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Add combined locking support for MMContainer
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through withEvictionIterator function.

Also, expose config option to enable and disable
combined locking.

withEvictionIterator is implemented as en extra function,
getEvictionIterator() is still there and it's behavior hasn't
changed.
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@igchor
igchor committed Jan 12, 2023
1 parent 227cac0 commit 3ad50fa
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Showing 16 changed files with 325 additions and 87 deletions.
2 changes: 1 addition & 1 deletion cachelib/allocator/CacheAllocator.h
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Original file line number Diff line number Diff line change
Expand Up @@ -1660,7 +1660,7 @@ class CacheAllocator : public CacheBase {
// @return An evicted item or nullptr if there is no suitable candidate.
Item* findEviction(PoolId pid, ClassId cid);

using EvictionIterator = typename MMContainer::Iterator;
using EvictionIterator = typename MMContainer::LockedIterator;

// Advance the current iterator and try to evict a regular item
//
Expand Down
41 changes: 17 additions & 24 deletions cachelib/allocator/MM2Q-inl.h
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Original file line number Diff line number Diff line change
Expand Up @@ -241,29 +241,21 @@ bool MM2Q::Container<T, HookPtr>::add(T& node) noexcept {
}

template <typename T, MM2Q::Hook<T> T::*HookPtr>
typename MM2Q::Container<T, HookPtr>::Iterator
typename MM2Q::Container<T, HookPtr>::LockedIterator
MM2Q::Container<T, HookPtr>::getEvictionIterator() const noexcept {
// we cannot use combined critical sections with folly::DistributedMutex here
// because the lock is held for the lifetime of the eviction iterator. In
// other words, the abstraction of the iterator just does not lend itself well
// to combinable critical sections as the user can hold the lock for an
// arbitrary amount of time outside a lambda-friendly piece of code (eg. they
// can return the iterator from functions, pass it to functions, etc)
//
// it would be theoretically possible to refactor this interface into
// something like the following to allow combining
//
// mm2q.withEvictionIterator([&](auto iterator) {
// // user code
// });
//
// at the time of writing it is unclear if the gains from combining are
// reasonable justification for the codemod required to achieve combinability
// as we don't expect this critical section to be the hotspot in user code.
// This is however subject to change at some time in the future as and when
// this assertion becomes false.
LockHolder l(*lruMutex_);
return Iterator{std::move(l), lru_.rbegin()};
return LockedIterator{std::move(l), lru_.rbegin()};
}

template <typename T, MM2Q::Hook<T> T::*HookPtr>
template <typename F>
void MM2Q::Container<T, HookPtr>::withEvictionIterator(F&& fun) {
if (config_.useCombinedLockForIterators) {
lruMutex_->lock_combine([this, &fun]() { fun(Iterator{lru_.rbegin()}); });
} else {
LockHolder lck{*lruMutex_};
fun(Iterator{lru_.rbegin()});
}
}

template <typename T, MM2Q::Hook<T> T::*HookPtr>
Expand Down Expand Up @@ -462,8 +454,9 @@ void MM2Q::Container<T, HookPtr>::reconfigureLocked(const Time& currTime) {

// Iterator Context Implementation
template <typename T, MM2Q::Hook<T> T::*HookPtr>
MM2Q::Container<T, HookPtr>::Iterator::Iterator(
LockHolder l, const typename LruList::Iterator& iter) noexcept
: LruList::Iterator(iter), l_(std::move(l)) {}
MM2Q::Container<T, HookPtr>::LockedIterator::LockedIterator(
LockHolder l, const Iterator& iter) noexcept
: Iterator(iter), l_(std::move(l)) {}

} // namespace cachelib
} // namespace facebook
77 changes: 66 additions & 11 deletions cachelib/allocator/MM2Q.h
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Original file line number Diff line number Diff line change
Expand Up @@ -214,6 +214,50 @@ class MM2Q {
size_t hotPercent,
size_t coldPercent,
uint32_t mmReconfigureInterval)
: Config(time,
ratio,
updateOnW,
updateOnR,
tryLockU,
rebalanceOnRecordAccs,
hotPercent,
coldPercent,
mmReconfigureInterval,
false) {}

