sp-pifo: approximating push-in first-out behaviors using strict … · 2020. 5. 10. · sp-pifo...
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Approximating Push-In First-Out Behaviors
Albert Gran Alcoz,
Alexander Dietmüller,
Laurent Vanbever
SP-PIFO
1
4 5 4 4 3
1 2 1
sp-pifo.ethz.ch
SP-PIFO:
NSDI ’20
February, 25 2020
1
Using Strict-Priority Queues
2
[SIGCOMM ’92]
Minimize tail packet delays
[SIGCOMM ’13, NSDI ’15]
Minimize flow completion times
[ToN ’93, NSDI ’18]
Enforce max-min fairness
3
“You can’t have everything you want,
but you can have anything you want”
Flexibility
Customized algorithms
Generality
Universal packet scheduler
[NSDI ’16]
Is there a universal packet scheduler?
4
Generality
Universal packet scheduler
Programmable
scheduling
but you can have anything you want”
“You can’t have everything you want,
[NSDI ’16]
Is there a universal packet scheduler?
5
[SIGCOMM ’16]
PIFO abstraction for programmable scheduling
Assumes monotonic increase of ranks within flows
Supports ~1k flows and ~10 Gbps
Requires new ASIC implementation, which might take years
Deployability
Scalability
Flexibility
Implementing PIFO queues in hardware
is difficult
on existing devices?
Can we approximate PIFO queues…
at scale, and
at line rate,
6
Introducing…
SP-PIFO
7
A deployable, scalable and flexible
PIFO approximation
PIFO queues can be used as an abstraction
to make scheduling programmable
The PIFO queue Allows packets to be pushed into arbitrary locations
Only drains packets from the head
8
Packet ranks
PIFO Queue
4 145 32Input sequence Output sequence
PIFO queues can be used as an abstraction
to make scheduling programmable
The PIFO queue Allows packets to be pushed into arbitrary locations
Only drains packets from the head
9
Packet ranks
PIFO Queue
4 145 3Input sequence Output sequence
2
PIFO queues can be used as an abstraction
to make scheduling programmable
The PIFO queue Allows packets to be pushed into arbitrary locations
Only drains packets from the head
10
Packet ranks
PIFO Queue
4 145 3 2Input sequence Output sequence
PIFO queues can be used as an abstraction
to make scheduling programmable
The PIFO queue Allows packets to be pushed into arbitrary locations
Only drains packets from the head
11
Packet ranks
PIFO Queue
Input sequence
5 4 4 3 2 1Output sequence
2
Programmable scheduler
12
PIFO queues can be used as an abstraction
to make scheduling programmable
Rank computation (programmable)
PIFO queue (fixed logic)
Rank Computation
p.rank = f.size
f = flow(p)
Input sequence
PIFO Queue
1445 3
Programmable scheduler
13
PIFO queues can be used as an abstraction
to make scheduling programmable
Rank computation (programmable)
PIFO queue (fixed logic)
Rank Computation
p.rank = f.size
f = flow(p)
Input sequence
PIFO Queue
1445 32
445 3
Programmable scheduler
14
PIFO queues can be used as an abstraction
to make scheduling programmable
Rank computation (programmable)
PIFO queue (fixed logic)
Rank Computation
p.rank = f.size
f = flow(p)
Input sequence
PIFO Queue
12
15
SP-PIFO approximates PIFO behaviors using Strict-Priority queues
and a dynamic mapping strategy
Adaptation StrategyStrict-Priority Queues
12445
3
Rank Computation
p.rank = f.size
f = flow(p)
Programmable scheduler
Rank computation (programmable)
Adaptation strategy + strict-priority queues (fixed logic)
16
Ideal case Perfect PIFO if number of queues >= number of ranks
SP-PIFO approximates PIFO behaviors using Strict-Priority queues
and a dynamic mapping strategy
Strict-Priority Queues
12345
23
5
1
44
17
Ideal case Perfect PIFO if number of queues >= number of ranks
SP-PIFO approximates PIFO behaviors using Strict-Priority queues
and a dynamic mapping strategy
Output sequence
Strict-Priority Queues
12345
5 4 4 3 2 1
18
Strict-Priority Queues
In practice Number of queues < number of ranks
SP-PIFO approximates PIFO behaviors using Strict-Priority queues
and a dynamic mapping strategy
Problem Output sequence can have scheduling errors
42 3 1
Problem Output sequence can have scheduling errors
19
Low-ranked packets drained after high-ranked packets
suboptimal output
In practice Number of queues < number of ranks
SP-PIFO approximates PIFO behaviors using Strict-Priority queues
and a dynamic mapping strategy
Strict-Priority Queues
4 12 3
Problem Output sequence can have scheduling errors
20
In practice Number of queues < number of ranks
SP-PIFO approximates PIFO behaviors using Strict-Priority queues
and a dynamic mapping strategy
Design mapping strategies that
minimize scheduling errors
Opportunity
21
SP-PIFO defines mapping through ‘queue bounds'
Queue bounds scanned bottom-upMapping
Packet enqueued if rank >= queue bound
Strict-Priority Queues
134 2Input sequence Output sequence
?
