adjusted scheduler prioritization algorithm (#45367)

This fixes a number of issues where zed depends on the order of polling which changed when switching scheduler. We have adjusted the algorithm so it matches the previous order while keeping the prioritization feature.

Release Notes:
- N/A

crates/gpui/src/queue.rs

@@ -1,4 +1,5 @@
 use std::{
+    collections::VecDeque,
     fmt,
     iter::FusedIterator,
     sync::{Arc, atomic::AtomicUsize},
@@ -9,9 +10,9 @@ use rand::{Rng, SeedableRng, rngs::SmallRng};
 use crate::Priority;
 
 struct PriorityQueues<T> {
-    high_priority: Vec<T>,
-    medium_priority: Vec<T>,
-    low_priority: Vec<T>,
+    high_priority: VecDeque<T>,
+    medium_priority: VecDeque<T>,
+    low_priority: VecDeque<T>,
 }
 
 impl<T> PriorityQueues<T> {
@@ -42,9 +43,9 @@ impl<T> PriorityQueueState<T> {
         let mut queues = self.queues.lock();
         match priority {
             Priority::Realtime(_) => unreachable!(),
-            Priority::High => queues.high_priority.push(item),
-            Priority::Medium => queues.medium_priority.push(item),
-            Priority::Low => queues.low_priority.push(item),
+            Priority::High => queues.high_priority.push_back(item),
+            Priority::Medium => queues.medium_priority.push_back(item),
+            Priority::Low => queues.low_priority.push_back(item),
         };
         self.condvar.notify_one();
         Ok(())
@@ -141,9 +142,9 @@ impl<T> PriorityQueueReceiver<T> {
     pub(crate) fn new() -> (PriorityQueueSender<T>, Self) {
         let state = PriorityQueueState {
             queues: parking_lot::Mutex::new(PriorityQueues {
-                high_priority: Vec::new(),
-                medium_priority: Vec::new(),
-                low_priority: Vec::new(),
+                high_priority: VecDeque::new(),
+                medium_priority: VecDeque::new(),
+                low_priority: VecDeque::new(),
             }),
             condvar: parking_lot::Condvar::new(),
             receiver_count: AtomicUsize::new(1),
@@ -226,7 +227,7 @@ impl<T> PriorityQueueReceiver<T> {
         if !queues.high_priority.is_empty() {
             let flip = self.rand.random_ratio(P::High.probability(), mass);
             if flip {
-                return Ok(queues.high_priority.pop());
+                return Ok(queues.high_priority.pop_front());
             }
             mass -= P::High.probability();
         }
@@ -234,7 +235,7 @@ impl<T> PriorityQueueReceiver<T> {
         if !queues.medium_priority.is_empty() {
             let flip = self.rand.random_ratio(P::Medium.probability(), mass);
             if flip {
-                return Ok(queues.medium_priority.pop());
+                return Ok(queues.medium_priority.pop_front());
             }
             mass -= P::Medium.probability();
         }
@@ -242,7 +243,7 @@ impl<T> PriorityQueueReceiver<T> {
         if !queues.low_priority.is_empty() {
             let flip = self.rand.random_ratio(P::Low.probability(), mass);
             if flip {
-                return Ok(queues.low_priority.pop());
+                return Ok(queues.low_priority.pop_front());
             }
         }