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// Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved.
// This source code is licensed under both the GPLv2 (found in the
// COPYING file in the root directory) and Apache 2.0 License
// (found in the LICENSE.Apache file in the root directory).
#pragma once
#include <array>
#include <cstdint>
#include <memory>
#include <mutex>
#include "cache/cache_helpers.h"
#include "cache/cache_key.h"
#include "port/lang.h"
#include "rocksdb/cache.h"
#include "rocksdb/status.h"
#include "rocksdb/system_clock.h"
#include "test_util/sync_point.h"
#include "util/coding_lean.h"
namespace ROCKSDB_NAMESPACE {
// A generic helper object for gathering stats about cache entries by
// iterating over them with ApplyToAllEntries. This class essentially
// solves the problem of slowing down a Cache with too many stats
// collectors that could be sharing stat results, such as from multiple
// column families or multiple DBs sharing a Cache. We employ a few
// mitigations:
// * Only one collector for a particular kind of Stats is alive
// for each Cache. This is guaranteed using the Cache itself to hold
// the collector.
// * A mutex ensures only one thread is gathering stats for this
// collector.
// * The most recent gathered stats are saved and simply copied to
// satisfy requests within a time window (default: 3 minutes) of
// completion of the most recent stat gathering.
//
// Template parameter Stats must be copyable and trivially constructable,
// as well as...
// concept Stats {
// // Notification before applying callback to all entries
// void BeginCollection(Cache*, SystemClock*, uint64_t start_time_micros);
// // Get the callback to apply to all entries. `callback`
// // type must be compatible with Cache::ApplyToAllEntries
// callback GetEntryCallback();
// // Notification after applying callback to all entries
// void EndCollection(Cache*, SystemClock*, uint64_t end_time_micros);
// // Notification that a collection was skipped because of
// // sufficiently recent saved results.
// void SkippedCollection();
// }
template <class Stats>
class CacheEntryStatsCollector {
public:
// Gather and save stats if saved stats are too old. (Use GetStats() to
// read saved stats.)
//
// Maximum allowed age for a "hit" on saved results is determined by the
// two interval parameters. Both set to 0 forces a re-scan. For example
// with min_interval_seconds=300 and min_interval_factor=100, if the last
// scan took 10s, we would only rescan ("miss") if the age in seconds of
// the saved results is > max(300, 100*10).
// Justification: scans can vary wildly in duration, e.g. from 0.02 sec
// to as much as 20 seconds, so we want to be able to cap the absolute
// and relative frequency of scans.
void CollectStats(int min_interval_seconds, int min_interval_factor) {
// Waits for any pending reader or writer (collector)
std::lock_guard<std::mutex> lock(working_mutex_);
uint64_t max_age_micros =
static_cast<uint64_t>(std::max(min_interval_seconds, 0)) * 1000000U;
if (last_end_time_micros_ > last_start_time_micros_ &&
min_interval_factor > 0) {
max_age_micros = std::max(
max_age_micros, min_interval_factor * (last_end_time_micros_ -
last_start_time_micros_));
}
uint64_t start_time_micros = clock_->NowMicros();
if ((start_time_micros - last_end_time_micros_) > max_age_micros) {
last_start_time_micros_ = start_time_micros;
working_stats_.BeginCollection(cache_, clock_, start_time_micros);
cache_->ApplyToAllEntries(working_stats_.GetEntryCallback(), {});
TEST_SYNC_POINT_CALLBACK(
"CacheEntryStatsCollector::GetStats:AfterApplyToAllEntries", nullptr);
uint64_t end_time_micros = clock_->NowMicros();
last_end_time_micros_ = end_time_micros;
working_stats_.EndCollection(cache_, clock_, end_time_micros);
} else {
working_stats_.SkippedCollection();
}
// Save so that we don't need to wait for an outstanding collection in
// order to make of copy of the last saved stats
std::lock_guard<std::mutex> lock2(saved_mutex_);
saved_stats_ = working_stats_;
}
// Gets saved stats, regardless of age
void GetStats(Stats *stats) {
std::lock_guard<std::mutex> lock(saved_mutex_);
*stats = saved_stats_;
}
Cache *GetCache() const { return cache_; }
// Gets or creates a shared instance of CacheEntryStatsCollector in the
// cache itself, and saves into `ptr`. This shared_ptr will hold the
// entry in cache until all refs are destroyed.
static Status GetShared(Cache *cache, SystemClock *clock,
std::shared_ptr<CacheEntryStatsCollector> *ptr) {
const Slice &cache_key = GetCacheKey();
Cache::Handle *h = cache->Lookup(cache_key);
if (h == nullptr) {
// Not yet in cache, but Cache doesn't provide a built-in way to
// avoid racing insert. So we double-check under a shared mutex,
// inspired by TableCache.
STATIC_AVOID_DESTRUCTION(std::mutex, static_mutex);
std::lock_guard<std::mutex> lock(static_mutex);
h = cache->Lookup(cache_key);
if (h == nullptr) {
auto new_ptr = new CacheEntryStatsCollector(cache, clock);
// TODO: non-zero charge causes some tests that count block cache
// usage to go flaky. Fix the problem somehow so we can use an
// accurate charge.
size_t charge = 0;
Status s = cache->Insert(cache_key, new_ptr, charge, Deleter, &h,
Cache::Priority::HIGH);
if (!s.ok()) {
assert(h == nullptr);
delete new_ptr;
return s;
}
}
}
// If we reach here, shared entry is in cache with handle `h`.
assert(cache->GetDeleter(h) == Deleter);
// Build an aliasing shared_ptr that keeps `ptr` in cache while there
// are references.
*ptr = MakeSharedCacheHandleGuard<CacheEntryStatsCollector>(cache, h);
return Status::OK();
}
private:
explicit CacheEntryStatsCollector(Cache *cache, SystemClock *clock)
: saved_stats_(),
working_stats_(),
last_start_time_micros_(0),
last_end_time_micros_(/*pessimistic*/ 10000000),
cache_(cache),
clock_(clock) {}
static void Deleter(const Slice &, void *value) {
delete static_cast<CacheEntryStatsCollector *>(value);
}
static const Slice &GetCacheKey() {
// For each template instantiation
static CacheKey ckey = CacheKey::CreateUniqueForProcessLifetime();
static Slice ckey_slice = ckey.AsSlice();
return ckey_slice;
}
std::mutex saved_mutex_;
Stats saved_stats_;
std::mutex working_mutex_;
Stats working_stats_;
uint64_t last_start_time_micros_;
uint64_t last_end_time_micros_;
Cache *const cache_;
SystemClock *const clock_;
};
} // namespace ROCKSDB_NAMESPACE
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