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A Network Communication System for Large-Scale, High-Speed Games
Ashwin Bharambe, Jeff Pang, Srini Seshan, Xinyu ZhangCarnegie Mellon
John Douceur, Jacob R. Lorch, Thomas MoscibrodaMicrosoft Research
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P2P Games
InternetPrimary object
Local View
Replica objects
0
50
100
150
200
0 100 200 300 400 500
Players
To
tal
Ba
nd
wid
th (
Mb
ps
)Goal: 1000 players in the
same battle at the same time
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Problem 1: Insufficient Capacity
• Update Size: ~35 bytes/frame
• Update Rate: 20 frames/sec
Need 14 Mb/s upload BW per peer
• Real P2P topologies: – 800 Kb/s to 1.5Mb/s per peer (on average)
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Solution 1: Subscriber Set
• Players can only focus on ~5 objects at the same time
• Each player divides other peers into two classes:– Subscriber Set: players focused on me
• Update every frame– Best Effort: everyone else
• Update with “guidance” once per second
• Who goes in my Subscriber Set?– Attention(i,j) = how much a player i is focused on player j– Attention(i,j) sent from peer i to peer j periodically – Peers with the highest Attention(i,j) go in peer j’s Set
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Solution 1: Subscriber Set
• Who goes in my Subscriber Set?– Attention(i,j) = how much a player i is focused on player j– Attention(i,j) sent from peer i to peer j periodically – Peers with the highest Attention(i,j) go in peer j’s Set
d1d2
θ1
θ2
Attention(i,j) =
fproximity(d1) faim(θ1) finteraction-recency(t1)+ +
j i
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Projected Scalability
0
200
400
600
800
1000
1200
1400
0 128 256 384 512 640 768 896 1024 1152 1280 1408 1536
Upload bandwidth per peer (kbps)
Ma
x #
of
pla
ye
rs
With Donnybrook
Without Donnybrook
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Problem 2: Bandwidth Heterogeneity
• Best Effort updates still scale quadratically• Still need ~1 Mb/s per peer Limited by low bandwidth peers
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Solution 2: Guidance Forwarding
• Static Overlay Multicast– Need low latency limit tree depth to 2– Can’t queue updates have senders send to
forwarders at 20 updates/sec– Generally have centralized match making
(replicated) centralized coordinator picks forwarders
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• Some players more interesting than others More players focus on them They need to send more frequent updates
• Subscriber sets change dynamically (~60% turnover per second!) Can’t use static overlay multicast
Limited by BW of popular players
Problem 3: Interest Heterogeneity
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Solution 3: Update Dissemination
• Alterative 1:– Static forwarder groups
• Alternative 2:– Centralized forwarder
assignment
• Alternative 3:– Uncoordinated
forwarder assignment
A B
I need BW!
Use B
A BPick random
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Evaluation
• Detailed packet level simulation– Empirical simulation models
• E2E latency• Jitter• Access Bandwidth• Packet Loss• Quake III-like CTF workload• ... many other details …• But no churn
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• Metric: updates delivered roughly on time– updates delivered too late are useless
• 3-5% loss rate acceptable for games
Alternative Comparison