Buffer sizing and Video · 2020. 2. 10. · Buffer sizing and Video QoE Measurements at Netflix...
Transcript of Buffer sizing and Video · 2020. 2. 10. · Buffer sizing and Video QoE Measurements at Netflix...
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Buffer sizing and Video QoE Measurements at NetflixBruce Spang, Brady Walsh, Te-Yuan Huang,Tom Rusnock, Joe Lawrence, Nick McKeown
February 10, 2020
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What are we talking about?
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Server 1Server 2
Buffer
ISP
…
What are we talking about?
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How big should a buffer be?
Too big: packets wait for too long
Too small: too many packets thrown away
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“A buffer should be at least one BDP” [Villamizar, Song 1994]
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“A buffer should be at least one BDP” [Villamizar, Song 1994]BDP=Bandwidth x Delay
# of packets in a link for full utilization
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“A buffer should be at least one BDP” [Villamizar, Song 1994]
Time
Congestion Window
BDP=Bandwidth x Delay# of packets in a link for full utilization
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“A buffer should be at least one BDP” [Villamizar, Song 1994]
Time
Congestion Window
BDP + B
Loss happens when link and buffer are full
BDP=Bandwidth x Delay# of packets in a link for full utilization
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“A buffer should be at least one BDP” [Villamizar, Song 1994]
Time
Congestion Window
BDP + B
Loss happens when link and buffer are full
½(BDP + B)
TCP stops sending until ½(BDP+B) packets received
BDP=Bandwidth x Delay# of packets in a link for full utilization
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“A buffer should be at least one BDP” [Villamizar, Song 1994]
Time
Congestion Window
BDP + B
Loss happens when link and buffer are full
½(BDP + B)
TCP stops sending until ½(BDP+B) packets received
BDP=Bandwidth x Delay# of packets in a link for full utilization
} Buffer needs to hold this many packets
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How big should a buffer be?
BDP: Villamizar and Song 1994
BDP/√n: Appenzeller, McKeown, Keslassy 2004
O(n): Dhamdhere, Jiang, Dovrolis 2005
O(1): Enachescu, Ganjali, Goel, McKeown, Roughgarden 2006
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Which is correct?
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It’s complicated
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1. TCP New Reno (mostly) behaves as expected
2. Video performance varies3. Real routers complicate this story
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Our Experiment
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Catalog servers
Uses spinning disks, cheaply stores entire catalog
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Offload servers
Use SSDs to serve top ~30% of content faster
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These three racks are called a stack
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Make this buffer small…
…and this one large
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1. TCP New Reno (mostly) behaves as expected
2. Video performance varies3. Real routers complicate this story
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Large buffer has higher latency during congested hour
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Sometimes the large buffer has much higher latency
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Large buffer has lower loss during congested hour
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1. TCP New Reno (mostly) behaves as expected
2. Video performance varies3. Real routers complicate this story
![Page 26: Buffer sizing and Video · 2020. 2. 10. · Buffer sizing and Video QoE Measurements at Netflix Bruce Spang, Brady Walsh, Te-Yuan Huang, Tom Rusnock, Joe Lawrence, Nick McKeown February](https://reader033.fdocuments.us/reader033/viewer/2022061001/60b08dce77fd027ebe05e4f1/html5/thumbnails/26.jpg)
Good buffer size:+ Fewer rebuffers+ Better video quality+ Videos start faster
Bad buffer size: - More rebuffers- Worse video quality- Videos start slower
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Good buffer size:+ Fewer rebuffers+ Better video quality+ Videos start faster
Bad buffer size: - More rebuffers- Worse video quality- Videos start slower } This happens
when buffer is too large ortoo small.
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Site #2: A smaller buffer is better
Reducing the buffer from 500MB to 25MB-15.6% decrease in sessions with a rebuffer-5.3% decrease in low quality video-13.5% decrease in play delay
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Site #3: A smaller buffer is better
Reducing the buffer from 500MB to 50MB-22.1% decrease in sessions with a rebuffer-7.0% decrease in low quality video-14.8% decrease in play delay
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Site #1: A smaller buffer is worse
Reducing the buffer from 500MB to 50MB+46.3% increase in sessions with a rebuffer+5.7% increase in low quality video-5.9% decrease in play delay
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1. TCP New Reno (mostly) behaves as expected
2. Video performance varies3. Real routers complicate this story
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Large buffer has higher latency during congested hour
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Remember how the large buffer has much higher latency…
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Servers have different very latency distributions
Min RTT (ms)
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Server 1Server 2
Buffer
ISP
…
What I imagined
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Server 1Server 2
Buffer
ISP
…
What I imagined
LIES!
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Line card #1Line card #2Line card #3Line card #4
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VOQ #1 VOQ #2VOQ #3 VOQ #4VOQ #5 VOQ #6VOQ #7 VOQ #8
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Buffer architecture
Server #1
Server #2
“Offload” VOQ
“Catalog” VOQ ISP100Gbps
2/3
1/3Server #3
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Traffic is fairly split when load is equal
40 Gbps
40 Gbps
“Offload” VOQ
“Catalog” VOQ ISP100Gbps
67 Gbps
33 Gbps40 Gbps
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When one VOQ offers less than its fair share, it sees no congestion
50 Gbps
50 Gbps
“Offload” VOQ
“Catalog” VOQ ISP100Gbps
90 Gbps
10 Gbps10 Gbps No delay!
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VOQs explain the RTT differences
Min RTT (ms)
This VOQ is served faster
This VOQ is served slower
This VOQ is all over the place
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Switches prioritize long-tail content
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Switches prioritize long-tail content
Same latency during uncongested hours
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Switches prioritize long-tail content
Same latency during uncongested hours
Popular content is congested
Long-tail content not congested
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New scheduling algorithm!
Server #1
Server #2
“Offload” VOQ
“Catalog” VOQ ISP100Gbps
Load-dependent
Load-dependentServer #3
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Default schedulingalgorithm
New schedulingalgorithm is more
consistent
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1. TCP New Reno (mostly) behaves as expected
2. Video performance varies3. Real routers complicate this story
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How big should a buffer be?
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Thanks!For more details, please see: https://brucespang.com/papers/netflix-buffer-sizing.pdf