// @param time the LRU refresh time.
// An item will be promoted only once in each
// lru refresh time depite the
// number of accesses it gets.
// @param ratio the LRU refresh ratio. The ratio times the
// oldest element's lifetime in warm queue
// would be the minimum value of LRU refresh
// time.
// @param udpateOnW whether to promote the item on write
// @param updateOnR whether to promote the item on read
// @param tryLockU whether to use a try lock when doing
// update.
// @param rebalanceOnRecordAccs whether to do rebalance on access. If set
// to false, rebalance only happens when
// items are added or removed to the queue.
// @param hotPercent percentage number for the size of the hot
// queue in the overall size.
// @param coldPercent percentage number for the size of the cold
// queue in the overall size.
// @param mmReconfigureInterval Time interval for recalculating lru
// refresh time according to the ratio.
// useCombinedLockForIterators Whether to use combined locking for
// withEvictionIterator
Config(uint32_t time,
double ratio,
bool updateOnW,
bool updateOnR,
bool tryLockU,
bool rebalanceOnRecordAccs,
size_t hotPercent,
size_t coldPercent,
uint32_t mmReconfigureInterval,
bool useCombinedLockForIterators)
: defaultLruRefreshTime(time),
lruRefreshRatio(ratio),
updateOnWrite(updateOnW),
Expand All @@ -223,7 +267,8 @@ class MM2Q {
hotSizePercent(hotPercent),
coldSizePercent(coldPercent),
mmReconfigureIntervalSecs(
std::chrono::seconds(mmReconfigureInterval)) {
std::chrono::seconds(mmReconfigureInterval)),
useCombinedLockForIterators(useCombinedLockForIterators) {
checkLruSizes();
}

Expand Down Expand Up @@ -306,6 +351,9 @@ class MM2Q {
// Minimum interval between reconfigurations. If 0, reconfigure is never
// called.
std::chrono::seconds mmReconfigureIntervalSecs{};

// Whether to use combined locking for withEvictionIterator.
bool useCombinedLockForIterators{false};
};

// The container object which can be used to keep track of objects of type
Expand Down Expand Up @@ -347,22 +395,24 @@ class MM2Q {
Container(const Container&) = delete;
Container& operator=(const Container&) = delete;

using Iterator = typename LruList::Iterator;

// context for iterating the MM container. At any given point of time,
// there can be only one iterator active since we need to lock the LRU for
// iteration. we can support multiple iterators at same time, by using a
// shared ptr in the context for the lock holder in the future.
class Iterator : public LruList::Iterator {
class LockedIterator : public Iterator {
public:
// noncopyable but movable.
Iterator(const Iterator&) = delete;
Iterator& operator=(const Iterator&) = delete;
LockedIterator(const LockedIterator&) = delete;
LockedIterator& operator=(const LockedIterator&) = delete;

Iterator(Iterator&&) noexcept = default;
LockedIterator(LockedIterator&&) noexcept = default;

// 1. Invalidate this iterator
// 2. Unlock
void destroy() {
LruList::Iterator::reset();
Iterator::reset();
if (l_.owns_lock()) {
l_.unlock();
}
Expand All @@ -373,15 +423,15 @@ class MM2Q {
if (!l_.owns_lock()) {
l_.lock();
}
LruList::Iterator::resetToBegin();
Iterator::resetToBegin();
}

private:
// private because it's easy to misuse and cause deadlock for MM2Q
Iterator& operator=(Iterator&&) noexcept = default;
LockedIterator& operator=(LockedIterator&&) noexcept = default;

// create an lru iterator with the lock being held.
Iterator(LockHolder l, const typename LruList::Iterator& iter) noexcept;
LockedIterator(LockHolder l, const Iterator& iter) noexcept;