22
SP-PIFO defines mapping through ‘queue bounds'
Strategy A
14
134 2Input sequence Output sequence
Mapping Queue bounds scanned bottom-up
Packet enqueued if rank >= queue bound
23
SP-PIFO defines mapping through ‘queue bounds'
Strategy A
14
34 12Input sequence Output sequence
Mapping Queue bounds scanned bottom-up
Packet enqueued if rank >= queue bound
Input sequence Output sequence
34 2
24
SP-PIFO defines mapping through ‘queue bounds'
Strategy A
14
1
Is rank >= queue bound ?
Input sequence Output sequence
34 2
25
SP-PIFO defines mapping through ‘queue bounds'
Strategy A
14
1
26
SP-PIFO defines mapping through ‘queue bounds'
Strategy A
114
Input sequence Output sequence
Mapping
34 2
Queue bounds scanned bottom-up
Packet enqueued if rank >= queue bound
27
SP-PIFO defines mapping through ‘queue bounds'
Strategy A
114
4 2 3suboptimal output
Input sequence Output sequence
Mapping Queue bounds scanned bottom-up
Packet enqueued if rank >= queue bound
28
SP-PIFO defines mapping through ‘queue bounds'
Strategy A
114
4 2 3suboptimal output
13Strategy B
134 2
Input sequence Output sequence
Mapping Queue bounds scanned bottom-up
Packet enqueued if rank >= queue bound
29
SP-PIFO defines mapping through ‘queue bounds'
Strategy A
Input sequence Output sequence
114
4 2 3suboptimal output
13Strategy B
134 2optimal output
Mapping Queue bounds scanned bottom-up
Packet enqueued if rank >= queue bound
30
How can we design a mapping strategy
that minimizes scheduling errors?
1 Adaptation design
How does it work
2 Implementation
How can it be deployed
3 Evaluation
How well does it perform
31
Approximating Push-In First-Out Behaviors SP-PIFO:
Using Strict-Priority Queues
1 Adaptation design
How does it work
32
Approximating Push-In First-Out Behaviors SP-PIFO:
Using Strict-Priority Queues
2 Implementation
How can it be deployed
3 Evaluation
How well does it perform
33
Problem formulation
q* = argmin E [ U(q, r) ]q ∈ Q r ∼ R
Objective Find optimal queue bounds q*
That minimize the expected loss U for all ranks
Unpifoness (U) quantifies the scheduling errors
34
SP-PIFO adapts the mapping of packet ranks
to strict-priority queues
1 42
Strict-Priority Queues
Input sequence Output sequence
03
0
Initialization
Queue bounds set to zero
Zero traffic knowledge
35
SP-PIFO adapts the mapping of packet ranks
to strict-priority queues
1 2
Strict-Priority Queues
Input sequence Output sequence
03
0
Rank >= queue bound ?
