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//! Index/slot allocator policies for the pooling allocator.
use crate::CompiledModuleId;
use std::collections::hash_map::{Entry, HashMap};
use std::mem;
use std::sync::Mutex;
/// A slot index. The job of this allocator is to hand out these
/// indices.
#[derive(Hash, Clone, Copy, Debug, PartialEq, Eq)]
pub struct SlotId(pub u32);
impl SlotId {
/// The index of this slot.
pub fn index(self) -> usize {
self.0 as usize
}
}
#[derive(Debug)]
pub struct IndexAllocator(Mutex<Inner>);
#[derive(Debug)]
struct Inner {
/// Maximum number of "unused warm slots" which will be allowed during
/// allocation.
///
/// This is a user-configurable knob which can be used to influence the
/// maximum number of unused slots at any one point in time. A "warm slot"
/// is one that's considered having been previously allocated.
max_unused_warm_slots: u32,
/// Current count of "warm slots", or those that were previously allocated
/// which are now no longer in use.
///
/// This is the size of the `warm` list.
unused_warm_slots: u32,
/// A linked list (via indices) which enumerates all "warm and unused"
/// slots, or those which have previously been allocated and then free'd.
warm: List,
/// Last slot that was allocated for the first time ever.
///
/// This is initially 0 and is incremented during `pick_cold`. If this
/// matches `max_cold`, there are no more cold slots left.
last_cold: u32,
/// The state of any given slot.
///
/// Records indices in the above list (empty) or two lists (with affinity),
/// and these indices are kept up-to-date to allow fast removal.
slot_state: Vec<SlotState>,
/// Affine slot management which tracks which slots are free and were last
/// used with the specified `CompiledModuleId`.
///
/// The `List` here is appended to during deallocation and removal happens
/// from the tail during allocation.
module_affine: HashMap<CompiledModuleId, List>,
}
/// A helper "linked list" data structure which is based on indices.
#[derive(Default, Debug)]
struct List {
head: Option<SlotId>,
tail: Option<SlotId>,
}
/// A helper data structure for an intrusive linked list, coupled with the
/// `List` type.
#[derive(Default, Debug, Copy, Clone)]
struct Link {
prev: Option<SlotId>,
next: Option<SlotId>,
}
#[derive(Clone, Debug)]
enum SlotState {
/// This slot is currently in use and is affine to the specified module.
Used(Option<CompiledModuleId>),
/// This slot is not currently used, and has never been used.
UnusedCold,
/// This slot is not currently used, but was previously allocated.
///
/// The payload here is metadata about the lists that this slot is contained
/// within.
UnusedWarm(Unused),
}
impl SlotState {
fn unwrap_unused(&mut self) -> &mut Unused {
match self {
SlotState::UnusedWarm(u) => u,
_ => unreachable!(),
}
}
}
#[derive(Default, Copy, Clone, Debug)]
struct Unused {
/// Which module this slot was historically affine to, if any.
affinity: Option<CompiledModuleId>,
/// Metadata about the linked list for all slots affine to `affinity`.
affine_list_link: Link,
/// Metadata within the `warm` list of the main allocator.
unused_list_link: Link,
}
enum AllocMode {
ForceAffineAndClear,
AnySlot,
}
impl IndexAllocator {
/// Create the default state for this strategy.
pub fn new(max_instances: u32, max_unused_warm_slots: u32) -> Self {
IndexAllocator(Mutex::new(Inner {
last_cold: 0,
max_unused_warm_slots,
unused_warm_slots: 0,
module_affine: HashMap::new(),
slot_state: (0..max_instances).map(|_| SlotState::UnusedCold).collect(),
warm: List::default(),
}))
}
/// Allocate a new index from this allocator optionally using `id` as an
/// affinity request if the allocation strategy supports it.
///
/// Returns `None` if no more slots are available.
pub fn alloc(&self, module_id: Option<CompiledModuleId>) -> Option<SlotId> {
self._alloc(module_id, AllocMode::AnySlot)
}
/// Attempts to allocate a guaranteed-affine slot to the module `id`
/// specified.
