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Merge pull request #4976 from meilisearch/fix-append-only-vec

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Fix append only `Vec` by using a `LinkedList`
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Many the fish 2024-10-03 17:26:00 +02:00 committed by GitHub
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6 changed files with 165 additions and 334 deletions
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158
milli/src/update/new/append_only_linked_list.rs Normal file
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@ -0,0 +1,158 @@
use std::sync::atomic::AtomicPtr;
use std::{fmt, mem};
/// An append-only linked-list that returns a mutable references to the pushed items.
pub struct AppendOnlyLinkedList<T> {
head: AtomicPtr<Node<T>>,
}
struct Node<T> {
item: T,
next: AtomicPtr<Node<T>>,
}
impl<T> AppendOnlyLinkedList<T> {
/// Creates an empty list.
pub fn new() -> AppendOnlyLinkedList<T> {
AppendOnlyLinkedList { head: AtomicPtr::default() }
}
/// Pushes the item at the front of the linked-list and returns a unique and mutable reference to it.
#[allow(clippy::mut_from_ref)] // the mut ref is derived from T and unique each time
pub fn push(&self, item: T) -> &mut T {
use std::sync::atomic::Ordering::{Relaxed, SeqCst};
let node = Box::leak(Box::new(Node { item, next: AtomicPtr::default() }));
let mut head = self.head.load(SeqCst);
loop {
std::hint::spin_loop();
node.next = AtomicPtr::new(head);
match self.head.compare_exchange_weak(head, node, SeqCst, Relaxed) {
Ok(_) => break,
Err(new) => head = new,
}
}
&mut node.item
}
}
impl<T> Default for AppendOnlyLinkedList<T> {
fn default() -> Self {
Self::new()
}
}
impl<T> Drop for AppendOnlyLinkedList<T> {
fn drop(&mut self) {
// Let's use the drop implementation of the IntoIter struct
IntoIter(mem::take(&mut self.head));
}
}
impl<T> fmt::Debug for AppendOnlyLinkedList<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("AppendOnlyLinkedList").finish()
}
}
impl<T> IntoIterator for AppendOnlyLinkedList<T> {
type Item = T;
type IntoIter = IntoIter<T>;
fn into_iter(mut self) -> Self::IntoIter {
IntoIter(mem::take(&mut self.head))
}
}
pub struct IntoIter<T>(AtomicPtr<Node<T>>);
impl<T> Iterator for IntoIter<T> {
type Item = T;
fn next(&mut self) -> Option<Self::Item> {
let ptr = *self.0.get_mut();
if ptr.is_null() {
None
} else {
let node = unsafe { Box::from_raw(ptr) };
// Let's set the next node to read to be the next of this one
self.0 = node.next;
Some(node.item)
}
}
}
impl<T> Drop for IntoIter<T> {
fn drop(&mut self) {
let mut ptr = *self.0.get_mut();
while !ptr.is_null() {
let mut node = unsafe { Box::from_raw(ptr) };
// Let's set the next node to read to be the next of this one
ptr = *node.next.get_mut();
}
}
}
#[test]
fn test_parallel_pushing() {
use std::sync::Arc;
let v = Arc::new(AppendOnlyLinkedList::<u64>::new());
let mut threads = Vec::new();
const N: u64 = 100;
for thread_num in 0..N {
let v = v.clone();
threads.push(std::thread::spawn(move || {
let which1 = v.push(thread_num);
let which2 = v.push(thread_num);
assert_eq!(*which1, thread_num);
assert_eq!(*which2, thread_num);
}));
}
for t in threads {
t.join().unwrap();
}
let v = Arc::into_inner(v).unwrap().into_iter().collect::<Vec<_>>();
for thread_num in (0..N).rev() {
assert_eq!(2, v.iter().copied().filter(|&x| x == thread_num).count());
}
}
#[test]
fn test_into_vec() {
struct SafeToDrop(bool);
impl Drop for SafeToDrop {
fn drop(&mut self) {
assert!(self.0);
}
}
let v = AppendOnlyLinkedList::new();
for _ in 0..50 {
v.push(SafeToDrop(false));
}
let mut v = v.into_iter().collect::<Vec<_>>();
assert_eq!(v.len(), 50);
for i in v.iter_mut() {
i.0 = true;
}
}
#[test]
fn test_push_then_index_mut() {
let v = AppendOnlyLinkedList::<usize>::new();
let mut w = Vec::new();
for i in 0..1024 {
*v.push(i) += 1;
w.push(i + 1);
}
let mut v = v.into_iter().collect::<Vec<_>>();
v.reverse();
assert_eq!(v, w);
}

327
milli/src/update/new/append_only_vec.rs
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@ -1,327 +0,0 @@
// Code taken from <https://github.com/droundy/append-only-vec/blob/main/src/lib.rs>
// and modified in order to get a ref mut instead of the index of newly inserted items.
