1. Migrate when necessary: toward partitioned reclaiming for soft real-time systems
Houssam-Eddine Zahaf, Giuseppe Lipari, Luca Abeni
RTNS’17
Presenter: Haoran Li

2. Motivation: Decrease Soft Real-Time Deadline misses
Undesirable to miss deadlines but not crucial
Task Model
Release job aperiodically
Execution time varies (“highly dynamic”)
Jobs: Possibly unbounded inter-arrival time
Tasks may enter and leave the system dynamically
Consequence
Non-guarantee to respect all deadlines
Tasks with long execution times may monopolize the processor, and then increase deadline miss rate.
Insight: CPU reservation  guarantee CPU share in a timely fashion
reclaiming unused CPU cycles  exploit unused CPU cycles

3. Reservation: Bandwidth Preserving Servers
a server is a scheduling entity, which specify its <budget, period>
The server consumes budget when executing jobs
When the budget exhausted, the server stop executing jobs
Different budget burn/replenishment algorithms  Different servers
Deferrable Server
GRUB Server (*)
Then, the server is treated as if it were a periodic task, which will be scheduled by the host scheduler.
* This paper adopt GRUB approach

4. Example: Deferrable Server
Preserve unused capacity till the end of the current period
Get better utilization?  Assign another server and sched the server by EDF!

5. Example: EDF + Deferrable Server
Better Utilization: suppose we have ONE JOB with very long execution time
Reservation! As long as the servers are schedulable, the budget is guaranteed in every period.

6. Issue: “Wasted CPU Cycles”
Server 1: Task 1 releases a job with a long execution time
Server 2: No pending jobs
Server (4, 6)’s cycle is wasted. How to RECLAME those unused budget?

7. GRUB, another server: A rough “insight”
Note: The algorithm showed here is NOT accurate, just for an insight
Server utilization (aka. bandwidth) 𝑢𝑖= 𝐵𝑖 𝑃𝑖
If the server has pending job, mark it as “Active” Ua= 𝑖∈𝑎𝑐𝑡𝑖𝑣𝑒 𝑢𝑖
Each period, Virtual Budget is replenished as its Period.
Burns at Different rate, according to active servers only r= 𝑈𝑎 𝑢𝑖
Key Features of GRUB
Servers scheduled by “EDF-like” policy
For any t, If 𝐔𝐚(𝐭)≤𝟏,
then no server misses its scheduling deadline
regardless of the behavior of the served tasks

8. Different Burn Rate
Server 1: Virtual Budget start at 3, r = 1 / (1/3) = 3
Server 2: Virtual Budget start at 6, r = 1 / (4/6) = 1.5
“EDF-Like”
r = 1.5
r = 3

9. Auto Reclaiming
Server 1: Only active server: Ua= 1/3, r = (1/3) / (1/3) = 1
Server 2: Not active
r = 1
Auto Reclaiming unused CPU cycles!

10. Architecture Partitioned Reservation
Partitioned: GRUB is a single core server + scheduling policy (pEDF like)
Each task has a server (no task scheduling within a server)
Reservation: Server can guarantee at least Budget execution every Period
Global Reclaiming
Global: Job can (temporarily) migrate among different PCPU
Reclaiming: the migrated job is put in a migrated server, which can exploit the unused PCPU cycles

11. Architecture Preset 𝑢𝑚 as migrating utilization
Total utilization of servers on core Uj
Total utilization of other migrating server 𝑈 𝑗 𝑚
Find the core j with smallest 𝑈 𝑗 𝑎 j
𝑢𝑚 + 𝑈𝑗 + 𝑈 𝑗 𝑚 ≤1
Other features:
Permanent Migration:
If one task is always tend to missing deadlines,
do permanent migration
Dynamic Load Balance
Different partitioned algorithm (e.g. Worst-Fit Best-Fit)
If cannot satisfied: do it with best effort:
𝑢𝑚′ =min{𝑢𝑚 ,1- 𝑈𝑗 - 𝑈 𝑗 𝑚 }

12. Evaluation: Two core platform (simulation)
A high CPU demand task t=2000, t=6000. Its server’s utilization = 0.3
Enable Load Balance/Migration
Disable Load Balance/Migration

13. Evaluation: Utilization
Enable Load Balance/Migration
Disable Load Balance/Migration

14. Contribution We proposed:
heuristics to partition a set of soft real-tasks to a set of identical cores
The execution time of each job is not know apriori
Tasks may enter and leave the system dynamically
We allow:
Jobs to migrate to reclaim extra-budget
Tasks to migrate to load balance

15. Critiques & Lesson Learned
Reclaiming: “Work Conserving”  Exploit unused cycles
Trade off:
Partitioned Reservation: EDF-Like utilization feature
Global Reclaiming: Opportunistic
Temporarily Migration  Guarantee the pre assigned servers,
while give best effort to temporarily job.
Challenge: How to choose um??? By profiling?
GRUB complexity? Challenge of adopting it in real system? (Marked as Future work)

16. Thank You