///
/// Returns `None` if there are no slots affine to `id`. The allocation of
/// this slot will not record the affinity to `id`, instead simply listing
/// it as taken. This is intended to be used for clearing out all affine
/// slots to a module.
pub fn alloc_affine_and_clear_affinity(&self, module_id: CompiledModuleId) -> Option<SlotId> {
self._alloc(Some(module_id), AllocMode::ForceAffineAndClear)
}
fn _alloc(&self, module_id: Option<CompiledModuleId>, mode: AllocMode) -> Option<SlotId> {
let mut inner = self.0.lock().unwrap();
let inner = &mut *inner;
// As a first-pass always attempt an affine allocation. This will
// succeed if any slots are considered affine to `module_id` (if it's
// specified). Failing that something else is attempted to be chosen.
let slot_id = inner.pick_affine(module_id).or_else(|| {
match mode {
// If any slot is requested then this is a normal instantiation
// looking for an index. Without any affine candidates there are
// two options here:
//
// 1. Pick a slot amongst previously allocated slots
// 2. Pick a slot that's never been used before
//
// The choice here is guided by the initial configuration of
// `max_unused_warm_slots`. If our unused warm slots, which are
// likely all affine, is below this threshold then the affinity
// of the warm slots isn't tampered with and first a cold slot
// is chosen. If the cold slot allocation fails, however, a warm
// slot is evicted.
//
// The opposite happens when we're above our threshold for the
// maximum number of warm slots, meaning that a warm slot is
// attempted to be picked from first with a cold slot following
// that. Note that the warm slot allocation in this case should
// only fail of `max_unused_warm_slots` is 0, otherwise
// `pick_warm` will always succeed.
AllocMode::AnySlot => {
if inner.unused_warm_slots < inner.max_unused_warm_slots {
inner.pick_cold().or_else(|| inner.pick_warm())
} else {
inner.pick_warm().or_else(|| {
debug_assert!(inner.max_unused_warm_slots == 0);
inner.pick_cold()
})
}
}
// In this mode an affinity-based allocation is always performed
// as the purpose here is to clear out slots relevant to
// `module_id` during module teardown. This means that there's
// no consulting non-affine slots in this path.
AllocMode::ForceAffineAndClear => None,
}
})?;
inner.slot_state[slot_id.index()] = SlotState::Used(match mode {
AllocMode::ForceAffineAndClear => None,
AllocMode::AnySlot => module_id,
});
Some(slot_id)
}
pub(crate) fn free(&self, index: SlotId) {
let mut inner = self.0.lock().unwrap();
let inner = &mut *inner;
let module = match inner.slot_state[index.index()] {
SlotState::Used(module) => module,
_ => unreachable!(),
};
// Bump the number of warm slots since this slot is now considered
// previously used. Afterwards append it to the linked list of all
// unused and warm slots.
inner.unused_warm_slots += 1;
let unused_list_link = inner
.warm
.append(index, &mut inner.slot_state, |s| &mut s.unused_list_link);
let affine_list_link = match module {
// If this slot is affine to a particular module then append this
// index to the linked list for the affine module. Otherwise insert
// a new one-element linked list.
Some(module) => match inner.module_affine.entry(module) {
Entry::Occupied(mut e) => e
.get_mut()
.append(index, &mut inner.slot_state, |s| &mut s.affine_list_link),
Entry::Vacant(v) => {
v.insert(List::new(index));
Link::default()
}
},
// If this slot has no affinity then the affine link is empty.
None => Link::default(),
};
inner.slot_state[index.index()] = SlotState::UnusedWarm(Unused {
affinity: module,
affine_list_link,
unused_list_link,
});
}
/// For testing only, we want to be able to assert what is on the
/// single freelist, for the policies that keep just one.
#[cfg(test)]
pub(crate) fn testing_freelist(&self) -> Vec<SlotId> {
let inner = self.0.lock().unwrap();
inner
.warm
.iter(&inner.slot_state, |s| &s.unused_list_link)
.collect()
}
/// For testing only, get the list of all modules with at least
/// one slot with affinity for that module.
#[cfg(test)]
pub(crate) fn testing_module_affinity_list(&self) -> Vec<CompiledModuleId> {
let inner = self.0.lock().unwrap();
inner.module_affine.keys().copied().collect()
}
}
impl Inner {
/// Attempts to allocate a slot already affine to `id`, returning `None` if
/// `id` is `None` or if there are no affine slots.
fn pick_affine(&mut self, module_id: Option<CompiledModuleId>) -> Option<SlotId> {
// Note that the `tail` is chosen here of the affine list as it's the
// most recently used, which for affine allocations is what we want --
// maximizing temporal reuse.
let ret = self.module_affine.get(&module_id?)?.tail?;
self.remove(ret);
Some(ret)
}
fn pick_warm(&mut self) -> Option<SlotId> {
// Insertions into the `unused` list happen at the `tail`, so the
// least-recently-used item will be at the head. That's our goal here,
// pick the least-recently-used slot since something "warm" is being
// evicted anyway.
let head = self.warm.head?;
self.remove(head);
Some(head)
}
fn remove(&mut self, slot: SlotId) {
// Decrement the size of the warm list, and additionally remove it from
// the `warm` linked list.