//! AppendOnlyVec
//!
//! This is a pretty simple type, which is a vector that you can push into and
//! receive a reference to the item you just inserted. The data structure never
//! moves an element once allocated, so you can push to the vec even while holding
//! mutable references to elements that have already been pushed.
//!
//! ### Scaling
//!
//! 1. Accessing an element is O(1), but slightly more expensive than for a
//! standard `Vec`.
//!
//! 2. Pushing a new element amortizes to O(1), but may require allocation of a
//! new chunk.
//!
//! ### Example
//!
//! ```
//! use append_only_vec::AppendOnlyVec;
//!
//! static V: AppendOnlyVec<String> = AppendOnlyVec::<String>::new();
//! let mut threads = Vec::new();
//! for thread_num in 0..10 {
//! threads.push(std::thread::spawn(move || {
//! for n in 0..100 {
//! let s = format!("thread {} says {}", thread_num, n);
//! let which = V.push(s.clone());
//! assert_eq!(&which, &s);
//! }
//! }));
//! }
//!
//! for t in threads {
//! t.join();
//! }
//!
//! assert_eq!(V.len(), 1000);
//! ```
use std::cell::UnsafeCell;
use std::fmt::Debug;
use std::ptr;
use std::sync::atomic::{AtomicUsize, Ordering};
pub struct AppendOnlyVec<T> {
count: AtomicUsize,
_reserved: AtomicUsize,
data: [UnsafeCell<*mut T>; BITS_USED - 1 - 3],
}
unsafe impl<T: Send> Send for AppendOnlyVec<T> {}
unsafe impl<T: Sync + Send> Sync for AppendOnlyVec<T> {}
const BITS: usize = std::mem::size_of::<usize>() * 8;
#[cfg(target_arch = "x86_64")]
const BITS_USED: usize = 48;
#[cfg(all(not(target_arch = "x86_64"), target_pointer_width = "64"))]
const BITS_USED: usize = 64;
#[cfg(target_pointer_width = "32")]
const BITS_USED: usize = 32;
// This takes an index into a vec, and determines which data array will hold it
// (the first return value), and what the index will be into that data array
// (second return value)
//
// The ith data array holds 1<<i values.
const fn indices(i: usize) -> (u32, usize) {
let i = i + 8;
let bin = BITS as u32 - 1 - i.leading_zeros();
let bin = bin - 3;
let offset = i - bin_size(bin);
(bin, offset)
}
const fn bin_size(array: u32) -> usize {
(1 << 3) << array
}
#[test]
fn test_indices() {
for i in 0..32 {
println!("{:3}: {} {}", i, indices(i).0, indices(i).1);
}
let mut array = 0;
let mut offset = 0;
let mut index = 0;
while index < 1000 {
index += 1;
offset += 1;
if offset >= bin_size(array) {
offset = 0;
array += 1;
}
assert_eq!(indices(index), (array, offset));
}
}
impl<T> AppendOnlyVec<T> {
const EMPTY: UnsafeCell<*mut T> = UnsafeCell::new(ptr::null_mut());
/// Allocate a new empty array.
pub const fn new() -> Self {
AppendOnlyVec {
count: AtomicUsize::new(0),
_reserved: AtomicUsize::new(0),
data: [Self::EMPTY; BITS_USED - 1 - 3],
}
}
/// Find the length of the array.
#[inline]
pub fn len(&self) -> usize {
self.count.load(Ordering::Acquire)
}
fn layout(array: u32) -> std::alloc::Layout {
std::alloc::Layout::array::<T>(bin_size(array)).unwrap()
}
/// Append an element to the array and get a mutable ref to it.
