universal packet scheduling - peopleradhika/ups-hotnets.pdf · 2016. 5. 18. · universal packet...
TRANSCRIPT
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Universal Packet Scheduling
Radhika Mittal, Rachit Agarwal, Sylvia Ratnasamy, Scott Shenker
UC Berkeley
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Packet Scheduling
• Active research literature with many – Algorithms • FIFO, DRR, virtual clocks, priorities…
– Goals • fairness, small packet delay, small FCT…
– Contexts • WAN, datacenters, cellular…
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Is there a universal packet scheduling algorithm?
We are asking a new question…..
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UPS: Universal Packet Scheduling Algorithm
A single scheduling algorithm that can imitate the network-wide output
produced by any other algorithm.
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How can a single algorithm imitate all
others?
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Network-wide Model
Input Traffic
INGRESS
CORE NETWORK
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Scheduling Algorithm
Network-wide Model
Input Traffic
INGRESS
CORE NETWORK
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Network-wide Model
INGRESS
Input Traffic
(Optional) Header
Initialization
Scheduling Algorithm
Output Traffic
CORE NETWORK
EGRESS
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Network-wide Model
INGRESS
Input Traffic
(Optional) Header
Initialization
Scheduling Algorithm
Output Traffic
CORE NETWORK
EGRESS
Output Traffic tied to Scheduling Algorithm
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Network-wide Model
INGRESS
Input Traffic
(Optional) Header
Initialization
Priority Scheduling
Output Traffic
CORE NETWORK
EGRESS
Goal: Minimize Mean FCT
Priority Value
Flow Size
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Network-wide Model
INGRESS
Input Traffic
(Optional) Header
Initialization FQ Output Traffic
CORE NETWORK
EGRESS
Goal: Fairness
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Network-wide Model
INGRESS
Input Traffic
(Optional) Header
Initialization WFQ Output Traffic
CORE NETWORK
EGRESS
Goal: Weighted Fairness
Flow Weights
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Network-wide Model
* Uses packet header state to make scheduling decisions
INGRESS
Input Traffic
Header
Initialization
Scheduling Algorithm* Output Traffic
CORE NETWORK
EGRESS
Output Traffic tied to Header Initialization
Header Initialization
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Network-wide Model
INGRESS
Input Traffic
Smart Header
Initialization UPS? Output Traffic
CORE NETWORK
EGRESS
Header Initialization
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Universality vs
Programmability
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Programmability: • Single hardware for multiple algorithms
to meet multiple goals
Universality:
• Single algorithm with varying header initializations for multiple goals
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How do we formally define and evaluate
a UPS?
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Defining a UPS
Theoretical Viewpoint: Can it replay a given schedule?
Practical Viewpoint:
Can it achieve a given objective?
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Theoretical Viewpoint Can it replay a given schedule?
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Original Schedule
Input Traffic
(Optional) Header
Initialization
INGRESS CORE NETWORK
State Oracle
* *
* *
Arbitrary Scheduling Algorithm
Output Timings
o(p) for a packet p EGRESS
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Replaying the Schedule, given o(p)
Input Traffic
Header Initialization (using o(p))
INGRESS CORE NETWORK
Output Timings
o’(p) for a packet p EGRESS
For every packet p, o’(p) ≤ o(p)
UPS Header Initialization
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Pragmatic Constraints on a UPS
Input Traffic
Output Timings Header
Initialization (using o(p)) o’(p) for a packet p
Obliviousness: For initializing p’s header, use only o(p) and path(p)
INGRESS CORE NETWORK EGRESS
UPS Header Initialization
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Pragmatic Constraints on a UPS
Input Traffic
Output Timings Header
Initialization (using o(p)) o’(p) for a packet p
Obliviousness: For initializing p’s header, use only o(p) and path(p)
INGRESS CORE NETWORK EGRESS
UPS Header Initialization
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Pragmatic Constraints on a UPS
Input Traffic
Output Timings
o’(p) for a packet p
Obliviousness: For initializing p’s header, use only o(p) and path(p)
INGRESS CORE NETWORK EGRESS
UPS Header
Initialization (using o(p))
Header Initialization
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Pragmatic Constraints on a UPS
Input Traffic
Output Timings
o’(p) for a packet p
Obliviousness: For initializing p’s header, use only o(p) and path(p)
INGRESS CORE NETWORK EGRESS
UPS Header
Initialization (using o(p))
Header Initialization
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Pragmatic Constraints on a UPS
Input Traffic
