Implement an algorithm to make this crate no_std, take 2 #33

notgull · 2022-11-01T23:34:13Z

I'm not satisfied with the direction #30 has taken, so I've written this PR. It's less destructive, less verbose, and faster than the previous one. I haven't implemented portable_atomic or loom support here, I'll do that in future PRs.

In this PR, rather than using a Mutex to wrap an unsync linked list (what's currently on master) or using ConcurrentQueue (as in #30), I use an atomic singly linked list to store listeners. Similarly to #30, listeners in this list can be marked as "orphaned" to indicate that they should not be notified (e.g. when the EventListener is dropped). As opposed to #30, it only exhibits a 20% slowdown as compared to master on my PC. With the increased number of atomic operations involved, I think that that is about as low as we can go without using a locking mechanism of some kind.

Future work:

Add portable_atomic and loom support.
It may be possible to write this crate using only safe code. I intentionally separated out RacyArc because we may be able to split it off into another crate, the interior mutability in Listener can be resolved using crossbeam_utils::AtomicCell<Wakeup> or similar, and the linked list could probably be emulated using AtomicCell and Arc as well. The main concern is that we would have to not use the cache in ListenerQueue, and this (among other things) would likely cause significant performance issues (Arc is expensive).
Another way of doing this is using a spinlock, but making sure we never actually spin on it. We keep the master branch, but replace the Mutex with a spinlock. notify and listen both try to access the spinlock. If notify sees the spinlock active, it places the notification counts in an atomic variable and whoever holds the spinlock preforms the notification once they're about the drop the lock. If listen (or any of EventListener's methods) see the spinlock locked, they put their Wakers (or Unparkers) into a short-term ConcurrentQueue which is then used to set up listeners once the current holder tries to drop the lock. This may be a better way of doing it, but I don't know whether it would be faster or slower.

fogti · 2022-11-02T14:23:19Z

src/queue.rs

+            unsafe {
+                // Initialize the node.
+                node_ptr.write(Node::new(listener));
+


why isn't .assume_init() used here?

I bypass it by just casting node.get() into an *mut Node, which allows me to write the Node straight into the slot without worrying about MaybeUninit.

fogti · 2022-11-02T14:23:27Z

src/listener.rs

+    ///
+    /// This is kept synchronized by the `state` variable; therefore, it's
+    /// technically `Sync`.
+    waker: Cell<Wakeup>,


wouldn't it make more sense to use UnsafeCell here?

I originally used UnsafeCell, but given that most of the semantics involved are just "replace the waker with something else", I ended up using Cell instead.

I tried replacing it with UnsafeCell<MaybeUninit<Wakeup>> (and also getting rid of the Wakeup::None variant), but any gains are minimal at best. I figure it's better to just use Cell and avoid the unsafe code.

fogti · 2022-11-02T14:26:04Z

src/queue.rs

+    /// Orphan a node in the queue.
+    ///
+    /// The return value is Some if the node was notified, and true is the notification
+    /// was an additional notification.
+    pub(crate) unsafe fn orphan(&self, node: NonNull<Node>) -> Option<bool> {
+        let (needs_drop, notified) = unsafe { node.as_ref().listener.orphan() };
+
+        // If we need to deallocate the node, do so.
+        if needs_drop {
+            unsafe { self.dealloc(node) };
+        }
+
+        notified
+    }
+
+    /// Allocate a new node for a listener.
+    fn alloc(&self, listener: Listener) -> NonNull<Node> {
+        // Try to install a cached node.
+        if !self.cache.occupied.swap(true, Ordering::Acquire) {
+            // We can now initialize the node.
+            let node_ptr = self.cache.node.get() as *mut Node;
+
+            unsafe {
+                // Initialize the node.
+                node_ptr.write(Node::new(listener));
+
+                // Return the node.
+                NonNull::new_unchecked(node_ptr)
+            }
+        } else {
+            // We failed to install a cached node, so we need to allocate a new
+            // one.
+            unsafe { NonNull::new_unchecked(Box::into_raw(Box::new(Node::new(listener)))) }
+        }
+    }
+
+    /// Deallocate a node.
+    unsafe fn dealloc(&self, node: NonNull<Node>) {
+        // Is this node the current cache node?
+        if ptr::eq(self.cache.node.get() as *const Node, node.as_ptr()) {
+            // We can now clear the cache.
+            unsafe { (self.cache.node.get() as *mut Node).drop_in_place() };
+            self.cache.occupied.store(false, Ordering::Release);
+        } else {
+            // We need to deallocate the node on the heap.
+            unsafe { Box::from_raw(node.as_ptr()) };
+        }
+    }