///
/// This is notable in that it doesn't require a `&mut self`, because it
/// does appropriate atomic synchronization.
pub fn push(&self, val: T) -> &mut T {
let idx = self._reserved.fetch_add(1, Ordering::Relaxed);
let (array, offset) = indices(idx);
let ptr = if self.len() < 1 + idx - offset {
// We are working on a new array, which may not have been allocated...
if offset == 0 {
// It is our job to allocate the array! The size of the array
// is determined in the self.layout method, which needs to be
// consistent with the indices function.
let layout = Self::layout(array);
let ptr = unsafe { std::alloc::alloc(layout) } as *mut T;
unsafe {
*self.data[array as usize].get() = ptr;
}
ptr
} else {
// We need to wait for the array to be allocated.
while self.len() < 1 + idx - offset {
std::hint::spin_loop();
}
// The Ordering::Acquire semantics of self.len() ensures that
// this pointer read will get the non-null pointer allocated
// above.
unsafe { *self.data[array as usize].get() }
}
} else {
// The Ordering::Acquire semantics of self.len() ensures that
// this pointer read will get the non-null pointer allocated
// above.
unsafe { *self.data[array as usize].get() }
};
// The contents of this offset are guaranteed to be unused (so far)
// because we got the idx from our fetch_add above, and ptr is
// guaranteed to be valid because of the loop we used above, which used
// self.len() which has Ordering::Acquire semantics.
unsafe { (ptr.add(offset)).write(val) };
// Now we need to increase the size of the vec, so it can get read. We
// use Release upon success, to ensure that the value which we wrote is
// visible to any thread that has confirmed that the count is big enough
// to read that element. In case of failure, we can be relaxed, since
// we don't do anything with the result other than try again.
while self
.count
.compare_exchange(idx, idx + 1, Ordering::Release, Ordering::Relaxed)
.is_err()
{
// This means that someone else *started* pushing before we started,
// but hasn't yet finished. We have to wait for them to finish
// pushing before we can update the count. Note that using a
// spinloop here isn't really ideal, but except when allocating a
// new array, the window between reserving space and using it is
// pretty small, so contention will hopefully be rare, and having a
// context switch during that interval will hopefully be vanishingly
// unlikely.
std::hint::spin_loop();
}
unsafe { &mut *ptr }
}
/// Convert into a standard `Vec`.
pub fn into_vec(self) -> Vec<T> {
let mut vec = Vec::with_capacity(self.len());
for idx in 0..self.len() {
let (array, offset) = indices(idx);
// We use a Relaxed load of the pointer, because the loop above (which
// ends before `self.len()`) should ensure that the data we want is
// already visible, since it Acquired `self.count` which synchronizes
// with the write in `self.push`.
let ptr = unsafe { *self.data[array as usize].get() };
// Copy the element value. The copy remaining in the array must not
// be used again (i.e. make sure we do not drop it)
let value = unsafe { ptr.add(offset).read() };
vec.push(value);
}
// Prevent dropping the copied-out values by marking the count as 0 before
// our own drop is run
self.count.store(0, Ordering::Relaxed);
vec
}
}
impl<T> Default for AppendOnlyVec<T> {
fn default() -> Self {
Self::new()
}
}
impl<T> Debug for AppendOnlyVec<T> {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
f.debug_struct("AppendOnlyVec").field("len", &self.len()).finish()
}
}
impl<T> Drop for AppendOnlyVec<T> {
fn drop(&mut self) {
// First we'll drop all the `T` in a slightly sloppy way. FIXME this
// could be optimized to avoid reloading the `ptr`.
for idx in 0..self.len() {
let (array, offset) = indices(idx);
// We use a Relaxed load of the pointer, because the loop above (which
// ends before `self.len()`) should ensure that the data we want is
// already visible, since it Acquired `self.count` which synchronizes
// with the write in `self.push`.
let ptr = unsafe { *self.data[array as usize].get() };
unsafe {
ptr::drop_in_place(ptr.add(offset));
}
}
// Now we will free all the arrays.
for array in 0..self.data.len() as u32 {
// This load is relaxed because no other thread can have a reference
// to Self because we have a &mut self.
