// 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.
//
// 2022-10-15: These benchmarks are from the old regex crate and they aren't
// easy to reproduce because some rely on older implementations of Pool that
// are no longer around. I've left the results here for posterity, but any
// enterprising individual should feel encouraged to re-litigate the way Pool
// works. I am not at all certain it is the best approach.
//
// 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.
//
// Note that I am not entirely happy with this pool. It has some subtle
// implementation details and is overall still observable (even with the
// thread owner optimization) in benchmarks. If someone wants to take a crack
// at building something better, please file an issue. Even if it means a
// different API. The API exposed by this pool is not the minimal thing that
// something like a 'Regex' actually needs. It could adapt to, for example,
// an API more like what is found in the 'thread_local' crate. However, we do
// really need to support the no-std alloc-only context, or else the regex
// crate wouldn't be able to support no-std alloc-only. However, I'm generally
// okay with making the alloc-only context slower (as it is here), although I
// do find it unfortunate.
/*!
A thread safe memory pool.
The principal type in this module is a [`Pool`]. It main use case is for
holding a thread safe collection of mutable scratch spaces (usually called
`Cache` in this crate) that regex engines need to execute a search. This then
permits sharing the same read-only regex object across multiple threads while
having a quick way of reusing scratch space in a thread safe way. This avoids
needing to re-create the scratch space for every search, which could wind up
being quite expensive.
*/
/// A thread safe pool that works in an `alloc`-only context.
///
/// Getting a value out comes with a guard. When that guard is dropped, the
/// value is automatically put back in the pool. The guard provides both a
/// `Deref` and a `DerefMut` implementation for easy access to an underlying
/// `T`.
///
/// A `Pool` impls `Sync` when `T` is `Send` (even if `T` is not `Sync`). 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
/// maximum number of simultaneous uses. This may change in the future.
///
/// A `Pool` is a particularly useful data structure for this crate because
/// many of the regex engines require a mutable "cache" in order to execute
/// a search. Since regexes themselves tend to be global, the problem is then:
/// how do you get a mutable cache to execute a search? You could:
///
/// 1. Use a `thread_local!`, which requires the standard library and requires
/// that the regex pattern be statically known.
/// 2. Use a `Pool`.
/// 3. Make the cache an explicit dependency in your code and pass it around.
/// 4. Put the cache state in a `Mutex`, but this means only one search can
/// execute at a time.
/// 5. Create a new cache for every search.
///
/// A `thread_local!` is perhaps the best choice if it works for your use case.
/// Putting the cache in a mutex or creating a new cache for every search are
/// perhaps the worst choices. Of the remaining two choices, whether you use
/// this `Pool` or thread through a cache explicitly in your code is a matter
/// of taste and depends on your code architecture.
///
/// # Warning: may use a spin lock
///
/// When this crate is compiled _without_ the `std` feature, then this type
/// may used a spin lock internally. This can have subtle effects that may
/// be undesirable. See [Spinlocks Considered Harmful][spinharm] for a more
/// thorough treatment of this topic.
///
/// [spinharm]: https://matklad.github.io/2020/01/02/spinlocks-considered-harmful.html
///
/// # Example
///
/// This example shows how to share a single hybrid regex among multiple
/// threads, while also safely getting exclusive access to a hybrid's
/// [`Cache`](crate::hybrid::regex::Cache) without preventing other searches
/// from running while your thread uses the `Cache`.
///
/// ```
/// use regex_automata::{
/// hybrid::regex::{Cache, Regex},
/// util::{lazy::Lazy, pool::Pool},
/// Match,
/// };
///
/// static RE: Lazy<Regex> =
/// Lazy::new(|| Regex::new("foo[0-9]+bar").unwrap());
/// static CACHE: Lazy<Pool<Cache>> =
/// Lazy::new(|| Pool::new(|| RE.create_cache()));
///
/// let expected = Some(Match::must(0, 3..14));
/// assert_eq!(expected, RE.find(&mut CACHE.get(), b"zzzfoo12345barzzz"));
/// ```
pub struct Pool<T, F = fn() -> T>(alloc::boxed::Box<inner::Pool<T, F>>);
impl<T, F> Pool<T, F> {
/// Create a new pool. The given closure is used to create values in
/// the pool when necessary.
pub fn new(create: F) -> Pool<T, F> {
Pool(alloc::boxed::Box::new(inner::Pool::new(create)))
}
}
impl<T: Send, F: Fn() -> T> Pool<T, F> {
/// Get a value from the pool. The caller is guaranteed to have
/// exclusive access to the given value. Namely, it is guaranteed that
/// this will never return a value that was returned by another call to
/// `get` but was not put back into the pool.
