EE 382C - S11 - Lecture 13 1
EE382C
Lecture 13
Network Performance Analysis
5/12/11
EE 382C - S11 - Lecture 13 2
Announcements
• I will be out all next week (IPDPS in Alaska)
– No office hours
– George doing both lectures
• Project sign-up (May 24th and 26th)
– 15 min. presentation + 3 min. for questions
• Final report due (May 31st -- 5pm)
EE 382C - S11 - Lecture 13 3
So far
• Topology
• Routing
• Flow Control / Deadlock
• Microarchitecture – datapath/control
EE 382C - S11 - Lecture 13 4
Question of the day
What is the best way to evaluate an interconnection network?
a) analysis
b) simulation
c) experiment
EE 382C - S11 - Lecture 13 5
Question of the day II
What metrics should you be worried about when evaluating an
interconnection network?
EE 382C - S11 - Lecture 13 6
Network Performance
• Latency
• Throughput
• Steady state
• Open loop
EE 382C - S11 - Lecture 13 7
T
Applied Load-Latency
EE 382C - S11 - Lecture 13 8
Offered vs. Accepted
EE 382C - S11 - Lecture 13 9
Network StabilityThroughput at maximum load vs peak throughput
EE 382C - S11 - Lecture 13 10
BNF chart
(Why is this wrong?)
How can you design a stable network?
EE 382C - S11 - Lecture 13 11
Throughput vs Offered Traffic for Stable Network8x8 Mesh, DOR, Bit Complement
EE 382C - S11 - Lecture 13 12
EE 382C - S11 - Lecture 13 13
Simple network
0 1
1
0.1
01
10
EE 382C - S11 - Lecture 13 14
Simple network
0 1
1
0.1
01
10
At what load does the simple network saturate?
Measure the minimum throughput over
source-destination pairs, not the average
EE 382C - S11 - Lecture 13 15
0.10 1
1 0
0.9
0 0
1 0
EE 382C - S11 - Lecture 13 16
3 ways to measure performance
• Analysis
• Simulation
• Experiment
EE 382C - S11 - Lecture 13 17
Analysis
Example – switch speedup analysis
• Probability that an input requests a particular output
• Probability that an input doesn’t request a particular output
EE 382C - S11 - Lecture 13 18
Pi 1
k
PNi k 1
k
• Probability that no input requests a particular output
• Probability that some input requests a particular output
EE 382C - S11 - Lecture 13 19
PNa k 1
k
k
Pa 1k 1
k
k
• Pa is the throughput of a switch with unit speedup.
• If we add input speedup this number goes up – but is still
bounded by 1
EE 382C - S11 - Lecture 13 20
Pa 1k 1
k
si k
• Adding output speedup increases overall throughput by so
and makes input speedup effectively si/so
EE 382C - S11 - Lecture 13 21
Pa so 1k 1
k
si
so
k
EE 382C - S11 - Lecture 13 22
Analysis
Example – queuing theory
EE 382C - S11 - Lecture 13 23
Network to Analyze
0
1
2
3
Switch
00
Switch
01
Switch
10
Switch
11
0
1
2
3
EE 382C - S11 - Lecture 13 24
Model
T
0
T1
T2 T
T0
T1
T
0
T1
T0
T1
T2 T
T2 T
T2 T
Switch 00
EE 382C - S11 - Lecture 13 25
Result
EE 382C - S11 - Lecture 13 26
Folded Clos network example
0
5
10
15
20
25
30
35
40
0 0.2 0.4 0.6 0.8 1
Offered load
La
ten
cy
(c
yc
les
)
oblivious - folded Clos oblivious - Clos M/D/1 model
EE 382C - S11 - Lecture 13 27
Simulation Workload
• Application-Driven
• Trace-Driven
• Synthetic workload
– Traffic pattern
– Injection Process
EE 382C - S11 - Lecture 13 28
Injection process
time
Inje
ction
pro
cess
T = 1/r
time
Inje
ction
pro
cess
E[T] = 1/r
Simulation Setup
EE 382C - S11 - Lecture 13 29
Simulation Procedure
• Warm up the simulator with “no measurement” packets
– Make sure you are in steady-state before taking a steady state
measurement
• Once in steady state, inject “measurement packets” long
enough to get a statistically meaningful measurement
– Apply the usual tests and compute error bounds
• Drain the simulator while injecting “no measurement”
packets until all “measurement packets” arrive at their
destinations.
EE 382C - S11 - Lecture 13 30
Include an infinite Source Queue when
measuring Latency Do not do the following:
EE 382C - S11 - Lecture 13 31
EE 382C - S11 - Lecture 13 32
Simulation results – Virtual channels
EE 382C - S11 - Lecture 13 33
Packet sizes
EE 382C - S11 - Lecture 13 34
Age based priority
EE 382C - S11 - Lecture 13 35
Question of the day
What is the best way to evaluate an interconnection network?
a) analysis
b) simulation
c) experiment
EE 382C - S11 - Lecture 13 36
Question of the day II
What metrics should you be worried about when evaluating an
interconnection network?
Errors to Avoid
• Don’t forget the source queue
• Measure minimum traffic across pairs
• Don’t combine latency with accepted traffic
• Measure all “measurement packets” generated during test
interval
– May require a long draining period
• Use realistic traffic – not just UR
EE 382C - S11 - Lecture 13 37
EE 382C - S11 - Lecture 13 38
Summary of performance analysis
• Analysis, simulation, experiment
– Maximize insight / effort
• Measurements – steady state, open loop, on specified
– Warm up, source queue, min throughput
• Queueing theory
– M/M/1 N = r1r
• Simulation
– Workloads
– Errors – systematic and sampling
– Confidence intervals and ensemble averages