self.unused_warm_slots -= 1;
self.warm
.remove(slot, &mut self.slot_state, |u| &mut u.unused_list_link);
// If this slot is affine to a module then additionally remove it from
// that module's affinity linked list. Note that if the module's affine
// list is empty then the module's entry in the map is completely
// removed as well.
let module = self.slot_state[slot.index()].unwrap_unused().affinity;
if let Some(module) = module {
let mut list = match self.module_affine.entry(module) {
Entry::Occupied(e) => e,
Entry::Vacant(_) => unreachable!(),
};
list.get_mut()
.remove(slot, &mut self.slot_state, |u| &mut u.affine_list_link);
if list.get_mut().head.is_none() {
list.remove();
}
}
}
fn pick_cold(&mut self) -> Option<SlotId> {
if (self.last_cold as usize) == self.slot_state.len() {
None
} else {
let ret = Some(SlotId(self.last_cold));
self.last_cold += 1;
ret
}
}
}
impl List {
/// Creates a new one-element list pointing at `id`.
fn new(id: SlotId) -> List {
List {
head: Some(id),
tail: Some(id),
}
}
/// Appends the `id` to this list whose links are determined by `link`.
fn append(
&mut self,
id: SlotId,
states: &mut [SlotState],
link: fn(&mut Unused) -> &mut Link,
) -> Link {
// This `id` is the new tail...
let tail = mem::replace(&mut self.tail, Some(id));
// If the tail was present, then update its `next` field to ourselves as
// we've been appended, otherwise update the `head` since the list was
// previously empty.
match tail {
Some(tail) => link(states[tail.index()].unwrap_unused()).next = Some(id),
None => self.head = Some(id),
}
Link {
prev: tail,
next: None,
}
}
/// Removes `id` from this list whose links are determined by `link`.
fn remove(
&mut self,
id: SlotId,
slot_state: &mut [SlotState],
link: fn(&mut Unused) -> &mut Link,
) -> Unused {
let mut state = *slot_state[id.index()].unwrap_unused();
let next = link(&mut state).next;
let prev = link(&mut state).prev;
// If a `next` node is present for this link, then its previous was our
// own previous now. Otherwise we are the tail so the new tail is our
// previous.
match next {
Some(next) => link(slot_state[next.index()].unwrap_unused()).prev = prev,
None => self.tail = prev,
}
// Same as the `next` node, except everything is in reverse.
match prev {
Some(prev) => link(slot_state[prev.index()].unwrap_unused()).next = next,
None => self.head = next,
}
state
}
#[cfg(test)]
fn iter<'a>(
&'a self,
states: &'a [SlotState],
link: fn(&Unused) -> &Link,
) -> impl Iterator<Item = SlotId> + 'a {
let mut cur = self.head;
let mut prev = None;
std::iter::from_fn(move || {
if cur.is_none() {
assert_eq!(prev, self.tail);
}
let ret = cur?;
match &states[ret.index()] {
SlotState::UnusedWarm(u) => {
assert_eq!(link(u).prev, prev);
prev = Some(ret);
cur = link(u).next
}
_ => unreachable!(),
}
Some(ret)
})
}
}
#[cfg(test)]
mod test {
use super::{IndexAllocator, SlotId};
use crate::CompiledModuleIdAllocator;
#[test]
fn test_next_available_allocation_strategy() {
for size in 0..20 {
let state = IndexAllocator::new(size, 0);
for i in 0..size {
assert_eq!(state.alloc(None).unwrap().index(), i as usize);
}
assert!(state.alloc(None).is_none());
}
}
#[test]
fn test_affinity_allocation_strategy() {
let id_alloc = CompiledModuleIdAllocator::new();
let id1 = id_alloc.alloc();
let id2 = id_alloc.alloc();
let state = IndexAllocator::new(100, 100);
let index1 = state.alloc(Some(id1)).unwrap();
assert_eq!(index1.index(), 0);
let index2 = state.alloc(Some(id2)).unwrap();
assert_eq!(index2.index(), 1);
assert_ne!(index1, index2);
state.free(index1);
let index3 = state.alloc(Some(id1)).unwrap();
assert_eq!(index3, index1);
state.free(index3);
state.free(index2);
// Both id1 and id2 should have some slots with affinity.
let affinity_modules = state.testing_module_affinity_list();
assert_eq!(2, affinity_modules.len());
assert!(affinity_modules.contains(&id1));
assert!(affinity_modules.contains(&id2));
// Now there is 1 free instance for id2 and 1 free instance
// for id1, and 98 empty. Allocate 100 for id2. The first
// should be equal to the one we know was previously used for
// id2. The next 99 are arbitrary.