///
/// When the guard goes out of scope and its destructor is called, then
/// it will automatically be put back into the pool. Alternatively,
/// [`PoolGuard::put`] may be used to explicitly put it back in the pool
/// without relying on its destructor.
///
/// 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.
#[inline]
pub fn get(&self) -> PoolGuard<'_, T, F> {
PoolGuard(self.0.get())
}
}
impl<T: core::fmt::Debug, F> core::fmt::Debug for Pool<T, F> {
fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
f.debug_tuple("Pool").field(&self.0).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.
pub struct PoolGuard<'a, T: Send, F: Fn() -> T>(inner::PoolGuard<'a, T, F>);
impl<'a, T: Send, F: Fn() -> T> PoolGuard<'a, T, F> {
/// Consumes this guard and puts it back into the pool.
///
/// This circumvents the guard's `Drop` implementation. This can be useful
/// in circumstances where the automatic `Drop` results in poorer codegen,
/// such as calling non-inlined functions.
#[inline]
pub fn put(this: PoolGuard<'_, T, F>) {
inner::PoolGuard::put(this.0);
}
}
impl<'a, T: Send, F: Fn() -> T> core::ops::Deref for PoolGuard<'a, T, F> {
type Target = T;
#[inline]
fn deref(&self) -> &T {
self.0.value()
}
}
impl<'a, T: Send, F: Fn() -> T> core::ops::DerefMut for PoolGuard<'a, T, F> {
#[inline]
fn deref_mut(&mut self) -> &mut T {
self.0.value_mut()
}
}
impl<'a, T: Send + core::fmt::Debug, F: Fn() -> T> core::fmt::Debug
for PoolGuard<'a, T, F>
{
fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
f.debug_tuple("PoolGuard").field(&self.0).finish()
}
}
#[cfg(feature = "std")]
mod inner {
use core::{
cell::UnsafeCell,
panic::{RefUnwindSafe, UnwindSafe},
sync::atomic::{AtomicUsize, Ordering},
};
use alloc::{boxed::Box, vec, vec::Vec};
use std::{sync::Mutex, thread_local};
/// An atomic counter used to allocate thread IDs.
///
/// We specifically start our counter at 3 so that we can use the values
/// less than it as sentinels.
static COUNTER: AtomicUsize = AtomicUsize::new(3);
/// A thread ID indicating that there is no owner. This is the initial
/// state of a pool. Once a pool has an owner, there is no way to change
/// it.
static THREAD_ID_UNOWNED: usize = 0;
/// A thread ID indicating that the special owner value is in use and not
/// available. This state is useful for avoiding a case where the owner
/// of a pool calls `get` before putting the result of a previous `get`
/// call back into the pool.
static THREAD_ID_INUSE: usize = 1;
/// This sentinel is used to indicate that a guard has already been dropped
/// and should not be re-dropped. We use this because our drop code can be
/// called outside of Drop and thus there could be a bug in the internal
/// implementation that results in trying to put the same guard back into
/// the same pool multiple times, and *that* could result in UB if we
/// didn't mark the guard as already having been put back in the pool.
///
/// So this isn't strictly necessary, but this let's us define some
/// routines as safe (like PoolGuard::put_imp) that we couldn't otherwise
/// do.
static THREAD_ID_DROPPED: usize = 2;
/// The number of stacks we use inside of the pool. These are only used for
/// non-owners. That is, these represent the "slow" path.
///
/// In the original implementation of this pool, we only used a single
/// stack. While this might be okay for a couple threads, the prevalence of
/// 32, 64 and even 128 core CPUs has made it untenable. The contention
/// such an environment introduces when threads are doing a lot of searches
/// on short haystacks (a not uncommon use case) is palpable and leads to
/// huge slowdowns.
///
/// This constant reflects a change from using one stack to the number of
/// stacks that this constant is set to. The stack for a particular thread
/// is simply chosen by `thread_id % MAX_POOL_STACKS`. The idea behind
/// this setup is that there should be a good chance that accesses to the
/// pool will be distributed over several stacks instead of all of them
/// converging to one.
///
/// This is not a particularly smart or dynamic strategy. Fixing this to a
/// specific number has at least two downsides. First is that it will help,
/// say, an 8 core CPU more than it will a 128 core CPU. (But, crucially,
/// it will still help the 128 core case.) Second is that this may wind
/// up being a little wasteful with respect to memory usage. Namely, if a
/// regex is used on one thread and then moved to another thread, then it
/// could result in creating a new copy of the data in the pool even though
/// only one is actually needed.
///
/// And that memory usage bit is why this is set to 8 and not, say, 64.
