CompSci 514: Computer Networks Lecture 14 Datacenter Transport - - PowerPoint PPT Presentation

CompSci 514: Computer Networks Lecture 14 Datacenter Transport protocols II

Xiaowei Yang

Roadmap

• Clos topology
• Datacenter TCP
• Re-architecting datacenter networks and

stacks for low latency and high performance

– Best Paper award, SIGCOMM’17

Motivation for Clos topology

• Clos topology aims to achieve the

performance of a cross-bar switch

• When the number of ports n is large, it is hard

to build such a nxn switch

Clos topology

• A multi-stage switching network
• A path from any input port to any output port
• Each switch has a small number of ports

Roadmap

• Clos topology
• Datacenter TCP
• Re-architecting datacenter networks and

stacks for low latency and high performance

– Best Paper award, SIGCOMM’17

Datacenter Impairments

• Incast
• Queue Buildup
• Buffer Pressure

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Queue Buildup

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Sender 1 Sender 2 Receiver

• Big flows buildup queues.

Ø Increased latency for short flows.

• Measurements in Bing cluster

Ø For 90% packets: RTT < 1ms Ø For 10% packets: 1ms < RTT < 15ms

Data Center Transport Requirements

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• 1. High Burst Tolerance

– Incast due to Partition/Aggregate is common.

• 2. Low Latency

– Short flows, queries

• 3. High Throughput

– Continuous data updates, large file transfers

The challenge is to achieve these three together.

Tension Between Requirements

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High Burst Tolerance High Throughput Low Latency

DCTCP

Deep Buffers: Ø Queuing Delays Increase Latency Shallow Buffers: Ø Bad for Bursts & Throughput Reduced RTOmin (SIGCOMM ‘09) Ø Doesn’t Help Latency AQM – RED: Ø Avg Queue Not Fast Enough for Incast

Objective: Low Queue Occupancy & High Throughput

The DCTCP Algorithm

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Review: The TCP/ECN Control Loop

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Sender 1 Sender 2 Receiver

ECN Mark (1 bit)

ECN = Explicit Congestion Notification

Small Queues & TCP Throughput:

The Buffer Sizing Story

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• Bandwidth-delay product rule of thumb:

– A single flow needs buffers for 100% Throughput.

B Cwnd Buffer Size Throughput 100%

Small Queues & TCP Throughput:

The Buffer Sizing Story

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• Bandwidth-delay product rule of thumb:

– A single flow needs buffers for 100% Throughput.

• Appenzeller rule of thumb (SIGCOMM ‘04):

– Large # of flows: is enough.

B Cwnd Buffer Size Throughput 100%

Small Queues & TCP Throughput:

The Buffer Sizing Story

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• Bandwidth-delay product rule of thumb:

– A single flow needs buffers for 100% Throughput.

• Appenzeller rule of thumb (SIGCOMM ‘04):

– Large # of flows: is enough.

• Can’t rely on stat-mux benefit in the DC.

– Measurements show typically 1-2 big flows at each server, at most 4.

Small Queues & TCP Throughput:

The Buffer Sizing Story

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• Bandwidth-delay product rule of thumb:

– A single flow needs buffers for 100% Throughput.

• Appenzeller rule of thumb (SIGCOMM ‘04):

– Large # of flows: is enough.

• Can’t rely on stat-mux benefit in the DC.

– Measurements show typically 1-2 big flows at each server, at most 4.

B

Real Rule of Thumb: Low Variance in Sending Rate → Small Buffers Suffice

Two Key Ideas

• 1. React in proportion to the extent of congestion, not its presence.

ü Reduces variance in sending rates, lowering queuing requirements.

• 2. Mark based on instantaneous queue length.

ü Fast feedback to better deal with bursts.

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ECN Marks TCP DCTCP 1 0 1 1 1 1 0 1 1 1 Cut window by 50% Cut window by 40% 0 0 0 0 0 0 0 0 0 1 Cut window by 50% Cut window by 5%

Data Center TCP Algorithm

Switch side:

– Mark packets when Queue Length > K.

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Sender side:

– Maintain running average of fraction of packets marked (α). In each RTT: Ø Adaptive window decreases: – Note: decrease factor between 1 and 2. B K

Mark Don’t Mark

Working with delayed acks

• Figure 10: Two state ACK generation state machine.

DCTCP in Action

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Setup: Win 7, Broadcom 1Gbps Switch Scenario: 2 long-lived flows, K = 30KB

(Kbytes)

Why it Works

1.High Burst Tolerance

ü Large buffer headroom → bursts fit.

ü Aggressive marking → sources react before packets are

dropped.

• 2. Low Latency

ü Small buffer occupancies → low queuing delay.

• 3. High Throughput

ü ECN averaging → smooth rate adjustments, low variance.