// only the container can create iterators
friend Container<T, HookPtr>;
Expand Down Expand Up @@ -422,7 +472,7 @@ class MM2Q {

// same as the above but uses an iterator context. The iterator is updated
// on removal of the corresponding node to point to the next node. The
// iterator context holds the lock on the lru.
// iterator context is responsible for locking.
//
// iterator will be advanced to the next node after removing the node
//
Expand All @@ -445,7 +495,12 @@ class MM2Q {
// Obtain an iterator that start from the tail and can be used
// to search for evictions. This iterator holds a lock to this
// container and only one such iterator can exist at a time
Iterator getEvictionIterator() const noexcept;
LockedIterator getEvictionIterator() const noexcept;

// Execute provided function under container lock. Function gets
// Iterator passed as parameter.
template <typename F>
void withEvictionIterator(F&& f);

// get the current config as a copy
Config getConfig() const;
Expand Down
22 changes: 17 additions & 5 deletions cachelib/allocator/MMLru-inl.h
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Original file line number Diff line number Diff line change
Expand Up @@ -212,10 +212,21 @@ bool MMLru::Container<T, HookPtr>::add(T& node) noexcept {
}

template <typename T, MMLru::Hook<T> T::*HookPtr>
typename MMLru::Container<T, HookPtr>::Iterator
typename MMLru::Container<T, HookPtr>::LockedIterator
MMLru::Container<T, HookPtr>::getEvictionIterator() const noexcept {
LockHolder l(*lruMutex_);
return Iterator{std::move(l), lru_.rbegin()};
return LockedIterator{std::move(l), lru_.rbegin()};
}

template <typename T, MMLru::Hook<T> T::*HookPtr>
template <typename F>
void MMLru::Container<T, HookPtr>::withEvictionIterator(F&& fun) {
if (config_.useCombinedLockForIterators) {
lruMutex_->lock_combine([this, &fun]() { fun(Iterator{lru_.rbegin()}); });
} else {
LockHolder lck{*lruMutex_};
fun(Iterator{lru_.rbegin()});
}
}

template <typename T, MMLru::Hook<T> T::*HookPtr>
Expand Down Expand Up @@ -360,8 +371,9 @@ void MMLru::Container<T, HookPtr>::reconfigureLocked(const Time& currTime) {

// Iterator Context Implementation
template <typename T, MMLru::Hook<T> T::*HookPtr>
MMLru::Container<T, HookPtr>::Iterator::Iterator(
LockHolder l, const typename LruList::Iterator& iter) noexcept
: LruList::Iterator(iter), l_(std::move(l)) {}
MMLru::Container<T, HookPtr>::LockedIterator::LockedIterator(
LockHolder l, const Iterator& iter) noexcept
: Iterator(iter), l_(std::move(l)) {}

} // namespace cachelib
} // namespace facebook
68 changes: 56 additions & 12 deletions cachelib/allocator/MMLru.h
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Original file line number Diff line number Diff line change
Expand Up @@ -145,14 +145,49 @@ class MMLru {
bool tryLockU,
uint8_t ipSpec,
uint32_t mmReconfigureInterval)
: Config(time,
ratio,
updateOnW,
updateOnR,
tryLockU,
ipSpec,
mmReconfigureInterval,
false) {}

