Upload
askarudin-tamam-eddy-noveka
View
219
Download
0
Embed Size (px)
Citation preview
7/31/2019 0414 Deficit Round Robin Scheduler
1/16
5/23/12
Deficit Round Robin
Scheduler
7/31/2019 0414 Deficit Round Robin Scheduler
2/16
5/23/12
Outline
Introduction
Ordinary Problems
Deficit Round Robin Latency of DRR
Improvement of latencies
7/31/2019 0414 Deficit Round Robin Scheduler
3/16
5/23/12
Introduction
Implementation complexity is low
Bandwidth guarantees are still robust
Appealing whenever the flows to bescheduled have no stringent delayrequirements.
7/31/2019 0414 Deficit Round Robin Scheduler
4/16
5/23/12
Ordinary Problems
Not fair, if node A,B,C,D does notdiscriminate flows, and so give serveto source flows by half
F1 gets 1/8
F2 gets 1/8
F3 gets 1/4
7/31/2019 0414 Deficit Round Robin Scheduler
5/16
7/31/2019 0414 Deficit Round Robin Scheduler
6/16
5/23/12
Deficit Round Robin
1. Add quantum size to Deficit counterof corresponding flow
2.
Process the first job of flow andminus job count from counter
3. Serve other flows if current counter
is not enough for next job4. Set counter to zero if job is all done
in a flow
5. Maintain a link list of active flows to
7/31/2019 0414 Deficit Round Robin Scheduler
7/16
5/23/12
Deficit Round Robin
7/31/2019 0414 Deficit Round Robin Scheduler
8/16
5/23/12
Deficit Round Robin
7/31/2019 0414 Deficit Round Robin Scheduler
9/16
5/23/12
Deficit Round Robin
Round Robin Listhead
Flow 1
Flow 2
Flow 3
Flow 4
7/31/2019 0414 Deficit Round Robin Scheduler
10/16
5/23/12
Deficit Round Robin
7/31/2019 0414 Deficit Round Robin Scheduler
11/16
5/23/12
Demers-Keshav-Shenkerbound
Let packet p of flow i arrive with s bits,and other flows have no tasks
Uses bit-by-bit round robin Assume there is no more than n active
flows in any time
Suppose B is bandwidth of output line(bit/s)
The latency bound is about n*s/B
n:number of flows, s=packet size,
7/31/2019 0414 Deficit Round Robin Scheduler
12/16
5/23/12
Latency of DRR
Reducing quantum size does not helpimprove the worst-case latency
Solution:
use separate
fair queuingalgorithm
7/31/2019 0414 Deficit Round Robin Scheduler
13/16
5/23/12
Improvement of latencies
Define two classes of flows:
latency critical
best effort Latency critical must contract send
no more than x bytes in some period
T If a latency critical flow f meet the
contract, put the flow in the head of
round-robin list
7/31/2019 0414 Deficit Round Robin Scheduler
14/16
5/23/12
Improvement of latencies
Define a state bit for flows, and atimer with countdown time T
Set when a packet is arrived Cleared if the timer expires
Reset timer to T if a packet arrives
If a packet arrive and the bit isalready set, put the flow to the endof the round-robin list (move to best-
effort class)
7/31/2019 0414 Deficit Round Robin Scheduler
15/16
5/23/12
Improvement of latencies
Delayed by
n is number of latency critical flows
MAX is max bit length of package inlist
B is speed of output line
s is size of current package
Only delayed by other latency critical
7/31/2019 0414 Deficit Round Robin Scheduler
16/16
5/23/12
Improvement of latencies