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Analysis

• How low can DCTCP maintain queues without loss of throughput?
• How do we set the DCTCP parameters?

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Ø Need to quantify queue size oscillations (Stability).

Time

(W*+1)(1-α/2) W*

Window Size

W*+1

Packets sent in this RTT are marked.

Analysis

• How low can DCTCP maintain queues without loss of throughput?
• How do we set the DCTCP parameters?

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Ø Need to quantify queue size oscillations (Stability).

Time

(W*+1)(1-α/2) W*

Window Size

W*+1

Analysis

• Q(t) = NW(t) − C × RTT
• The key observation is that with synchronized senders,

the queue size exceeds the marking threshold K for exactly one RTT in each period of the saw-tooth, before the sources receive ECN marks and reduce their window sizes accordingly.

• S(W1,W2)=(W22 −W12)/2
• Critical window size when ECN marking occurs: W∗

=(C×RTT+K)/N

• α = S(W∗,W∗ + 1)/S((W∗ + 1)(1 − α/2),W∗ + 1)
• α2(1 − α/4) = (2W∗ + 1)/(W∗ + 1)2 ≈ 2/W∗
• α ≈ sqrt(2/W∗)
• Single flow oscillation

– D = (W∗ +1)−(W∗ +1)(1−α/2)

A = ND = N(W ∗ + 1)α/2 ≈ N 2 √2W ∗ = 1 2 p2N(C × RT T + K), (8) TC = D = 1 2 p2(C × RT T + K)/N (in RTTs). (9) Finally, using (3), we have: Qmax = N(W ∗ + 1) − C × RT T = K + N. (10)

Analysis

• How low can DCTCP maintain queues without loss of throughput?
• How do we set the DCTCP parameters?

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Ø Need to quantify queue size oscillations (Stability).

85% Less Buffer than TCP

Qmin = Qmax − A (11) = K + N − 1 2 p 2N(C × RTT + K). (12)

Minimizing Qmin

Evaluation

• Implemented in Windows stack.
• Real hardware, 1Gbps and 10Gbps experiments

– 90 server testbed – Broadcom Triumph 48 1G ports – 4MB shared memory – Cisco Cat4948 48 1G ports – 16MB shared memory – Broadcom Scorpion 24 10G ports – 4MB shared memory

• Numerous micro-benchmarks

– Throughput and Queue Length – Multi-hop – Queue Buildup – Buffer Pressure

• Cluster traffic benchmark

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– Fairness and Convergence – Incast – Static vs Dynamic Buffer Mgmt

Cluster Traffic Benchmark

• Emulate traffic within 1 Rack of Bing cluster

– 45 1G servers, 10G server for external traffic

• Generate query, and background traffic

– Flow sizes and arrival times follow distributions seen in Bing

• Metric:

– Flow completion time for queries and background flows.

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We use RTOmin = 10ms for both TCP & DCTCP.

Baseline

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Background Flows Query Flows

Baseline

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Background Flows Query Flows

ü Low latency for short flows.

Baseline

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Background Flows Query Flows

ü Low latency for short flows. ü High throughput for long flows.

Baseline

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Background Flows Query Flows

ü Low latency for short flows. ü High throughput for long flows. ü High burst tolerance for query flows.

Scaled Background & Query

10x Background, 10x Query

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Query Short messages

Conclusions

• DCTCP satisfies all our requirements for Data Center

packet transport.

ü Handles bursts well ü Keeps queuing delays low ü Achieves high throughput

• Features:

ü Very simple change to TCP and a single switch parameter. ü Based on mechanisms already available in Silicon.

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Comments

• Real world data
• A novel idea
• Comprehensive evaluation
• Didn’t compare with the scheme of

eliminating RTOmin and microsecond RTT measurement

• Deadline-based scheduling research

Discussion

• How does DCTCP differ from TCP?
• Will DCTCP work well on the Internet? Why?
• Is there a tradeoff between generality and

performance?

Re-architecting datacenter networks and stacks for low latency and high performance

Mark Handley, Costin Raiciu, Alexandru Agache, Andrei Voinescu, Andrew W. Moore, Gianni Antichi, and Marcin Wójcik

Motivation

• Low latency
• High throughput

Design assumptions

• Clos Topology
• Designer can change end system protocol

stacks as well as switches

• https://www.youtube.com/watch?v=OI3mh1V

x8xI

Discussion

• Will NDP work well on the Internet? Why?
• Is there a tradeoff between generality and

performance?

• Will it work well on non-clos topology?

Summary

• How to overcome the transport challenges in

DC networks

• DCTCP

– Use the fraction of CE marked packets to estimate congestion – Smoothing sending rates

• NDP

– Start, spray, trim