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// Copyright 2012 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#include <stddef.h>
#include <vector>
#include "net/base/net_export.h"
#include "net/base/priority_queue.h"
namespace net {
// A priority-based dispatcher of jobs. Dispatch order is by priority (highest
// first) and then FIFO. The dispatcher enforces limits on the number of running
// jobs. It never revokes a job once started. The job must call OnJobFinished
// once it finishes in order to dispatch further jobs.
// This class is NOT thread-safe which is enforced by the underlying
// non-thread-safe PriorityQueue. All operations are O(p) time for p priority
// levels. It is safe to execute any method, including destructor, from within
// Job::Start.
class NET_EXPORT_PRIVATE PrioritizedDispatcher {
class Job;
typedef PriorityQueue<Job*>::Priority Priority;
// Describes the limits for the number of jobs started by the dispatcher.
// For example, |total_jobs| = 30 and |reserved_slots| = { 0, 5, 10, 5 } allow
// for at most 30 running jobs in total. Jobs at priority 0 can't use slots
// reserved for higher priorities, so they are limited to 10.
// If there are already 24 jobs running, then only 6 more jobs can start. No
// jobs at priority 1 or below can start. After one more job starts, no jobs
// at priority 2 or below can start, since the remaining 5 slots are reserved
// for priority 3 or above.
struct NET_EXPORT_PRIVATE Limits {
Limits(Priority num_priorities, size_t total_jobs);
Limits(const Limits& other);
// Total allowed running jobs.
size_t total_jobs;
// Number of slots reserved for each priority and higher.
// Sum of |reserved_slots| must be no greater than |total_jobs|.
std::vector<size_t> reserved_slots;
// An interface to the job dispatched by PrioritizedDispatcher. The dispatcher
// does not own the Job but expects it to live as long as the Job is queued.
// Use Cancel to remove Job from queue before it is dispatched. The Job can be
// deleted after it is dispatched or canceled, or the dispatcher is destroyed.
class Job {
// Note: PrioritizedDispatcher will never delete a Job.
virtual ~Job() = default;
// Called when the dispatcher starts the job. Once the job finishes, it must
// call OnJobFinished.
virtual void Start() = 0;
// A handle to the enqueued job. The handle becomes invalid when the job is
// canceled, updated, or started.
typedef PriorityQueue<Job*>::Pointer Handle;
// Creates a dispatcher enforcing |limits| on number of running jobs.
explicit PrioritizedDispatcher(const Limits& limits);
PrioritizedDispatcher(const PrioritizedDispatcher&) = delete;
PrioritizedDispatcher& operator=(const PrioritizedDispatcher&) = delete;
size_t num_running_jobs() const { return num_running_jobs_; }
size_t num_queued_jobs() const { return queue_.size(); }
size_t num_priorities() const { return max_running_jobs_.size(); }
// Adds |job| with |priority| to the dispatcher. If limits permit, |job| is
// started immediately. Returns handle to the job or null-handle if the job is
// started. The dispatcher does not own |job|, but |job| must live as long as
// it is queued in the dispatcher.
Handle Add(Job* job, Priority priority);
// Just like Add, except that it adds Job at the font of queue of jobs with
// priorities of |priority|.
Handle AddAtHead(Job* job, Priority priority);
// Removes the job with |handle| from the queue. Invalidates |handle|.
// Note: a Handle is valid iff the job is in the queue, i.e. has not Started.
void Cancel(const Handle& handle);
// Cancels and returns the oldest-lowest-priority Job invalidating any
// handles to it. Returns NULL if the queue is empty.
Job* EvictOldestLowest();
// Moves the queued job with |handle| to the end of all values with priority
// |priority| and returns the updated handle, or null-handle if it starts the
// job. Invalidates |handle|. No-op if priority did not change.
Handle ChangePriority(const Handle& handle, Priority priority);
// Notifies the dispatcher that a running job has finished. Could start a job.
void OnJobFinished();
// Retrieves the Limits that |this| is currently using. This may not exactly
// match the Limits this was created with. In particular, the number of slots
// reserved for the lowest priority will always be 0, even if it was non-zero
// in the Limits passed to the constructor or to SetLimits.
Limits GetLimits() const;
// Updates |max_running_jobs_| to match |limits|. Starts jobs if new limit
// allows. Does not stop jobs if the new limits are lower than the old ones.
void SetLimits(const Limits& limits);
// Set the limits to zero for all priorities, allowing no new jobs to start.
void SetLimitsToZero();
// Attempts to dispatch the job with |handle| at priority |priority| (might be
// different than |handle.priority()|. Returns true if successful. If so
// the |handle| becomes invalid.
bool MaybeDispatchJob(const Handle& handle, Priority priority);
// Attempts to dispatch the next highest priority job in the queue. Returns
// true if successful, and all handles to that job become invalid.
bool MaybeDispatchNextJob();
// Queue for jobs that need to wait for a spare slot.
PriorityQueue<Job*> queue_;
// Maximum total number of running jobs allowed after a job at a particular
// priority is started. If a greater or equal number of jobs are running, then
// another job cannot be started.
std::vector<size_t> max_running_jobs_;
// Total number of running jobs.
size_t num_running_jobs_ = 0;
} // namespace net