let ptr = unsafe { *self.data[array as usize].get() };
if !ptr.is_null() {
let layout = Self::layout(array);
unsafe { std::alloc::dealloc(ptr as *mut u8, layout) };
} else {
break;
}
}
}
}
impl<T> IntoIterator for AppendOnlyVec<T> {
type Item = T;
type IntoIter = std::vec::IntoIter<T>;
fn into_iter(self) -> Self::IntoIter {
self.into_vec().into_iter()
}
}
#[test]
fn test_parallel_pushing() {
use std::sync::Arc;
let v = Arc::new(AppendOnlyVec::<u64>::new());
let mut threads = Vec::new();
const N: u64 = 100;
for thread_num in 0..N {
let v = v.clone();
threads.push(std::thread::spawn(move || {
let which1 = v.push(thread_num);
let which2 = v.push(thread_num);
assert_eq!(*which1, thread_num);
assert_eq!(*which2, thread_num);
}));
}
for t in threads {
t.join().unwrap();
}
let v = Arc::into_inner(v).unwrap().into_vec();
for thread_num in 0..N {
assert_eq!(2, v.iter().copied().filter(|&x| x == thread_num).count());
}
}
#[test]
fn test_into_vec() {
struct SafeToDrop(bool);
impl Drop for SafeToDrop {
fn drop(&mut self) {
assert!(self.0);
}
}
let v = AppendOnlyVec::new();
for _ in 0..50 {
v.push(SafeToDrop(false));
}
let mut v = v.into_vec();
assert_eq!(v.len(), 50);
for i in v.iter_mut() {
i.0 = true;
}
}
#[test]
fn test_push_then_index_mut() {
let v = AppendOnlyVec::<usize>::new();
for i in 0..1024 {
*v.push(i) += 1;
}
let v = v.into_vec();
for i in 0..1024 {
assert_eq!(v[i], 2 * i);
}
}

4
milli/src/update/new/extract/faceted/extract_facets.rs
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@ -12,7 +12,7 @@ use super::super::cache::CboCachedSorter;
use super::facet_document::extract_document_facets;
use super::FacetKind;
use crate::facet::value_encoding::f64_into_bytes;
use crate::update::new::append_only_vec::AppendOnlyVec;
use crate::update::new::append_only_linked_list::AppendOnlyLinkedList;
use crate::update::new::extract::DocidsExtractor;
use crate::update::new::parallel_iterator_ext::ParallelIteratorExt;
use crate::update::new::DocumentChange;
@ -210,7 +210,7 @@ impl DocidsExtractor for FacetedDocidsExtractor {
let attributes_to_extract = Self::attributes_to_extract(&rtxn, index)?;
let attributes_to_extract: Vec<_> =
attributes_to_extract.iter().map(|s| s.as_ref()).collect();
let caches = AppendOnlyVec::new();
let caches = AppendOnlyLinkedList::new();
{
let span =

4
milli/src/update/new/extract/searchable/extract_word_docids.rs
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@ -9,7 +9,7 @@ use rayon::iter::IntoParallelIterator;
use super::tokenize_document::{tokenizer_builder, DocumentTokenizer};
use super::SearchableExtractor;
use crate::update::new::append_only_vec::AppendOnlyVec;
use crate::update::new::append_only_linked_list::AppendOnlyLinkedList;
use crate::update::new::extract::cache::CboCachedSorter;
use crate::update::new::extract::perm_json_p::contained_in;
use crate::update::new::parallel_iterator_ext::ParallelIteratorExt;
@ -341,7 +341,7 @@ impl WordDocidsExtractors {
max_positions_per_attributes: MAX_POSITION_PER_ATTRIBUTE,
};
let caches = AppendOnlyVec::new();
let caches = AppendOnlyLinkedList::new();
{
let span =

4
milli/src/update/new/extract/searchable/mod.rs
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@ -14,7 +14,7 @@ use tokenize_document::{tokenizer_builder, DocumentTokenizer};
use super::cache::CboCachedSorter;
use super::DocidsExtractor;
use crate::update::new::append_only_vec::AppendOnlyVec;
use crate::update::new::append_only_linked_list::AppendOnlyLinkedList;
use crate::update::new::parallel_iterator_ext::ParallelIteratorExt;
use crate::update::new::DocumentChange;
use crate::update::{create_sorter, GrenadParameters, MergeDeladdCboRoaringBitmaps};
@ -58,7 +58,7 @@ pub trait SearchableExtractor {
localized_attributes_rules: &localized_attributes_rules,
max_positions_per_attributes: MAX_POSITION_PER_ATTRIBUTE,
};
let caches = AppendOnlyVec::new();
let caches = AppendOnlyLinkedList::new();
{
let span =

2
milli/src/update/new/mod.rs
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@ -4,7 +4,7 @@ pub use top_level_map::{CowStr, TopLevelMap};
use super::del_add::DelAdd;
use crate::FieldId;
mod append_only_vec;
mod append_only_linked_list;
mod channel;
mod document_change;
mod extract;