/// Keeping it at 8 limits, to an extent, how much unnecessary memory can
/// be allocated.
///
/// In an ideal world, we'd be able to have something like this:
///
/// * Grow the number of stacks as the number of concurrent callers
/// increases. I spent a little time trying this, but even just adding an
/// atomic addition/subtraction for each pop/push for tracking concurrent
/// callers led to a big perf hit. Since even more work would seemingly be
/// required than just an addition/subtraction, I abandoned this approach.
/// * The maximum amount of memory used should scale with respect to the
/// number of concurrent callers and *not* the total number of existing
/// threads. This is primarily why the `thread_local` crate isn't used, as
/// as some environments spin up a lot of threads. This led to multiple
/// reports of extremely high memory usage (often described as memory
/// leaks).
/// * Even more ideally, the pool should contract in size. That is, it
/// should grow with bursts and then shrink. But this is a pretty thorny
/// issue to tackle and it might be better to just not.
/// * It would be nice to explore the use of, say, a lock-free stack
/// instead of using a mutex to guard a `Vec` that is ultimately just
/// treated as a stack. The main thing preventing me from exploring this
/// is the ABA problem. The `crossbeam` crate has tools for dealing with
/// this sort of problem (via its epoch based memory reclamation strategy),
/// but I can't justify bringing in all of `crossbeam` as a dependency of
/// `regex` for this.
///
/// See this issue for more context and discussion:
/// https://github.com/rust-lang/regex/issues/934
const MAX_POOL_STACKS: usize = 8;
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. Even on a 32-bit
// system, it would require spawning 2^32 threads (although they
// wouldn't all need to run simultaneously, so it is in theory
// possible).
//
// 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
};
);
/// This puts each stack in the pool below into its own cache line. This is
/// an absolutely critical optimization that tends to have the most impact
/// in high contention workloads. Without forcing each mutex protected
/// into its own cache line, high contention exacerbates the performance
/// problem by causing "false sharing." By putting each mutex in its own
/// cache-line, we avoid the false sharing problem and the affects of
/// contention are greatly reduced.
#[derive(Debug)]
#[repr(C, align(64))]
struct CacheLine<T>(T);
/// A thread safe pool utilizing std-only features.
///
/// The main difference between this and the simplistic alloc-only pool is
/// the use of std::sync::Mutex and an "owner thread" optimization that
/// makes accesses by the owner of a pool faster than all other threads.
/// This makes the common case of running a regex within a single thread
/// faster by avoiding mutex unlocking.
pub(super) struct Pool<T, F> {
/// A function to create more T values when stack is empty and a caller
/// has requested a T.
create: F,
/// Multiple stacks of T values to hand out. These are used when a Pool
/// is accessed by a thread that didn't create it.
///
/// Conceptually this is `Mutex<Vec<Box<T>>>`, but sharded out to make
/// it scale better under high contention work-loads. We index into
/// this sequence via `thread_id % stacks.len()`.
stacks: Vec<CacheLine<Mutex<Vec<Box<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
/// first called `Pool::get`.
///
/// This is set to None when a Pool is first created, and set to Some
/// once the first thread calls Pool::get.
owner_val: UnsafeCell<Option<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 first called 'Pool::get'. 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.
//
// 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 value out of the pool will require unlocking a mutex.
//
// We require `F: Send + Sync` because we call `F` at any point on demand,
// potentially from multiple threads simultaneously.
unsafe impl<T: Send, F: Send + Sync> Sync for Pool<T, F> {}
// If T is UnwindSafe, then since we provide exclusive access to any
// particular value in the pool, the pool should therefore also be
// considered UnwindSafe.
//
// We require `F: UnwindSafe + RefUnwindSafe` because we call `F` at any
// point on demand, so it needs to be unwind safe on both dimensions for
// the entire Pool to be unwind safe.
impl<T: UnwindSafe, F: UnwindSafe + RefUnwindSafe> UnwindSafe for Pool<T, F> {}
// If T is UnwindSafe, then since we provide exclusive access to any
// particular value in the pool, the pool should therefore also be
// considered RefUnwindSafe.
//
// We require `F: UnwindSafe + RefUnwindSafe` because we call `F` at any
// point on demand, so it needs to be unwind safe on both dimensions for
// the entire Pool to be unwind safe.
impl<T: UnwindSafe, F: UnwindSafe + RefUnwindSafe> RefUnwindSafe
for Pool<T, F>
{
}
impl<T, F> Pool<T, F> {
/// Create a new pool. The given closure is used to create values in
/// the pool when necessary.
pub(super) fn new(create: F) -> Pool<T, F> {
// FIXME: Now that we require 1.65+, Mutex::new is available as
// const... So we can almost mark this function as const. But of
// course, we're creating a Vec of stacks below (we didn't when I
// originally wrote this code). It seems like the best way to work
// around this would be to use a `[Stack; MAX_POOL_STACKS]` instead
// of a `Vec<Stack>`. I refrained from making this change at time
// of writing (2023/10/08) because I was making a lot of other
// changes at the same time and wanted to do this more carefully.
