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Diffstat (limited to 'src/pool.rs')
| -rw-r--r-- | src/pool.rs | 333 |
1 files changed, 333 insertions, 0 deletions
diff --git a/src/pool.rs b/src/pool.rs new file mode 100644 index 0000000..a506ee9 --- /dev/null +++ b/src/pool.rs @@ -0,0 +1,333 @@ +// 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: 'a + 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 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()); + } +} |