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Fix side effect of Remove operation.
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In the original ARC paper, there was no explanation of how Remove should be
handled when the cache is full. The Remove operation would have a side effect
on subsequent Put operations when the cache is not full, which is still
evicting items even though it's clear that there is no need to evict when
the cache is not full.

Now, code has been added to check if the cache is full to avoid performance
degradation.

Also fix c_, it should be max_count.
Also add unittest.

Signed-off-by: Xu Yifeng <[email protected]>
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@skypexu
skypexu committed Dec 13, 2023
1 parent ba61d87 commit 500dbfe
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Showing 2 changed files with 234 additions and 51 deletions.
151 changes: 100 additions & 51 deletions src/common/arc_cache.h
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Original file line number Diff line number Diff line change
Expand Up @@ -26,6 +26,7 @@
#include <assert.h>
#include <stdint.h>

#include <algorithm>
#include <list>
#include <memory>
#include <unordered_map>
Expand All @@ -35,9 +36,17 @@ template <typename K, typename V, typename KeyTraits = CacheTraits<K>,
typename ValueTraits = CacheTraits<V>>
class ARCCache : public LRUCacheInterface<K, V> {
public:
ARCCache(int64_t max_count,
struct ArcSizeInfo {
uint64_t b1, t1;
uint64_t b2, t2;

uint64_t BSize() const { return b1 + b2; }
uint64_t TSize() const { return t1 + t2; }
};

ARCCache(uint64_t max_count,
std::shared_ptr<CacheMetrics> cacheMetrics = nullptr)
: c_(max_count/2), p_(0), cacheMetrics_(cacheMetrics) {}
: c_(max_count), p_(0), cacheMetrics_(cacheMetrics) {}

void Put(const K& key, const V& value);
bool Put(const K& key, const V& value, V* eliminated);
Expand All @@ -49,8 +58,13 @@ class ARCCache : public LRUCacheInterface<K, V> {
bool GetLast(const V value, K* key);
bool GetLast(K* key, V* value);
bool GetLast(K* key, V* value, bool (*f)(const V& value));
uint64_t Capacity() const { return c_; }
ArcSizeInfo ArcSize() const;

private:
ARCCache(const ARCCache&) = delete;
void operator=(const ARCCache&) = delete;

struct BMapVal;
struct TMapVal;
struct BListVal;
Expand All @@ -69,54 +83,41 @@ class ARCCache : public LRUCacheInterface<K, V> {
struct BMapVal {
blist_iter list_iter;

BMapVal() {}
BMapVal(const BMapVal& o) { list_iter = o.list_iter; }
BMapVal& operator=(const BMapVal& o) { list_iter = o.list_iter; }
BMapVal() = default;
BMapVal(const BMapVal& o) = default;
BMapVal& operator=(const BMapVal& o) = default;
BMapVal(const blist_iter& iter) // NOLINT
: list_iter(iter) {}
};
struct TMapVal {
tlist_iter list_iter;

TMapVal() {}
TMapVal(const TMapVal& o) { list_iter = o.list_iter; }
TMapVal& operator=(const TMapVal& o) {
list_iter = o.list_iter;
return *this;
}
TMapVal() = default;
TMapVal(const TMapVal& o) = default;
TMapVal& operator=(const TMapVal& o) = default;
TMapVal(const tlist_iter& iter) // NOLINT
: list_iter(iter) {}
};
struct BListVal {
bmap_iter map_iter;

BListVal() {}
BListVal(const BListVal& o) { map_iter = o.map_iter; }
BListVal& operator=(const BListVal& o) {
map_iter = o.map_iter;
return *this;
}
BListVal() = default;
BListVal(const BListVal& o) = default;
BListVal& operator=(const BListVal& o) = default;
BListVal(const bmap_iter& iter) // NOLINT
:map_iter(iter) {}
};
struct TListVal {
tmap_iter map_iter;
V value;

TListVal() {}
TListVal(const TListVal& o) {
map_iter = o.map_iter;
value = o.value;
}
TListVal() = default;
TListVal(const TListVal& o) = default;
TListVal(const tmap_iter& iter, const V& v)
: map_iter(iter), value(v) {}
TListVal(const V& v) // NOLINT
: value(v) {}
TListVal& operator=(const TListVal& o) {
map_iter = o.map_iter;
value = o.value;
return *this;
}
TListVal& operator=(const TListVal& o) = default;
};

struct B {
Expand Down Expand Up @@ -158,7 +159,7 @@ class ARCCache : public LRUCacheInterface<K, V> {
map.erase(map_iter);
}

int Count() const { return map.size(); }
uint64_t Count() const { return map.size(); }
};

struct T {
Expand Down Expand Up @@ -229,16 +230,15 @@ class ARCCache : public LRUCacheInterface<K, V> {
if (v != nullptr) list_iter->value = *v;
return;
}

list.splice(list.end(), list, map_iter->second.list_iter);
if (v != nullptr) {
list.push_back(*v);
} else {
list.push_back(map_iter->second.list_iter->value);
list.back().value = *v;
}
list.erase(map_iter->second.list_iter);
map_iter->second.list_iter = --list.end();
}

int Count() const { return map.size(); }
uint64_t Count() const { return map.size(); }

tmap_iter GetLRU() const { return list.begin()->map_iter; }
};
Expand All @@ -262,11 +262,33 @@ class ARCCache : public LRUCacheInterface<K, V> {
return true;
}

bool IsCacheFull() const {
return t1_.Count() + t2_.Count() == c_;
}

void IncreaseP(uint64_t delta) {
if (!IsCacheFull())
return;
if (delta > c_ - p_)
p_ = c_;
else
p_ += delta;
}

void DecreaseP(uint64_t delta) {
if (!IsCacheFull())
return;
if (delta > p_)
p_ = 0;
else
p_ -= delta;
}

bool Replace(const K& k, V* eliminated);