// Namely, because of the cache line optimization, that `[Stack;
// MAX_POOL_STACKS]` would be quite big. It's unclear how bad (if
// at all) that would be.
//
// Another choice would be to lazily allocate the stacks, but...
// I'm not so sure about that. Seems like a fair bit of complexity?
//
// Maybe there's a simple solution I'm missing.
//
// ... OK, I tried to fix this. First, I did it by putting `stacks`
// in an `UnsafeCell` and using a `Once` to lazily initialize it.
// I benchmarked it and everything looked okay. I then made this
// function `const` and thought I was just about done. But the
// public pool type wraps its inner pool in a `Box` to keep its
// size down. Blech.
//
// So then I thought that I could push the box down into this
// type (and leave the non-std version unboxed) and use the same
// `UnsafeCell` technique to lazily initialize it. This has the
// downside of the `Once` now needing to get hit in the owner fast
// path, but maybe that's OK? However, I then realized that we can
// only lazily initialize `stacks`, `owner` and `owner_val`. The
// `create` function needs to be put somewhere outside of the box.
// So now the pool is a `Box`, `Once` and a function. Now we're
// starting to defeat the point of boxing in the first place. So I
// backed out that change too.
//
// Back to square one. I maybe we just don't make a pool's
// constructor const and live with it. It's probably not a huge
// deal.
let mut stacks = Vec::with_capacity(MAX_POOL_STACKS);
for _ in 0..stacks.capacity() {
stacks.push(CacheLine(Mutex::new(vec![])));
}
let owner = AtomicUsize::new(THREAD_ID_UNOWNED);
let owner_val = UnsafeCell::new(None); // init'd on first access
Pool { create, stacks, owner, owner_val }
}
}
impl<T: Send, F: Fn() -> T> Pool<T, F> {
/// Get a value from the pool. This may block if another thread is also
/// attempting to retrieve a value from the pool.
#[inline]
pub(super) fn get(&self) -> PoolGuard<'_, T, F> {
// 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 if the caller's thread
// ID is equal to the owner, then only one thread may receive this
// value. This is also why we can get away with what looks like a
// racy load and a store. We know that if 'owner == caller', then
// only one thread can be here, so we don't need to worry about any
// other thread setting the owner to something else.
let caller = THREAD_ID.with(|id| *id);
let owner = self.owner.load(Ordering::Acquire);
if caller == owner {
// N.B. We could also do a CAS here instead of a load/store,
// but ad hoc benchmarking suggests it is slower. And a lot
// slower in the case where `get_slow` is common.
self.owner.store(THREAD_ID_INUSE, Ordering::Release);
return self.guard_owned(caller);
}
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, F> {
if owner == THREAD_ID_UNOWNED {
// This sentinel 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.
//
// Note that we set the owner to a different sentinel that
// indicates that the owned value is in use. The owner ID will
// get updated to the actual ID of this thread once the guard
// returned by this function is put back into the pool.
let res = self.owner.compare_exchange(
THREAD_ID_UNOWNED,
THREAD_ID_INUSE,
Ordering::AcqRel,
Ordering::Acquire,
);
if res.is_ok() {
// SAFETY: A successful CAS above implies this thread is
// the owner and that this is the only such thread that
// can reach here. Thus, there is no data race.
unsafe {
*self.owner_val.get() = Some((self.create)());
}
return self.guard_owned(caller);
}
}
let stack_id = caller % self.stacks.len();
// We try to acquire exclusive access to this thread's stack, and
// if so, grab a value from it if we can. We put this in a loop so
// that it's easy to tweak and experiment with a different number
// of tries. In the end, I couldn't see anything obviously better
// than one attempt in ad hoc testing.
for _ in 0..1 {
let mut stack = match self.stacks[stack_id].0.try_lock() {
Err(_) => continue,
Ok(stack) => stack,
};
if let Some(value) = stack.pop() {
return self.guard_stack(value);
}
// Unlock the mutex guarding the stack before creating a fresh
// value since we no longer need the stack.
drop(stack);
let value = Box::new((self.create)());
return self.guard_stack(value);
}
// We're only here if we could get access to our stack, so just
// create a new value. This seems like it could be wasteful, but
// waiting for exclusive access to a stack when there's high
// contention is brutal for perf.
self.guard_stack_transient(Box::new((self.create)()))
}
/// 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.