Output Timings
o’(p) for a packet p
Obliviousness: For initializing p’s header, use only o(p) and path(p)
Limited State: Scheduling can use only header state and static information
INGRESS CORE NETWORK EGRESS
UPS Header
Initialization (using o(p))
Header Initialization
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Pragmatic Constraints on a UPS
Input Traffic
Output Timings
o’(p) for a packet p
Obliviousness: For initializing p’s header, use only o(p) and path(p)
Limited State: Scheduling can use only header state and static information
INGRESS CORE NETWORK EGRESS
UPS Header
Initialization (using o(p))
Header Initialization
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We call this Blackbox Initialization
Input Traffic
Output Timings
o’(p) for a packet p
Limited State: Scheduling can use only header state and static information
INGRESS CORE NETWORK EGRESS
Obliviousness: For initializing p’s header, use only o(p) and path(p)
UPS Header
Initialization (using o(p))
Header Initialization
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Basic Existence and Non-existence Results
There exists a UPS under Omniscient Initialization
when scheduling time at every hop is known
No UPS exists under Blackbox Initialization
when only the final output time is known
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How close can we get to a UPS?
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Key Result: Depends on congestion points
No. of Congestion Points per Packet General
1 ✓
2 ✓
3 ✗
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Can we achieve this upper bound?
Yes, LSTF!
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Can we achieve this upper bound?
Yes, LSTF!
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Least Slack Time First
• Packet header initialized with a slack value – slack = maximum tolerable queuing delay
• At the routers – Schedule packet with least slack time first
– Update the slack by subtracting the wait time
Alternate EDF-based implementation: Static deadlines in packet headers with
additional state in the routers.
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Key Results
No. of Congestion Points per Packet General LSTF
1 ✓ ✓
2 ✓ ✓
3 ✗
✗
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Not all algorithms achieve upper bound
No. of Congestion Points per Packet General LSTF Priorities
1 ✓ ✓ ✓
2 ✓ ✓ ✗
3 ✗
✗
✗
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How well does LSTF perform empirically?
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Empirically, LSTF is (almost) universal
• ns-2 simulation results on realistic network settings - Less than 3% packets missed their output times
- Less than 0.1% packets are late by more than one transmission time
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Summarizing the theoretical viewpoint
• Evaluate the ability to replay a schedule, given its final output times
• Analytical Results: - No UPS exists - LSTF comes as close to a UPS as possible
• Empirical Results: LSTF is almost universal!
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Practical Viewpoint Can it achieve a given objective?
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Achieving various network objectives
• Slack assignment based on heuristics • Comparison with state-of-the-art • Three objective functions - Tail packet delays - Mean Flow Completion Time - Fairness
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Tail Packet Delays
Slack Assignment: Same slack for all packets State-of-the-art: FIFO, FIFO+
Smaller Tail Packet Delays with LSTF (FIFO+)
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Mean Flow Completion Time
Slack Assignment: Proportional to flow size State-of-the-art: SJF, SRPT
Mean FCT with LSTF comparable to SRPT and SJF
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Fairness
Slack Assignment: Based on Virtual Clocks
State-of-the-art: Fair Queuing
Result: Eventual convergence to fairness
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Results Summary
• Theoretical results show that - There is no UPS under blackbox initialization - LSTF comes as close to a UPS as possible - Empirically, LSTF is very close
• LSTF can be used in practice to achieve a variety of network-wide objectives
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Implication
• Less need for many different scheduling algorithms.
• Can just use LSTF, with varying initializations.
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There are still a bunch of
open questions!
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Open Questions • What is the least amount of information
needed to achieve universality?
• Are there tractable bounds for the degree of lateness with LSTF?
• What is the class of objectives that can be
achieved with LSTF in practice?
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Summary
• Theoretical results show that - There is no UPS under blackbox initialization - LSTF comes as close to a UPS as possible - Empirically, LSTF is very close
• LSTF can be used in practice to achieve a variety of network-wide objectives