seastar::fair_queue Class Reference

Detailed Description

Fair queuing class.

This is a fair queue, allowing multiple request producers to queue requests that will then be served proportionally to their classes' shares.

To each request, a weight can also be associated. A request of weight 1 will consume 1 share. Higher weights for a request will consume a proportionally higher amount of shares.

The user of this interface is expected to register multiple priority_class_data objects, which will each have a shares attribute.

Internally, each priority class may keep a separate queue of requests. Requests pertaining to a class can go through even if they are over its share limit, provided that the other classes have empty queues.

When the classes that lag behind start seeing requests, the fair queue will serve them first, until balance is restored. This balancing is expected to happen within a certain time window that obeys an exponential decay.

#include <seastar/core/fair_queue.hh>

Classes
struct	config
	Fair Queue configuration structure. More...

Public Types
using	class_id = unsigned int
using	capacity_t = fair_group::capacity_t
using	signed_capacity_t = std::make_signed< capacity_t >::type

Public Member Functions
	fair_queue (fair_group &shared, config cfg)
	fair_queue (fair_queue &&)
sstring	label () const noexcept
void	register_priority_class (class_id c, uint32_t shares)
void	unregister_priority_class (class_id c)
void	update_shares_for_class (class_id c, uint32_t new_shares)
size_t	waiters () const
size_t	requests_currently_executing () const
fair_queue_ticket	resources_currently_waiting () const
fair_queue_ticket	resources_currently_executing () const
void	queue (class_id c, fair_queue_entry &ent) noexcept
void	plug_class (class_id c) noexcept
void	unplug_class (class_id c) noexcept
void	notify_request_finished (fair_queue_ticket desc) noexcept
void	notify_request_cancelled (fair_queue_entry &ent) noexcept
void	dispatch_requests (std::function< void(fair_queue_entry &)> cb)
	Try to execute new requests if there is capacity left in the queue.
clock_type::time_point	next_pending_aio () const noexcept
std::vector< seastar::metrics::impl::metric_definition_impl >	metrics (class_id c)

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Class Documentation

seastar::fair_queue::config

struct seastar::fair_queue::config

Class Members
sstring	label
microseconds	tau

Constructor & Destructor Documentation

fair_queue()

seastar::fair_queue::fair_queue ( fair_group &  shared, config  cfg  )

explicit

Constructs a fair queue with configuration parameters cfg.

Parameters

cfg	an instance of the class config

Member Function Documentation

notify_request_finished()

void seastar::fair_queue::notify_request_finished ( fair_queue_ticket  desc )

noexcept

Notifies that ont request finished

Parameters

desc	an instance of fair_queue_ticket structure describing the request that just finished.

queue()

void seastar::fair_queue::queue ( class_id  c, fair_queue_entry &  ent  )

noexcept

Queue the entry ent through this class' fair_queue

The user of this interface is supposed to call notify_requests_finished when the request finishes executing - regardless of success or failure.

register_priority_class()

void seastar::fair_queue::register_priority_class	(	class_id	c,
		uint32_t	shares
	)

Registers a priority class against this fair queue.

Parameters

shares

how many shares to create this class with

requests_currently_executing()

size_t seastar::fair_queue::requests_currently_executing

(

)

const

Returns

the number of requests currently executing

resources_currently_executing()

fair_queue_ticket seastar::fair_queue::resources_currently_executing

(

)

const

Returns

the amount of resources (weight, size) currently executing

resources_currently_waiting()

fair_queue_ticket seastar::fair_queue::resources_currently_waiting

(

)

const

Returns

how much resources (weight, size) are currently queued for all classes.

unregister_priority_class()

void seastar::fair_queue::unregister_priority_class

(

class_id

c

)

Unregister a priority class.

It is illegal to unregister a priority class that still have pending requests.

waiters()

size_t seastar::fair_queue::waiters

(

)

const

Returns

how many waiters are currently queued for all classes.

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The documentation for this class was generated from the following file:

• seastar/core/fair_queue.hh