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+// This module provides a relatively simple thread-safe pool of reusable
+// objects. For the most part, it's implemented by a stack represented by a
+// Mutex<Vec<T>>. It has one small trick: because unlocking a mutex is somewhat
+// costly, in the case where a pool is accessed by the first thread that tried
+// to get a value, we bypass the mutex. Here are some benchmarks showing the
+// difference.
+//
+// 1) misc::anchored_literal_long_non_match 21 (18571 MB/s)
+// 2) misc::anchored_literal_long_non_match 107 (3644 MB/s)
+// 3) misc::anchored_literal_long_non_match 45 (8666 MB/s)
+// 4) misc::anchored_literal_long_non_match 19 (20526 MB/s)
+//
+// (1) represents our baseline: the master branch at the time of writing when
+// using the 'thread_local' crate to implement the pool below.
+//
+// (2) represents a naive pool implemented completely via Mutex<Vec<T>>. There
+// is no special trick for bypassing the mutex.
+//
+// (3) is the same as (2), except it uses Mutex<Vec<Box<T>>>. It is twice as
+// fast because a Box<T> is much smaller than the T we use with a Pool in this
+// crate. So pushing and popping a Box<T> from a Vec is quite a bit faster
+// than for T.
+//
+// (4) is the same as (3), but with the trick for bypassing the mutex in the
+// case of the first-to-get thread.
+//
+// Why move off of thread_local? Even though (4) is a hair faster than (1)
+// above, this was not the main goal. The main goal was to move off of
+// thread_local and find a way to *simply* re-capture some of its speed for
+// regex's specific case. So again, why move off of it? The *primary* reason is
+// because of memory leaks. See https://github.com/rust-lang/regex/issues/362
+// for example. (Why do I want it to be simple? Well, I suppose what I mean is,
+// "use as much safe code as possible to minimize risk and be as sure as I can
+// be that it is correct.")
+//
+// My guess is that the thread_local design is probably not appropriate for
+// regex since its memory usage scales to the number of active threads that
+// have used a regex, where as the pool below scales to the number of threads
+// that simultaneously use a regex. While neither case permits contraction,
+// since we own the pool data structure below, we can add contraction if a
+// clear use case pops up in the wild. More pressingly though, it seems that
+// there are at least some use case patterns where one might have many threads
+// sitting around that might have used a regex at one point. While thread_local
+// does try to reuse space previously used by a thread that has since stopped,
+// its maximal memory usage still scales with the total number of active
+// threads. In contrast, the pool below scales with the total number of threads
+// *simultaneously* using the pool. The hope is that this uses less memory
+// overall. And if it doesn't, we can hopefully tune it somehow.
+//
+// It seems that these sort of conditions happen frequently
+// in FFI inside of other more "managed" languages. This was
+// mentioned in the issue linked above, and also mentioned here:
+// https://github.com/BurntSushi/rure-go/issues/3. And in particular, users
+// confirm that disabling the use of thread_local resolves the leak.
+//
+// There were other weaker reasons for moving off of thread_local as well.
+// Namely, at the time, I was looking to reduce dependencies. And for something
+// like regex, maintenance can be simpler when we own the full dependency tree.
+
+use std::panic::{RefUnwindSafe, UnwindSafe};
+use std::sync::atomic::{AtomicUsize, Ordering};
+use std::sync::Mutex;
+
+/// An atomic counter used to allocate thread IDs.
+static COUNTER: AtomicUsize = AtomicUsize::new(1);
+
+thread_local!(
+ /// A thread local used to assign an ID to a thread.
+ static THREAD_ID: usize = {
+ let next = COUNTER.fetch_add(1, Ordering::Relaxed);
+ // SAFETY: We cannot permit the reuse of thread IDs since reusing a
+ // thread ID might result in more than one thread "owning" a pool,
+ // and thus, permit accessing a mutable value from multiple threads
+ // simultaneously without synchronization. The intent of this panic is
+ // to be a sanity check. It is not expected that the thread ID space
+ // will actually be exhausted in practice.
+ //
+ // This checks that the counter never wraps around, since atomic
+ // addition wraps around on overflow.
+ if next == 0 {
+ panic!("regex: thread ID allocation space exhausted");
+ }
+ next
+ };
+);
+
+/// The type of the function used to create values in a pool when the pool is
+/// empty and the caller requests one.
+type CreateFn<T> =
+ Box<dyn Fn() -> T + Send + Sync + UnwindSafe + RefUnwindSafe + 'static>;
+
+/// A simple thread safe pool for reusing values.
+///
+/// Getting a value out comes with a guard. When that guard is dropped, the
+/// value is automatically put back in the pool.
+///
+/// A Pool<T> impls Sync when T is Send (even if it's not Sync). This means
+/// that T can use interior mutability. This is possible because a pool is
+/// guaranteed to provide a value to exactly one thread at any time.