::curve::common::RWLock lock_;
int64_t c_;
int64_t p_;
mutable ::curve::common::RWLock lock_;
uint64_t c_;
uint64_t p_;
B b1_, b2_;
T t1_, t2_;
std::shared_ptr<CacheMetrics> cacheMetrics_;
Expand Down Expand Up @@ -315,21 +337,23 @@ bool ARCCache<K, V, KeyTraits, ValueTraits>::Put(const K& key, const V& value,
if (t1_.Find(key, &it)) {
t2_.Insert(key, value, nullptr);
t1_.Remove(std::move(it), nullptr);
if (cacheMetrics_ != nullptr) {
cacheMetrics_->OnCacheHit();
}
return false;
}
if (t2_.Find(key, &it)) {
t2_.Touch(it, &value, cacheMetrics_.get());
if (cacheMetrics_ != nullptr) {
cacheMetrics_->OnCacheHit();
}
return false;
}

bmap_iter b_it;
if (b1_.Find(key, &b_it)) {
if (b1_.Count() >= b2_.Count()) {
p_ += 1;
} else {
p_ += b2_.Count() / b1_.Count();
}
if (p_ > c_) p_ = c_;
uint64_t delta = std::min((uint64_t)1, b2_.Count() / b1_.Count());
IncreaseP(delta);

ret = Replace(key, eliminated);
b1_.Remove(std::move(b_it), cacheMetrics_.get());
Expand All @@ -338,20 +362,16 @@ bool ARCCache<K, V, KeyTraits, ValueTraits>::Put(const K& key, const V& value,
}

if (b2_.Find(key, &b_it)) {
if (b2_.Count() >= b1_.Count()) {
p_ -= 1;
} else {
p_ -= b1_.Count() / b2_.Count();
}
if (p_ < 0) p_ = 0;
uint64_t delta = std::max((uint64_t)1, b1_.Count() / b2_.Count());
DecreaseP(delta);

ret = Replace(key, eliminated);
b2_.Remove(std::move(b_it), cacheMetrics_.get());
t2_.Insert(key, value, cacheMetrics_.get());
return ret;
}

if (t1_.Count() + b1_.Count() == c_) {
if (IsCacheFull() && t1_.Count() + b1_.Count() == c_) {
if (t1_.Count() < c_) {
b1_.RemoveLRU(cacheMetrics_.get());
ret = Replace(key, eliminated);
Expand All @@ -361,7 +381,13 @@ bool ARCCache<K, V, KeyTraits, ValueTraits>::Put(const K& key, const V& value,
} else if (t1_.Count() + b1_.Count() < c_) {
auto total = t1_.Count() + b1_.Count() + t2_.Count() + b2_.Count();
if (total >= c_) {
if (total == 2 * c_) b2_.RemoveLRU(cacheMetrics_.get());
if (total == 2 * c_) {
if (b2_.Count() > 0) {
b2_.RemoveLRU(cacheMetrics_.get());
} else {
b1_.RemoveLRU(cacheMetrics_.get());
}
}
Replace(key, eliminated);
}
}
Expand All @@ -372,11 +398,16 @@ bool ARCCache<K, V, KeyTraits, ValueTraits>::Put(const K& key, const V& value,
template <typename K, typename V, typename KeyTraits, typename ValueTraits>
bool ARCCache<K, V, KeyTraits, ValueTraits>::Replace(const K& k,
V* eliminated) {
if (!IsCacheFull()) {
return false;
}
if (t1_.Count() != 0 &&
((t1_.Count() > p_) || (b2_.Find(k, nullptr) && t1_.Count() == p_))) {
return Move_T_B(&t1_, &b1_, eliminated);
} else {
} else if (t2_.Count() > 0) {
return Move_T_B(&t2_, &b2_, eliminated);
} else {
return Move_T_B(&t1_, &b1_, eliminated);
}
}

Expand All @@ -386,6 +417,16 @@ uint64_t ARCCache<K, V, KeyTraits, ValueTraits>::Size() {
return t1_.Count() + t2_.Count();
}

template <typename K, typename V, typename KeyTraits, typename ValueTraits>
typename ARCCache<K, V, KeyTraits, ValueTraits>::ArcSizeInfo
ARCCache<K, V, KeyTraits, ValueTraits>::ArcSize() const {
::curve::common::ReadLockGuard guard(lock_);

return {b1_.Count(), t1_.Count(),
b2_.Count(), t2_.Count()};
}

// This operation detach the key from cache
template <typename K, typename V, typename KeyTraits, typename ValueTraits>
void ARCCache<K, V, KeyTraits, ValueTraits>::Remove(const K& key) {
::curve::common::WriteLockGuard guard(lock_);
Expand All @@ -397,6 +438,14 @@ void ARCCache<K, V, KeyTraits, ValueTraits>::Remove(const K& key) {
return;
}
}
B* bs[]{&b1_, &b2_};
for (auto b : bs) {
bmap_iter it;
if (b->Find(key, &it)) {
b->Remove(std::move(it), cacheMetrics_.get());
return;
}
}
}

template <typename K, typename V, typename KeyTraits, typename ValueTraits>
Expand Down
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