static> Deque<T> {
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}
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unsafe fn pop(&self) -> Option<T> {
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let b = self.bottom.load(SeqCst);
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let b = self.bottom.load(Relaxed);
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let a = self.array.load(SeqCst);
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let b = b - 1;
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self.bottom.store(b, SeqCst);
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let t = self.top.load(SeqCst);
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let t = self.top.load(Relaxed);
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let size = b - t;
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if size < 0 {
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self.bottom.store(t, SeqCst);
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@ -282,12 +281,12 @@ impl<T: Send + 'static> Deque<T> {
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}
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unsafe fn steal(&self) -> Stolen<T> {
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let t = self.top.load(SeqCst);
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let old = self.array.load(SeqCst);
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let b = self.bottom.load(SeqCst);
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let a = self.array.load(SeqCst);
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let t = self.top.load(Relaxed);
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let old = self.array.load(Relaxed);
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let b = self.bottom.load(Relaxed);
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let size = b - t;
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if size <= 0 { return Empty }
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let a = self.array.load(SeqCst);
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if size % (*a).size() == 0 {
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if a == old && t == self.top.load(SeqCst) {
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return Empty
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@ -76,14 +76,26 @@ struct WorkerThread<QueueData: 'static, WorkData: 'static> {
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unsafe impl<QueueData: 'static, WorkData: 'static> Send for WorkerThread<QueueData, WorkData> {}
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static SPIN_COUNT: u32 = 128;
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static SPINS_UNTIL_BACKOFF: u32 = 100;
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static BACKOFF_INCREMENT_IN_US: u32 = 5;
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const SPINS_UNTIL_BACKOFF: u32 = 128;
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const BACKOFF_INCREMENT_IN_US: u32 = 5;
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const BACKOFFS_UNTIL_CONTROL_CHECK: u32 = 6;
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fn next_power_of_two(mut v: u32) -> u32 {
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v -= 1;
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v |= v >> 1;
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v |= v >> 2;
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v |= v >> 4;
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v |= v >> 8;
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v |= v >> 16;
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v += 1;
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v
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}
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impl<QueueData: Send, WorkData: Send> WorkerThread<QueueData, WorkData> {
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/// The main logic. This function starts up the worker and listens for
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/// messages.
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fn start(&mut self) {
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let deque_index_mask = next_power_of_two(self.other_deques.len() as u32) - 1;
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loop {
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// Wait for a start message.
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let (mut deque, ref_count, queue_data) = match self.port.recv().unwrap() {
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@ -110,8 +122,16 @@ impl<QueueData: Send, WorkData: Send> WorkerThread<QueueData, WorkData> {
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let mut i = 0;
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let mut should_continue = true;
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loop {
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let victim = (self.rng.next_u32() as usize) % self.other_deques.len();
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match self.other_deques[victim].steal() {
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// Don't just use `rand % len` because that's slow on ARM.
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let mut victim;
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loop {
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victim = self.rng.next_u32() & deque_index_mask;
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if (victim as usize) < self.other_deques.len() {
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break
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}
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}
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match self.other_deques[victim as usize].steal() {
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Empty | Abort => {
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// Continue.
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}
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@ -123,23 +143,23 @@ impl<QueueData: Send, WorkData: Send> WorkerThread<QueueData, WorkData> {
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}
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if i > SPINS_UNTIL_BACKOFF {
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if back_off_sleep >= BACKOFF_INCREMENT_IN_US *
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BACKOFFS_UNTIL_CONTROL_CHECK {
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match self.port.try_recv() {
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Ok(WorkerMsg::Stop) => {
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should_continue = false;
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break
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}
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Ok(WorkerMsg::Exit) => return,
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Ok(_) => panic!("unexpected message"),
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_ => {}
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}
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}
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unsafe {
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usleep(back_off_sleep as u32);
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}
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back_off_sleep += BACKOFF_INCREMENT_IN_US;
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}
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if i == SPIN_COUNT {
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match self.port.try_recv() {
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Ok(WorkerMsg::Stop) => {
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should_continue = false;
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break
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}
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Ok(WorkerMsg::Exit) => return,
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Ok(_) => panic!("unexpected message"),
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_ => {}
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}
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i = 0
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} else {
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i += 1
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@ -164,7 +184,7 @@ impl<QueueData: Send, WorkData: Send> WorkerThread<QueueData, WorkData> {
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// The work is done. Now decrement the count of outstanding work items. If this was
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// the last work unit in the queue, then send a message on the channel.
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unsafe {
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if (*ref_count).fetch_sub(1, Ordering::SeqCst) == 1 {
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if (*ref_count).fetch_sub(1, Ordering::Release) == 1 {
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self.chan.send(SupervisorMsg::Finished).unwrap()
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}
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}
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@ -189,7 +209,7 @@ impl<'a, QueueData: 'static, WorkData: Send + 'static> WorkerProxy<'a, QueueData
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#[inline]
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pub fn push(&mut self, work_unit: WorkUnit<QueueData, WorkData>) {
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unsafe {
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drop((*self.ref_count).fetch_add(1, Ordering::SeqCst));
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drop((*self.ref_count).fetch_add(1, Ordering::Relaxed));
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}
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self.worker.push(work_unit);
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}
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