#[inline]
fn put_value(&self, value: Box<T>) {
let caller = THREAD_ID.with(|id| *id);
let stack_id = caller % self.stacks.len();
// As with trying to pop a value from this thread's stack, we
// merely attempt to get access to push this value back on the
// stack. If there's too much contention, we just give up and throw
// the value away.
//
// Interestingly, in ad hoc benchmarking, it is beneficial to
// attempt to push the value back more than once, unlike when
// popping the value. I don't have a good theory for why this is.
// I guess if we drop too many values then that winds up forcing
// the pop operation to create new fresh values and thus leads to
// less reuse. There's definitely a balancing act here.
for _ in 0..10 {
let mut stack = match self.stacks[stack_id].0.try_lock() {
Err(_) => continue,
Ok(stack) => stack,
};
stack.push(value);
return;
}
}
/// Create a guard that represents the special owned T.
#[inline]
fn guard_owned(&self, caller: usize) -> PoolGuard<'_, T, F> {
PoolGuard { pool: self, value: Err(caller), discard: false }
}
/// Create a guard that contains a value from the pool's stack.
#[inline]
fn guard_stack(&self, value: Box<T>) -> PoolGuard<'_, T, F> {
PoolGuard { pool: self, value: Ok(value), discard: false }
}
/// Create a guard that contains a value from the pool's stack with an
/// instruction to throw away the value instead of putting it back
/// into the pool.
#[inline]
fn guard_stack_transient(&self, value: Box<T>) -> PoolGuard<'_, T, F> {
PoolGuard { pool: self, value: Ok(value), discard: true }
}
}
impl<T: core::fmt::Debug, F> core::fmt::Debug for Pool<T, F> {
fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
f.debug_struct("Pool")
.field("stacks", &self.stacks)
.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.
pub(super) struct PoolGuard<'a, T: Send, F: Fn() -> T> {
/// The pool that this guard is attached to.
pool: &'a Pool<T, F>,
/// This is Err when the guard represents the special "owned" value.
/// In which case, the value is retrieved from 'pool.owner_val'. And
/// in the special case of `Err(THREAD_ID_DROPPED)`, it means the
/// guard has been put back into the pool and should no longer be used.
value: Result<Box<T>, usize>,
/// When true, the value should be discarded instead of being pushed
/// back into the pool. We tend to use this under high contention, and
/// this allows us to avoid inflating the size of the pool. (Because
/// under contention, we tend to create more values instead of waiting
/// for access to a stack of existing values.)
discard: bool,
}
impl<'a, T: Send, F: Fn() -> T> PoolGuard<'a, T, F> {
/// Return the underlying value.
#[inline]
pub(super) fn value(&self) -> &T {
match self.value {
Ok(ref v) => &**v,
// SAFETY: This is safe because the only way a PoolGuard gets
// created for self.value=Err is when the current thread
// corresponds to the owning thread, of which there can only
// be one. Thus, we are guaranteed to be providing exclusive
// access here which makes this safe.
//
// Also, since 'owner_val' is guaranteed to be initialized
// before an owned PoolGuard is created, the unchecked unwrap
// is safe.
Err(id) => unsafe {
// This assert is *not* necessary for safety, since we
// should never be here if the guard had been put back into
// the pool. This is a sanity check to make sure we didn't
// break an internal invariant.
debug_assert_ne!(THREAD_ID_DROPPED, id);
(*self.pool.owner_val.get()).as_ref().unwrap_unchecked()
},
}
}
/// Return the underlying value as a mutable borrow.
#[inline]
pub(super) fn value_mut(&mut self) -> &mut T {
match self.value {
Ok(ref mut v) => &mut **v,
// SAFETY: This is safe because the only way a PoolGuard gets
// created for self.value=None is when the current thread
// corresponds to the owning thread, of which there can only
// be one. Thus, we are guaranteed to be providing exclusive
// access here which makes this safe.
//
// Also, since 'owner_val' is guaranteed to be initialized
// before an owned PoolGuard is created, the unwrap_unchecked
// is safe.
Err(id) => unsafe {
// This assert is *not* necessary for safety, since we
// should never be here if the guard had been put back into
// the pool. This is a sanity check to make sure we didn't
// break an internal invariant.
debug_assert_ne!(THREAD_ID_DROPPED, id);
(*self.pool.owner_val.get()).as_mut().unwrap_unchecked()
},
}
}
/// Consumes this guard and puts it back into the pool.