let mut indices = vec![];
for _ in 0..100 {
indices.push(state.alloc(Some(id2)).unwrap());
}
assert!(state.alloc(None).is_none());
assert_eq!(indices[0], index2);
for i in indices {
state.free(i);
}
// Now there should be no slots left with affinity for id1.
let affinity_modules = state.testing_module_affinity_list();
assert_eq!(1, affinity_modules.len());
assert!(affinity_modules.contains(&id2));
// Allocate an index we know previously had an instance but
// now does not (list ran empty).
let index = state.alloc(Some(id1)).unwrap();
state.free(index);
}
#[test]
fn clear_affine() {
let id_alloc = CompiledModuleIdAllocator::new();
let id = id_alloc.alloc();
for max_unused_warm_slots in [0, 1, 2] {
let state = IndexAllocator::new(100, max_unused_warm_slots);
let index1 = state.alloc(Some(id)).unwrap();
let index2 = state.alloc(Some(id)).unwrap();
state.free(index2);
state.free(index1);
assert!(state.alloc_affine_and_clear_affinity(id).is_some());
assert!(state.alloc_affine_and_clear_affinity(id).is_some());
assert_eq!(state.alloc_affine_and_clear_affinity(id), None);
}
}
#[test]
fn test_affinity_allocation_strategy_random() {
use rand::Rng;
let mut rng = rand::thread_rng();
let id_alloc = CompiledModuleIdAllocator::new();
let ids = std::iter::repeat_with(|| id_alloc.alloc())
.take(10)
.collect::<Vec<_>>();
let state = IndexAllocator::new(1000, 1000);
let mut allocated: Vec<SlotId> = vec![];
let mut last_id = vec![None; 1000];
let mut hits = 0;
for _ in 0..100_000 {
loop {
if !allocated.is_empty() && rng.gen_bool(0.5) {
let i = rng.gen_range(0..allocated.len());
let to_free_idx = allocated.swap_remove(i);
state.free(to_free_idx);
} else {
let id = ids[rng.gen_range(0..ids.len())];
let index = match state.alloc(Some(id)) {
Some(id) => id,
None => continue,
};
if last_id[index.index()] == Some(id) {
hits += 1;
}
last_id[index.index()] = Some(id);
allocated.push(index);
}
break;
}
}
// 10% reuse would be random chance (because we have 10 module
// IDs). Check for at least double that to ensure some sort of
// affinity is occurring.
assert!(
hits > 20000,
"expected at least 20000 (20%) ID-reuses but got {}",
hits
);
}
#[test]
fn test_affinity_threshold() {
let id_alloc = CompiledModuleIdAllocator::new();
let id1 = id_alloc.alloc();
let id2 = id_alloc.alloc();
let id3 = id_alloc.alloc();
let state = IndexAllocator::new(10, 2);
// Set some slot affinities
assert_eq!(state.alloc(Some(id1)), Some(SlotId(0)));
state.free(SlotId(0));
assert_eq!(state.alloc(Some(id2)), Some(SlotId(1)));
state.free(SlotId(1));
// Only 2 slots are allowed to be unused and warm, so we're at our
// threshold, meaning one must now be evicted.
assert_eq!(state.alloc(Some(id3)), Some(SlotId(0)));
state.free(SlotId(0));
// pickup `id2` again, it should be affine.
assert_eq!(state.alloc(Some(id2)), Some(SlotId(1)));
// with only one warm slot available allocation for `id1` should pick a
// fresh slot
assert_eq!(state.alloc(Some(id1)), Some(SlotId(2)));
state.free(SlotId(1));
state.free(SlotId(2));
// ensure everything stays affine
assert_eq!(state.alloc(Some(id1)), Some(SlotId(2)));
assert_eq!(state.alloc(Some(id2)), Some(SlotId(1)));
assert_eq!(state.alloc(Some(id3)), Some(SlotId(0)));
state.free(SlotId(1));
state.free(SlotId(2));
state.free(SlotId(0));
// LRU is 1, so that should be picked
assert_eq!(state.alloc(Some(id_alloc.alloc())), Some(SlotId(1)));
// Pick another LRU entry, this time 2
assert_eq!(state.alloc(Some(id_alloc.alloc())), Some(SlotId(2)));
// This should preserve slot `0` and pick up something new
assert_eq!(state.alloc(Some(id_alloc.alloc())), Some(SlotId(3)));
state.free(SlotId(1));
state.free(SlotId(2));
state.free(SlotId(3));
// for good measure make sure id3 is still affine
assert_eq!(state.alloc(Some(id3)), Some(SlotId(0)));
}
}