+///
+/// Currently, a pool never contracts in size. Its size is proportional to the
+/// number of simultaneous uses.
+pub struct Pool<T> {
+ /// A stack of T values to hand out. These are used when a Pool is
+ /// accessed by a thread that didn't create it.
+ stack: Mutex<Vec<Box<T>>>,
+ /// A function to create more T values when stack is empty and a caller
+ /// has requested a T.
+ create: CreateFn<T>,
+ /// The ID of the thread that owns this pool. The owner is the thread
+ /// that makes the first call to 'get'. When the owner calls 'get', it
+ /// gets 'owner_val' directly instead of returning a T from 'stack'.
+ /// See comments elsewhere for details, but this is intended to be an
+ /// optimization for the common case that makes getting a T faster.
+ ///
+ /// It is initialized to a value of zero (an impossible thread ID) as a
+ /// sentinel to indicate that it is unowned.
+ owner: AtomicUsize,
+ /// A value to return when the caller is in the same thread that created
+ /// the Pool.
+ owner_val: T,
+}
+
+// SAFETY: Since we want to use a Pool from multiple threads simultaneously
+// behind an Arc, we need for it to be Sync. In cases where T is sync, Pool<T>
+// would be Sync. However, since we use a Pool to store mutable scratch space,
+// we wind up using a T that has interior mutability and is thus itself not
+// Sync. So what we *really* want is for our Pool<T> to by Sync even when T is
+// not Sync (but is at least Send).
+//
+// The only non-sync aspect of a Pool is its 'owner_val' field, which is used
+// to implement faster access to a pool value in the common case of a pool
+// being accessed in the same thread in which it was created. The 'stack' field
+// is also shared, but a Mutex<T> where T: Send is already Sync. So we only
+// need to worry about 'owner_val'.
+//
+// The key is to guarantee that 'owner_val' can only ever be accessed from one
+// thread. In our implementation below, we guarantee this by only returning the
+// 'owner_val' when the ID of the current thread matches the ID of the thread
+// that created the Pool. Since this can only ever be one thread, it follows
+// that only one thread can access 'owner_val' at any point in time. Thus, it
+// is safe to declare that Pool<T> is Sync when T is Send.
+//
+// NOTE: It would also be possible to make the owning thread be the *first*
+// thread that tries to get a value out of a Pool. However, the current
+// implementation is a little simpler and it's not clear if making the first
+// thread (rather than the creating thread) is meaningfully better.
+//
+// If there is a way to achieve our performance goals using safe code, then
+// I would very much welcome a patch. As it stands, the implementation below
+// tries to balance safety with performance. The case where a Regex is used
+// from multiple threads simultaneously will suffer a bit since getting a cache
+// will require unlocking a mutex.
+unsafe impl<T: Send> Sync for Pool<T> {}
+
+impl<T: ::std::fmt::Debug> ::std::fmt::Debug for Pool<T> {
+ fn fmt(&self, f: &mut ::std::fmt::Formatter<'_>) -> ::std::fmt::Result {
+ f.debug_struct("Pool")
+ .field("stack", &self.stack)
+ .field("owner", &self.owner)
+ .field("owner_val", &self.owner_val)
+ .finish()
+ }
+}
+
+/// A guard that is returned when a caller requests a value from the pool.
+///
+/// The purpose of the guard is to use RAII to automatically put the value back
+/// in the pool once it's dropped.
+#[derive(Debug)]
+pub struct PoolGuard<'a, T: Send> {
+ /// The pool that this guard is attached to.
+ pool: &'a Pool<T>,
+ /// This is None when the guard represents the special "owned" value. In
+ /// which case, the value is retrieved from 'pool.owner_val'.
+ value: Option<Box<T>>,
+}
+
+impl<T: Send> Pool<T> {
+ /// Create a new pool. The given closure is used to create values in the
+ /// pool when necessary.
+ pub fn new(create: CreateFn<T>) -> Pool<T> {
+ let owner = AtomicUsize::new(0);
+ let owner_val = create();
+ Pool { stack: Mutex::new(vec![]), create, owner, owner_val }
+ }
+
+ /// Get a value from the pool. The caller is guaranteed to have exclusive
+ /// access to the given value.
+ ///
+ /// Note that there is no guarantee provided about which value in the
+ /// pool is returned. That is, calling get, dropping the guard (causing
+ /// the value to go back into the pool) and then calling get again is NOT
+ /// guaranteed to return the same value received in the first get call.
+ #[cfg_attr(feature = "perf-inline", inline(always))]
+ pub fn get(&self) -> PoolGuard<'_, T> {
+ // Our fast path checks if the caller is the thread that "owns" this
+ // pool. Or stated differently, whether it is the first thread that
+ // tried to extract a value from the pool. If it is, then we can return
+ // a T to the caller without going through a mutex.