#[inline]
pub(super) fn put(this: PoolGuard<'_, T, F>) {
// Since this is effectively consuming the guard and putting the
// value back into the pool, there's no reason to run its Drop
// impl after doing this. I don't believe there is a correctness
// problem with doing so, but there's definitely a perf problem
// by redoing this work. So we avoid it.
let mut this = core::mem::ManuallyDrop::new(this);
this.put_imp();
}
/// Puts this guard back into the pool by only borrowing the guard as
/// mutable. This should be called at most once.
#[inline(always)]
fn put_imp(&mut self) {
match core::mem::replace(&mut self.value, Err(THREAD_ID_DROPPED)) {
Ok(value) => {
// If we were told to discard this value then don't bother
// trying to put it back into the pool. This occurs when
// the pop operation failed to acquire a lock and we
// decided to create a new value in lieu of contending for
// the lock.
if self.discard {
return;
}
self.pool.put_value(value);
}
// If this guard has a value "owned" by the thread, then
// the Pool guarantees that this is the ONLY such guard.
// Therefore, in order to place it back into the pool and make
// it available, we need to change the owner back to the owning
// thread's ID. But note that we use the ID that was stored in
// the guard, since a guard can be moved to another thread and
// dropped. (A previous iteration of this code read from the
// THREAD_ID thread local, which uses the ID of the current
// thread which may not be the ID of the owning thread! This
// also avoids the TLS access, which is likely a hair faster.)
Err(owner) => {
// If we hit this point, it implies 'put_imp' has been
// called multiple times for the same guard which in turn
// corresponds to a bug in this implementation.
assert_ne!(THREAD_ID_DROPPED, owner);
self.pool.owner.store(owner, Ordering::Release);
}
}
}
}
impl<'a, T: Send, F: Fn() -> T> Drop for PoolGuard<'a, T, F> {
#[inline]
fn drop(&mut self) {
self.put_imp();
}
}
impl<'a, T: Send + core::fmt::Debug, F: Fn() -> T> core::fmt::Debug
for PoolGuard<'a, T, F>
{
fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
f.debug_struct("PoolGuard")
.field("pool", &self.pool)
.field("value", &self.value)
.finish()
}
}
}
// FUTURE: We should consider using Mara Bos's nearly-lock-free version of this
// here: https://gist.github.com/m-ou-se/5fdcbdf7dcf4585199ce2de697f367a4.
//
// One reason why I did things with a "mutex" below is that it isolates the
// safety concerns to just the Mutex, where as the safety of Mara's pool is a
// bit more sprawling. I also expect this code to not be used that much, and
// so is unlikely to get as much real world usage with which to test it. That
// means the "obviously correct" lever is an important one.
//
// The specific reason to use Mara's pool is that it is likely faster and also
// less likely to hit problems with spin-locks, although it is not completely
// impervious to them.
//
// The best solution to this problem, probably, is a truly lock free pool. That
// could be done with a lock free linked list. The issue is the ABA problem. It
// is difficult to avoid, and doing so is complex. BUT, the upshot of that is
// that if we had a truly lock free pool, then we could also use it above in
// the 'std' pool instead of a Mutex because it should be completely free the
// problems that come from spin-locks.
#[cfg(not(feature = "std"))]
mod inner {
use core::{
cell::UnsafeCell,
panic::{RefUnwindSafe, UnwindSafe},
sync::atomic::{AtomicBool, Ordering},
};
use alloc::{boxed::Box, vec, vec::Vec};
/// A thread safe pool utilizing alloc-only features.
///
/// Unlike the std version, it doesn't seem possible(?) to implement the
/// "thread owner" optimization because alloc-only doesn't have any concept
/// of threads. So the best we can do is just a normal stack. This will
/// increase latency in alloc-only environments.
pub(super) struct Pool<T, F> {
/// 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: F,
}
// If T is UnwindSafe, then since we provide exclusive access to any
// particular value in the pool, it should therefore also be considered
// RefUnwindSafe.
impl<T: UnwindSafe, F: UnwindSafe> RefUnwindSafe for Pool<T, F> {}
impl<T, F> Pool<T, F> {
/// Create a new pool. The given closure is used to create values in
/// the pool when necessary.
pub(super) const fn new(create: F) -> Pool<T, F> {
Pool { stack: Mutex::new(vec![]), create }
}
}
impl<T: Send, F: Fn() -> T> Pool<T, F> {
/// Get a value from the pool. This may block if another thread is also
/// attempting to retrieve a value from the pool.
#[inline]
pub(super) fn get(&self) -> PoolGuard<'_, T, F> {
let mut stack = self.stack.lock();
let value = match stack.pop() {
None => Box::new((self.create)()),
Some(value) => value,
};
PoolGuard { pool: self, value: Some(value) }
}
#[inline]
fn put(&self, guard: PoolGuard<'_, T, F>) {
let mut guard = core::mem::ManuallyDrop::new(guard);
if let Some(value) = guard.value.take() {
self.put_value(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.
