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// Copyright Vespa.ai. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
/**
* \class MetricValueSet
* \ingroup metrics
*
* \brief Utility for doing lockless metric updates and reads.
*
* We don't want to use regular locking while updating metrics due to overhead.
* We use this class to make metric updates as safe as possible without
* requiring locks.
*
* It keeps the set of values a metric wants to set. Thus it is templated on
* the class keeping the values. All that is required of this class is that it
* has an empty constructor and a copy constructor.
*
* The locking works, by keeping a set of values, with an active pointer into
* the value vector. Assuming only one thread calls setValues(), it can update
* the active pointer safely. We assume updating the active pointer is a
* non-interruptable operations, such that other threads will see either the new
* or the old value correctly. This should be the case on our platforms.
*
* Due to the reset functionality, it is possible to miss out on a metrics
* added during the reset, but this is very unlikely. For that to happen, when
* someone sets the reset flag, the writer thread must be in setValues(),
* having already passed the check for the reset flag, but not finished setting
* the values yet.
*/
#pragma once
#include <vector>
#include <atomic>
#include <array>
#include <vespa/vespalib/stllike/string.h>
#include <vespa/vespalib/util/jsonstream.h>
namespace metrics {
struct MetricValueClass {
using UP = std::unique_ptr<MetricValueClass>;
using stringref = vespalib::stringref;
virtual ~MetricValueClass() {}
virtual double getDoubleValue(stringref id) const = 0;
virtual uint64_t getLongValue(stringref id) const = 0;
virtual void output(const std::string& id, std::ostream&) const = 0;
virtual void output(const std::string& id, vespalib::JsonStream&) const = 0;
std::string toString(const std::string& id);
};
template<typename ValueClass>
class MetricValueSet {
using AtomicValues = typename ValueClass::AtomicImpl;
std::array<AtomicValues, 3> _values;
std::atomic<uint32_t> _activeValueIndex;
std::atomic<uint32_t> _flags;
enum Flag { RESET = 1 };
bool isReset() const { return hasFlag(RESET); }
public:
MetricValueSet() noexcept;
MetricValueSet(const MetricValueSet&) noexcept;
MetricValueSet& operator=(const MetricValueSet& other) noexcept;
/** Get the current values. */
ValueClass getValues() const;
/**
* Set the current values for the metric. This function should not be
* called in parallel. Only call it from a single thread or use external
* locking. If it returns false, it means the metric have just been reset.
* In which case, redo getValues(), apply the update again, and call
* setValues() again.
*/
bool setValues(const ValueClass& values);
void reset() {
setFlag(RESET);
}
std::string toString();
uint32_t size() const { return _values.size(); }
bool hasFlag(uint32_t flags) const {
return ((_flags.load(std::memory_order_relaxed) & flags) != 0);
}
void setFlag(uint32_t flags) {
_flags.store(_flags.load(std::memory_order_relaxed) | flags,
std::memory_order_relaxed);
}
void removeFlag(uint32_t flags) {
_flags.store(_flags.load(std::memory_order_relaxed) & ~flags,
std::memory_order_relaxed);
}
};
} // metrics
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