+ //
+ // SAFETY: We must guarantee that only one thread gets access to this
+ // value. Since a thread is uniquely identified by the THREAD_ID thread
+ // local, it follows that is the caller's thread ID is equal to the
+ // owner, then only one thread may receive this value.
+ let caller = THREAD_ID.with(|id| *id);
+ let owner = self.owner.load(Ordering::Relaxed);
+ if caller == owner {
+ return self.guard_owned();
+ }
+ self.get_slow(caller, owner)
+ }
+
+ /// This is the "slow" version that goes through a mutex to pop an
+ /// allocated value off a stack to return to the caller. (Or, if the stack
+ /// is empty, a new value is created.)
+ ///
+ /// If the pool has no owner, then this will set the owner.
+ #[cold]
+ fn get_slow(&self, caller: usize, owner: usize) -> PoolGuard<'_, T> {
+ use std::sync::atomic::Ordering::Relaxed;
+
+ if owner == 0 {
+ // The sentinel 0 value means this pool is not yet owned. We
+ // try to atomically set the owner. If we do, then this thread
+ // becomes the owner and we can return a guard that represents
+ // the special T for the owner.
+ let res = self.owner.compare_exchange(0, caller, Relaxed, Relaxed);
+ if res.is_ok() {
+ return self.guard_owned();
+ }
+ }
+ let mut stack = self.stack.lock().unwrap();
+ let value = match stack.pop() {
+ None => Box::new((self.create)()),
+ Some(value) => value,
+ };
+ self.guard_stack(value)
+ }
+
+ /// Puts a value back into the pool. Callers don't need to call this. Once
+ /// the guard that's returned by 'get' is dropped, it is put back into the
+ /// pool automatically.
+ fn put(&self, value: Box<T>) {
+ let mut stack = self.stack.lock().unwrap();
+ stack.push(value);
+ }
+
+ /// Create a guard that represents the special owned T.
+ fn guard_owned(&self) -> PoolGuard<'_, T> {
+ PoolGuard { pool: self, value: None }
+ }
+
+ /// Create a guard that contains a value from the pool's stack.
+ fn guard_stack(&self, value: Box<T>) -> PoolGuard<'_, T> {
+ PoolGuard { pool: self, value: Some(value) }
+ }
+}
+
+impl<'a, T: Send> PoolGuard<'a, T> {
+ /// Return the underlying value.
+ pub fn value(&self) -> &T {
+ match self.value {
+ None => &self.pool.owner_val,
+ Some(ref v) => &**v,
+ }
+ }
+}
+
+impl<'a, T: Send> Drop for PoolGuard<'a, T> {
+ #[cfg_attr(feature = "perf-inline", inline(always))]
+ fn drop(&mut self) {
+ if let Some(value) = self.value.take() {
+ self.pool.put(value);
+ }
+ }
+}
+
+#[cfg(test)]
+mod tests {
+ use std::panic::{RefUnwindSafe, UnwindSafe};
+
+ use super::*;
+
+ #[test]
+ fn oibits() {
+ use crate::exec::ProgramCache;
+
+ fn has_oibits<T: Send + Sync + UnwindSafe + RefUnwindSafe>() {}
+ has_oibits::<Pool<ProgramCache>>();
+ }
+
+ // Tests that Pool implements the "single owner" optimization. That is, the
+ // thread that first accesses the pool gets its own copy, while all other
+ // threads get distinct copies.
+ #[test]
+ fn thread_owner_optimization() {
+ use std::cell::RefCell;
+ use std::sync::Arc;
+
+ let pool: Arc<Pool<RefCell<Vec<char>>>> =
+ Arc::new(Pool::new(Box::new(|| RefCell::new(vec!['a']))));
+ pool.get().value().borrow_mut().push('x');
+
+ let pool1 = pool.clone();
+ let t1 = std::thread::spawn(move || {
+ let guard = pool1.get();
+ let v = guard.value();
+ v.borrow_mut().push('y');
+ });
+
+ let pool2 = pool.clone();
+ let t2 = std::thread::spawn(move || {
+ let guard = pool2.get();
+ let v = guard.value();
+ v.borrow_mut().push('z');
+ });
+
+ t1.join().unwrap();
+ t2.join().unwrap();
+
+ // If we didn't implement the single owner optimization, then one of
+ // the threads above is likely to have mutated the [a, x] vec that
+ // we stuffed in the pool before spawning the threads. But since
+ // neither thread was first to access the pool, and because of the
+ // optimization, we should be guaranteed that neither thread mutates
+ // the special owned pool value.
+ //
+ // (Technically this is an implementation detail and not a contract of
+ // Pool's API.)
+ assert_eq!(vec!['a', 'x'], *pool.get().value().borrow());
+ }
+}