#[inline]
fn put_value(&self, value: Box<T>) {
let mut stack = self.stack.lock();
stack.push(value);
}
}
impl<T: core::fmt::Debug, F> core::fmt::Debug for Pool<T, F> {
fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
f.debug_struct("Pool").field("stack", &self.stack).finish()
}
}
/// A guard that is returned when a caller requests a value from the pool.
pub(super) struct PoolGuard<'a, T: Send, F: Fn() -> T> {
/// The pool that this guard is attached to.
pool: &'a Pool<T, F>,
/// This is None after the guard has been put back into the pool.
value: Option<Box<T>>,
}
impl<'a, T: Send, F: Fn() -> T> PoolGuard<'a, T, F> {
/// Return the underlying value.
#[inline]
pub(super) fn value(&self) -> &T {
self.value.as_deref().unwrap()
}
/// Return the underlying value as a mutable borrow.
#[inline]
pub(super) fn value_mut(&mut self) -> &mut T {
self.value.as_deref_mut().unwrap()
}
/// Consumes this guard and puts it back into the pool.
#[inline]
pub(super) fn put(this: PoolGuard<'_, T, F>) {
// Since this is effectively consuming the guard and putting the
// value back into the pool, there's no reason to run its Drop
// impl after doing this. I don't believe there is a correctness
// problem with doing so, but there's definitely a perf problem
// by redoing this work. So we avoid it.
let mut this = core::mem::ManuallyDrop::new(this);
this.put_imp();
}
/// Puts this guard back into the pool by only borrowing the guard as
/// mutable. This should be called at most once.
#[inline(always)]
fn put_imp(&mut self) {
if let Some(value) = self.value.take() {
self.pool.put_value(value);
}
}
}
impl<'a, T: Send, F: Fn() -> T> Drop for PoolGuard<'a, T, F> {
#[inline]
fn drop(&mut self) {
self.put_imp();
}
}
impl<'a, T: Send + core::fmt::Debug, F: Fn() -> T> core::fmt::Debug
for PoolGuard<'a, T, F>
{
fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
f.debug_struct("PoolGuard")
.field("pool", &self.pool)
.field("value", &self.value)
.finish()
}
}
/// A spin-lock based mutex. Yes, I have read spinlocks cosnidered
/// harmful[1], and if there's a reasonable alternative choice, I'll
/// happily take it.
///
/// I suspect the most likely alternative here is a Treiber stack, but
/// implementing one correctly in a way that avoids the ABA problem looks
/// subtle enough that I'm not sure I want to attempt that. But otherwise,
/// we only need a mutex in order to implement our pool, so if there's
/// something simpler we can use that works for our `Pool` use case, then
/// that would be great.
///
/// Note that this mutex does not do poisoning.
///
/// [1]: https://matklad.github.io/2020/01/02/spinlocks-considered-harmful.html
#[derive(Debug)]
struct Mutex<T> {
locked: AtomicBool,
data: UnsafeCell<T>,
}
// SAFETY: Since a Mutex guarantees exclusive access, as long as we can
// send it across threads, it must also be Sync.
unsafe impl<T: Send> Sync for Mutex<T> {}
impl<T> Mutex<T> {
/// Create a new mutex for protecting access to the given value across
/// multiple threads simultaneously.
const fn new(value: T) -> Mutex<T> {
Mutex {
locked: AtomicBool::new(false),
data: UnsafeCell::new(value),
}
}
/// Lock this mutex and return a guard providing exclusive access to
/// `T`. This blocks if some other thread has already locked this
/// mutex.
#[inline]
fn lock(&self) -> MutexGuard<'_, T> {
while self
.locked
.compare_exchange(
false,
true,
Ordering::AcqRel,
Ordering::Acquire,
)
.is_err()
{
core::hint::spin_loop();
}
// SAFETY: The only way we're here is if we successfully set
// 'locked' to true, which implies we must be the only thread here
// and thus have exclusive access to 'data'.
let data = unsafe { &mut *self.data.get() };
MutexGuard { locked: &self.locked, data }
}
}
/// A guard that derefs to &T and &mut T. When it's dropped, the lock is
/// released.