Initialization
Queue bounds set to zero
Zero traffic knowledge
4
36
SP-PIFO adapts the mapping of packet ranks
to strict-priority queues
1 2
Strict-Priority Queues
Input sequence Output sequence
0
0
Initialization
Queue bounds set to zero
Zero traffic knowledge
Rank >= queue bound ?
43
37
SP-PIFO adapts the mapping of packet ranks
to strict-priority queues
1 2
Strict-Priority Queues
Input sequence Output sequence
0
0 43
Initialization
Queue bounds set to zero
Zero traffic knowledge
38
SP-PIFO adapts the mapping of packet ranks
to strict-priority queues
1 2
Strict-Priority Queues
Input sequence Output sequence
0
043
scheduling error
Initialization
Queue bounds set to zero
Zero traffic knowledge
39
SP-PIFO adapts the mapping of packet ranks
to strict-priority queues
1 2
Strict-Priority Queues
Input sequence Output sequence
0
00 4
Initialization
Queue bounds set to zero
Zero traffic knowledge
Rank >= queue bound ?
3
40
SP-PIFO adapts the mapping of packet ranks
to strict-priority queues
1 2
Strict-Priority Queues
Input sequence Output sequence
0
03
0 4Push-up 4
41
SP-PIFO adapts the mapping of packet ranks
to strict-priority queues
1 2
Strict-Priority Queues
Input sequence Output sequence
03
44
42
SP-PIFO adapts the mapping of packet ranks
to strict-priority queues
1 2
Strict-Priority Queues
Input sequence Output sequence
0
00 44
After enqueue, queue bound set to
the rank of the packet enqueued
Push-up
(future low-rank packets to
higher-priority queues)
Rank >= queue bound ?
3
43
SP-PIFO adapts the mapping of packet ranks
to strict-priority queues
1 2
Strict-Priority Queues
Input sequence Output sequence
0
0
3
0 44
Push-up 3
After enqueue, queue bound set to
the rank of the packet enqueued
Push-up
(future low-rank packets to
higher-priority queues)
44
SP-PIFO adapts the mapping of packet ranks
to strict-priority queues
1 2
Strict-Priority Queues
Input sequence Output sequence
3
0
3
0 44
45
SP-PIFO adapts the mapping of packet ranks
to strict-priority queues
1
Strict-Priority Queues
Input sequence Output sequence
3
0
3
0 44
!
Output sequence
After enqueue, queue bound set to
the rank of the packet enqueued
Push-up
(future low-rank packets to
higher-priority queues)
2
46
SP-PIFO adapts the mapping of packet ranks
to strict-priority queues
1
Strict-Priority Queues
Input sequence Output sequence
3
00 44
Output sequence
2 3
Error cost = (queue bound - rank) = 1
2
47
SP-PIFO adapts the mapping of packet ranks
to strict-priority queues
1
Strict-Priority Queues
Input sequence Output sequence
00 4
Output sequence
3Push-down 3 3
4
For all queue bounds:
decrease error cost (1)
2
2
48
SP-PIFO adapts the mapping of packet ranks
to strict-priority queues
1
Strict-Priority Queues
Input sequence Output sequence
00 4
Output sequence
Push-down 32 3
4
2
For all queue bounds:
decrease error cost (1)
2
49
SP-PIFO adapts the mapping of packet ranks
to strict-priority queues
1
Strict-Priority Queues
Input sequence Output sequence
00 4
Output sequence
32 3
3Push-down
2
For all queue bounds:
decrease error cost (1)
50
SP-PIFO adapts the mapping of packet ranks
to strict-priority queues
Strict-Priority Queues
Input sequence Output sequence
00 4
Output sequence
32 3
3
2
After potential error detected,
all queue bounds decreased the error cost
Push-down
(future high-rank packets to
lower-priority queues)
3
51
SP-PIFO adapts the mapping of packet ranks
to strict-priority queues
Objective Find optimal queue bounds q*
That minimize the expected loss U for all ranks
Strict-Priority Queues
00 4
3 32
3
2
3Push-up Push-down
Low-rank packets High-rank packets
Result Packet-level adaptation of q
2 Implementation
How can it be deployed
3 Evaluation
How well does it perform
52
Approximating Push-In First-Out Behaviors SP-PIFO:
Using Strict-Priority Queues
1 Adaptation design
How does it work
53
SP-PIFO has been fully implemented on
existing programmable hardware
Parser Traffic ManagerIngress pipeline
…
Priority Queues
Queue Bound n
Registers
Queue Bound n-1 Queue Bound 1
Metadata
Queue ID
Queue Bound 1 - Rank
2 Implementation
How can it be deployed
3 Evaluation
How well does it perform
54
Approximating Push-In First-Out Behaviors SP-PIFO:
Using Strict-Priority Queues
1 Adaptation design
How does it work
How well does SP-PIFO approximate
well-known scheduling objectives under
realistic traffic workloads?