// @param time the LRU refresh time in seconds.
// An item will be promoted only once in each lru refresh
// time depite the number of accesses it gets.
// @param ratio the lru refresh ratio. The ratio times the
// oldest element's lifetime in warm queue
// would be the minimum value of LRU refresh time.
// @param udpateOnW whether to promote the item on write
// @param updateOnR whether to promote the item on read
// @param tryLockU whether to use a try lock when doing update.
// @param ipSpec insertion point spec, which is the inverse power of
// length from the end of the queue. For example, value 1
// means the insertion point is 1/2 from the end of LRU;
// value 2 means 1/4 from the end of LRU.
// @param mmReconfigureInterval Time interval for recalculating lru
// refresh time according to the ratio.
// useCombinedLockForIterators Whether to use combined locking for
// withEvictionIterator
Config(uint32_t time,
double ratio,
bool updateOnW,
bool updateOnR,
bool tryLockU,
uint8_t ipSpec,
uint32_t mmReconfigureInterval,
bool useCombinedLockForIterators)
: defaultLruRefreshTime(time),
lruRefreshRatio(ratio),
updateOnWrite(updateOnW),
updateOnRead(updateOnR),
tryLockUpdate(tryLockU),
lruInsertionPointSpec(ipSpec),
mmReconfigureIntervalSecs(
std::chrono::seconds(mmReconfigureInterval)) {}
std::chrono::seconds(mmReconfigureInterval)),
useCombinedLockForIterators(useCombinedLockForIterators) {}

Config() = default;
Config(const Config& rhs) = default;
Expand Down Expand Up @@ -198,6 +233,9 @@ class MMLru {
// Minimum interval between reconfigurations. If 0, reconfigure is never
// called.
std::chrono::seconds mmReconfigureIntervalSecs{};

// Whether to use combined locking for withEvictionIterator.
bool useCombinedLockForIterators{false};
};

// The container object which can be used to keep track of objects of type
Expand Down Expand Up @@ -234,22 +272,24 @@ class MMLru {
Container(const Container&) = delete;
Container& operator=(const Container&) = delete;

using Iterator = typename LruList::Iterator;

// context for iterating the MM container. At any given point of time,
// there can be only one iterator active since we need to lock the LRU for
// iteration. we can support multiple iterators at same time, by using a
// shared ptr in the context for the lock holder in the future.
class Iterator : public LruList::Iterator {
class LockedIterator : public Iterator {
public:
// noncopyable but movable.
Iterator(const Iterator&) = delete;
Iterator& operator=(const Iterator&) = delete;
LockedIterator(const LockedIterator&) = delete;
LockedIterator& operator=(const LockedIterator&) = delete;

Iterator(Iterator&&) noexcept = default;
LockedIterator(LockedIterator&&) noexcept = default;

// 1. Invalidate this iterator
// 2. Unlock
void destroy() {
LruList::Iterator::reset();
Iterator::reset();
if (l_.owns_lock()) {
l_.unlock();
}
Expand All @@ -260,15 +300,15 @@ class MMLru {
if (!l_.owns_lock()) {
l_.lock();
}
LruList::Iterator::resetToBegin();
Iterator::resetToBegin();
}

private:
// private because it's easy to misuse and cause deadlock for MMLru
Iterator& operator=(Iterator&&) noexcept = default;
LockedIterator& operator=(LockedIterator&&) noexcept = default;

// create an lru iterator with the lock being held.
Iterator(LockHolder l, const typename LruList::Iterator& iter) noexcept;
LockedIterator(LockHolder l, const Iterator& iter) noexcept;

// only the container can create iterators
friend Container<T, HookPtr>;
Expand Down Expand Up @@ -307,10 +347,9 @@ class MMLru {
// state of node is unchanged.
bool remove(T& node) noexcept;

using Iterator = Iterator;
// same as the above but uses an iterator context. The iterator is updated
// on removal of the corresponding node to point to the next node. The
// iterator context holds the lock on the lru.
// iterator context is responsible for locking.
//
// iterator will be advanced to the next node after removing the node
//
Expand All @@ -330,7 +369,12 @@ class MMLru {
// Obtain an iterator that start from the tail and can be used
// to search for evictions. This iterator holds a lock to this
// container and only one such iterator can exist at a time
Iterator getEvictionIterator() const noexcept;
LockedIterator getEvictionIterator() const noexcept;

// Execute provided function under container lock. Function gets
// iterator passed as parameter.
template <typename F>
void withEvictionIterator(F&& f);

// get copy of current config
Config getConfig() const;
Expand Down
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