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// Copyright Yahoo. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
#pragma once
#include "time.h"
#include <functional>
#include <memory>
#include <optional>
#include <limits.h>
namespace vespalib {
/**
* Operation throttler that is intended to provide global throttling of
* async operations across multiple threads. A throttler wraps a logical
* max pending window size of in-flight operations. Depending on the
* throttler implementation, the window size may expand and shrink dynamically.
* Exactly how and when this happens is unspecified.
*
* Offers both polling and (timed, non-timed) blocking calls for acquiring
* a throttle token. If the returned token is valid, the caller may proceed
* to invoke the asynchronous operation.
*
* The window slot taken up by a valid throttle token is implicitly freed up
* when the token is destroyed.
*
* All operations on the throttler are thread safe.
*/
class SharedOperationThrottler {
protected:
struct TokenCtorTag {}; // Make available to subclasses for token construction.
public:
class Token {
SharedOperationThrottler* _throttler;
public:
constexpr Token(SharedOperationThrottler* throttler, TokenCtorTag) noexcept : _throttler(throttler) {}
constexpr Token() noexcept : _throttler(nullptr) {}
constexpr Token(Token&& rhs) noexcept
: _throttler(rhs._throttler)
{
rhs._throttler = nullptr;
}
Token& operator=(Token&& rhs) noexcept;
~Token();
Token(const Token&) = delete;
Token& operator=(const Token&) = delete;
[[nodiscard]] constexpr bool valid() const noexcept { return (_throttler != nullptr); }
void reset() noexcept;
};
virtual ~SharedOperationThrottler() = default;
// Acquire a valid throttling token, uninterruptedly blocking until one can be obtained.
[[nodiscard]] virtual Token blocking_acquire_one() noexcept = 0;
// Attempt to acquire a valid throttling token, waiting up to `timeout` for one to be
// available. If the deadline is reached without any tokens becoming available, an
// invalid token will be returned.
[[nodiscard]] virtual Token blocking_acquire_one(vespalib::steady_time deadline) noexcept = 0;
// Attempt to acquire a valid throttling token if one is immediately available.
// An invalid token will be returned if none is available. Never blocks (other than
// when contending for the internal throttler mutex).
[[nodiscard]] virtual Token try_acquire_one() noexcept = 0;
// May return 0, in which case the window size is unlimited.
[[nodiscard]] virtual uint32_t current_window_size() const noexcept = 0;
[[nodiscard]] virtual uint32_t current_active_token_count() const noexcept = 0;
[[nodiscard]] virtual uint32_t waiting_threads() const noexcept = 0;
struct DynamicThrottleParams {
uint32_t window_size_increment = 20;
uint32_t min_window_size = 20;
uint32_t max_window_size = INT_MAX; // signed max to be 1-1 compatible with Java defaults
double resize_rate = 3.0;
double window_size_decrement_factor = 1.2;
double window_size_backoff = 0.95;
bool operator==(const DynamicThrottleParams&) const noexcept = default;
bool operator!=(const DynamicThrottleParams&) const noexcept = default;
};
// No-op if underlying throttler does not use a dynamic policy, or if the supplied
// parameters are equal to the current configuration.
// FIXME leaky abstraction alert!
virtual void reconfigure_dynamic_throttling(const DynamicThrottleParams& params) noexcept = 0;
// Creates a throttler that does exactly zero throttling (but also has zero overhead and locking)
static std::unique_ptr<SharedOperationThrottler> make_unlimited_throttler();
// Creates a throttler that uses a DynamicThrottlePolicy under the hood
static std::unique_ptr<SharedOperationThrottler> make_dynamic_throttler(const DynamicThrottleParams& params);
static std::unique_ptr<SharedOperationThrottler> make_dynamic_throttler(const DynamicThrottleParams& params,
std::function<steady_time()> time_provider);
private:
// Exclusively called from a valid Token. Thread safe.
virtual void release_one() noexcept = 0;
};
}
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