Max-min fairness
Minimizing Flow Completion Time
pFabric* (8 queues)
Start-Time Fair Queuing (32 queues)
Ranks based on a fluid model
Ranks are set to the remaining flow size
55
Evaluation
* without starvation prevention
Scheduling objectives
Question
56
We generate traffic following pFabricRealistic workloads web-search workload
Methodology
We use a leaf-spine topology with 144 servers,Topology
links of 1Gbps and 4Gbps
Packet-level simulator
We integrated SP-PIFO in Netbench
[SIGCOMM 2017]
50
57
SP-PIFO closely approximates pFabric,
minimizing FCTs for both small and big flows
Load
Small flows <100KB Big flows ≥1MB
0
100
200
300
400
500
0.2 0.3 0.4 0.5 0.6 0.7 0.8
Flow
Com
plet
ion
Tim
e (m
s)
Load
PIFOSP-PIFODCTCP
TCP
0
10
20
30
40
50
0.2 0.3 0.4 0.5 0.6 0.7 0.8
Flow
Com
plet
ion
Tim
e (m
s)
Load
PIFOSP-PIFODCTCP
TCP
0.2 0.3 0.4 0.5 0.6 0.7 0.80
10
20
30
40
Load
0.2 0.3 0.4 0.5 0.6 0.7 0.8
500
0
100
200
300
400
99th percentile FCT (ms) Average FCT (ms)
0
10
20
30
40
50
0.2 0.3 0.4 0.5 0.6 0.7 0.8
Flow
Com
plet
ion
Tim
e (m
s)
Load
PIFOSP-PIFODCTCP
TCP
58
SP-PIFO closely approximates state-of-the-art
fair-queuing algorithms
101
103
105
107
�2M 0.2M-1M 80K 50K 30K 20K 10K
Flow
Com
plet
ion
Tim
e (m
s)
Flow Size
PIFOAFQ
SP-PIFODCTCP
TCP
0
2
4
6
8
10
0.2 0.3 0.4 0.5 0.6 0.7 0.8
Flow
Com
plet
ion
Tim
e (m
s)
Load
PIFOAFQ
SP-PIFODCTCP
TCP
Flow size
All flows @ Load 0.7
Average FCT (ms)
2
4
6
8
10
0.2 0.3 0.4 0.5 0.6 0.7 0.8
Load
≥2M
10
105
107
103
0≤1M 80K 50K 30K 20K 10K
Small flows <100KB
Average FCT (ms)
0
59
SP-PIFO characterization, comparison with gradient
Check our website!
Limitations and future improvements
sp-pifo.ethz.ch
All the code is availableAll our experiments are reproducible
Hardware evaluation on Barefoot Tofino
60
SP-PIFO approximates the behavior of PIFO queues
Making scheduling programmable, today!SP-PIFO:
at line rate, at scale and on existing devices
It adapts the mapping between packet ranks and
strict-priority queues to minimize the scheduling errors
It reacts per-packet to traffic variations,
without traffic knowledge required
sp-pifo.ethz.ch
Albert Gran Alcoz Alexander Dietmüller Laurent Vanbever
61
Approximating Push-In First-Out Behaviors SP-PIFO:
Using Strict-Priority Queues
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