these methods (alloc, dealloc, orphan) appear to only act on the CachedNode and imo should be moved into an impl block associated with that. (and if these methods are still necessary, should be replaced with forwarding methods)

I've moved alloc and dealloc, but orphan is, semantically, an important operation on the Queue so I've kept it there.

fogti · 2022-11-02T14:28:02Z

src/util.rs

+
+/// A wrapper around an `Arc` that is initialized atomically in a racy manner.
+#[derive(Default)]
+pub(crate) struct RacyArc<T> {


How big is the difference between this and OnceCell impls?

Two main differences:

1). While the once cells are synchronized, this one is racy. While the once cells only run one closure at a time, this one runs all closures at once and sets the value to whichever one comes first.

2). It's only one system word in length, while once cells are usually at least three.

In addition, since our once cells are backed by this crate, we can't use once cells here.

JFYI, the once_cell crate also provides a racy "OnceCell": https://docs.rs/once_cell/latest/once_cell/race/struct.OnceNonZeroUsize.html

We're trying to avoid the once_cell crate due to its recent MSRV bump, see smol-rs/async-io#93

I'd like to ask about the performance of this, because in the past I implemented https://github.com/zseri/revenq,which I originally based upon a similar pattern and hit a huge perf bottleneck. Thus I ask: How much does this get "hammered" during its lifetime or the lifetime of the chain?

After initialization (which is a one-time process), loads from the AtomicPtr are a simple atomic load. This is opposed to a OnceCell, which consists of a load as well. I doubt that there's much overhead aside from at initialization time. At worst, this leads to the initialization code being run once per thread.

I could replace it with the proper cell to see what happens, but I doubt it would make a difference. Besides, this is what this code used prior to this change.

fogti · 2022-11-02T22:01:37Z

src/queue.rs

+            // We can now initialize the node.
+            let node_ptr = self.node.get() as *mut Node;
+
+            unsafe {
+                // Initialize the node.
+                node_ptr.write(Node::new(listener));
+
+                // Return the node.
+                NonNull::new_unchecked(node_ptr)
+            }


I don't like this part at all, especially since it appears to misuse MaybeUninit and/or UnsafeCell somewhat. how about:

unsafe { let node_ptr: &mut MaybeUninit<_> = &mut *self.node.get(); // Initialize the node node_ptr.write(Node::new(listener)); // Return the node node_ptr.assume_init_mut().into() }

I've replaced the node cast to an as_mut_ptr() usage. Unfortunately I can't use MaybeUninit::write() and MaybeUninit::assume_init_mut() since they were introduced in Rust 1.55, which would lead to a significant MSRV bump

notgull · 2022-11-06T00:41:30Z

Closing in favor of #34

notgull added 2 commits November 1, 2022 15:58

New, no_std algorithm

034912b

Fix a bug where the queue tail isn't set

3c69b09

notgull mentioned this pull request Nov 1, 2022

Implement a new algorithm to make this crate no_std #30

Closed

Adjust lints

bc79907

notgull force-pushed the no_std_2 branch from bf52c88 to bc79907 Compare November 2, 2022 01:22

fogti reviewed Nov 2, 2022

View reviewed changes

Move queue methods to CachedNode

3c81f04

fogti reviewed Nov 2, 2022

View reviewed changes

Fix MaybeUninit usage

1de1adc

notgull mentioned this pull request Nov 5, 2022

Implement an algorithm to make this crate no_std, take 3 #34

Merged

notgull closed this Nov 6, 2022

notgull deleted the no_std_2 branch November 10, 2022 23:50

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notgull commented Nov 6, 2022

Implement an algorithm to make this crate no_std, take 2 #33

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