#[derive(Debug)]
struct MutexGuard<'a, T> {
locked: &'a AtomicBool,
data: &'a mut T,
}
impl<'a, T> core::ops::Deref for MutexGuard<'a, T> {
type Target = T;
#[inline]
fn deref(&self) -> &T {
self.data
}
}
impl<'a, T> core::ops::DerefMut for MutexGuard<'a, T> {
#[inline]
fn deref_mut(&mut self) -> &mut T {
self.data
}
}
impl<'a, T> Drop for MutexGuard<'a, T> {
#[inline]
fn drop(&mut self) {
// Drop means 'data' is no longer accessible, so we can unlock
// the mutex.
self.locked.store(false, Ordering::Release);
}
}
}
#[cfg(test)]
mod tests {
use core::panic::{RefUnwindSafe, UnwindSafe};
use alloc::{boxed::Box, vec, vec::Vec};
use super::*;
#[test]
fn oibits() {
fn assert_oitbits<T: Send + Sync + UnwindSafe + RefUnwindSafe>() {}
assert_oitbits::<Pool<Vec<u32>>>();
assert_oitbits::<Pool<core::cell::RefCell<Vec<u32>>>>();
assert_oitbits::<
Pool<
Vec<u32>,
Box<
dyn Fn() -> Vec<u32>
+ Send
+ Sync
+ UnwindSafe
+ RefUnwindSafe,
>,
>,
>();
}
// 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.
#[cfg(feature = "std")]
#[test]
fn thread_owner_optimization() {
use std::{cell::RefCell, sync::Arc, vec};
let pool: Arc<Pool<RefCell<Vec<char>>>> =
Arc::new(Pool::new(|| RefCell::new(vec!['a'])));
pool.get().borrow_mut().push('x');
let pool1 = pool.clone();
let t1 = std::thread::spawn(move || {
let guard = pool1.get();
guard.borrow_mut().push('y');
});
let pool2 = pool.clone();
let t2 = std::thread::spawn(move || {
let guard = pool2.get();
guard.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().borrow());
}
// This tests that if the "owner" of a pool asks for two values, then it
// gets two distinct values and not the same one. This test failed in the
// course of developing the pool, which in turn resulted in UB because it
// permitted getting aliasing &mut borrows to the same place in memory.
#[test]
fn thread_owner_distinct() {
let pool = Pool::new(|| vec!['a']);
{
let mut g1 = pool.get();
let v1 = &mut *g1;
let mut g2 = pool.get();
let v2 = &mut *g2;
v1.push('b');
v2.push('c');
assert_eq!(&mut vec!['a', 'b'], v1);
assert_eq!(&mut vec!['a', 'c'], v2);
}
// This isn't technically guaranteed, but we
// expect to now get the "owned" value (the first
// call to 'get()' above) now that it's back in
// the pool.
assert_eq!(&mut vec!['a', 'b'], &mut *pool.get());
}
// This tests that we can share a guard with another thread, mutate the
// underlying value and everything works. This failed in the course of
// developing a pool since the pool permitted 'get()' to return the same
// value to the owner thread, even before the previous value was put back
// into the pool. This in turn resulted in this test producing a data race.
#[cfg(feature = "std")]
#[test]
fn thread_owner_sync() {
let pool = Pool::new(|| vec!['a']);
{
let mut g1 = pool.get();
let mut g2 = pool.get();
std::thread::scope(|s| {
s.spawn(|| {
g1.push('b');
});
s.spawn(|| {
g2.push('c');
});
});
let v1 = &mut *g1;
let v2 = &mut *g2;
assert_eq!(&mut vec!['a', 'b'], v1);
assert_eq!(&mut vec!['a', 'c'], v2);
}
// This isn't technically guaranteed, but we
// expect to now get the "owned" value (the first
// call to 'get()' above) now that it's back in
// the pool.
assert_eq!(&mut vec!['a', 'b'], &mut *pool.get());
}
// This tests that if we move a PoolGuard that is owned by the current
// thread to another thread and drop it, then the thread owner doesn't
// change. During development of the pool, this test failed because the
// PoolGuard assumed it was dropped in the same thread from which it was
// created, and thus used the current thread's ID as the owner, which could
// be different than the actual owner of the pool.
#[cfg(feature = "std")]
#[test]
fn thread_owner_send_drop() {
let pool = Pool::new(|| vec!['a']);
// Establishes this thread as the owner.
{
pool.get().push('b');
}
std::thread::scope(|s| {
// Sanity check that we get the same value back.
// (Not technically guaranteed.)
let mut g = pool.get();
assert_eq!(&vec!['a', 'b'], &*g);
// Now push it to another thread and drop it.
s.spawn(move || {
g.push('c');
})
.join()
.unwrap();
});
// Now check that we're still the owner. This is not technically
// guaranteed by the API, but is true in practice given the thread
// owner optimization.
assert_eq!(&vec!['a', 'b', 'c'], &*pool.get());
}
}
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