BSc CS graduation project (March 2008)

1. Analysis of
interrupt latencies
in a real-time
Università degli Studi di Trento kernel
Facoltà di Scienze Matematiche Fisiche e Naturali
Real-time
operating systems
Real-time Linux
CFS
Analysis of interrupt latencies in a Recap
real-time kernel
March 26, 2008
Laureando: Gabriele Modena (Mat. 108742)
Relatore: dott. Luigi Palopoli
Correlatore: dott. Luca Abeni

2. Analysis of
Real-Time Operating Systems (RTOS) interrupt latencies
in a real-time
kernel
Real-time
operating systems
Real-time Linux
CFS
Recap
Ensure that an operation completes within a fixed amount
of time (deadline) =⇒ predictability.
When dealing with device drivers this implies
managing race conditions;
fulfill temporal constraints.
This thesis focuses on shared mixed resources.

3. Analysis of
Latency interrupt latencies
in a real-time
kernel
Real-time
Latency is the measure of the difference between the operating systems
theoretical (algorithm) and actual (implementation) Real-time Linux
schedule. CFS
Recap
In a RTOS latency:
should be bounded;
the bound should not be too high.
The goal of a real-time kernel is to minimize latency by
using algorithms and technology that properly
implements them:
preemptible kernel.

4. Analysis of
Linux as an RTOS interrupt latencies
in a real-time
kernel
Real-time
operating systems
Linux vanilla:
Real-time Linux
since 2.6 preemptible kernel; CFS
The rt-preempt patch: Recap
developed by Ingo Molnar;
makes Linux hard real-time;
uses the new (2.6.21) high resolution timers;
preemptible critical sections;
real-time mutexes;
threaded interrput handlers.

5. Analysis of
Benchmarking rt-preempt interrupt latencies
in a real-time
kernel
Real-time
operating systems
Real-time Linux
CFS
Recap
Two goals:
test the actual real-time performance;
test the performance of I/O scheduling.
We look for a trade-off between latency and throughput.

6. Analysis of
Cyclictest interrupt latencies
in a real-time
kernel
Real-time
operating systems
Real-time Linux
CFS
high resolution test program written by Thomas Recap
Gleixner;
measure latency;
schedules periodic threads;
When it comes to real-time applications rt-preempt gives
the expected performance.

7. Analysis of
Vanilla vs. rt-preempt (1) interrupt latencies
in a real-time
kernel
Probability of the latency (num. of samples / total samples)
Real-time
Preemptible Linux
operating systems
1
Real-time Linux
CFS
0.1
Recap
0.01
0.001
0.0001
1e-05
1e-06
0 1000 2000 3000 4000 5000 6000
Latency (usecs)

8. Analysis of
Vanilla vs. rt-preempt (2) interrupt latencies
in a real-time
kernel
Probability of the latency (num. of samples / total samples)
Real-time
RT Linux
operating systems
1
Real-time Linux
CFS
0.1
Recap
0.01
0.001
0.0001
1e-05
1e-06
0 10 20 30 40 50 60
Latency (usecs)

9. Analysis of
I/O scheduling interrupt latencies
in a real-time
kernel
Real-time
operating systems
Real-time Linux
CFS
In traditional operating systems interrupt handler Recap
have priority above everything.
rt-preempt =⇒ soft and hard irq handlers are
threads:
scheduling I/O ≡ scheduling tasks.

10. Analysis of
Benchmark network throughput interrupt latencies
in a real-time
kernel
Real-time
operating systems
Real-time Linux
CFS
netperf; Recap
UDP traffic;
750B datagrams;
schedule the real-time network threads;
30 minutes execution.

11. Analysis of
Fixed priority scheduling interrupt latencies
in a real-time
kernel
It is not possibile to obtain low latency and high
throughput; Real-time
operating systems
Real-time Linux
Net priority min. lat (µs) max. lat (µs) Tp (Mbps) CFS
1 20 94 38.9 Recap
50 22 96 39.9
80 20 148 77.4
99 22 164 73.4
Table: Linux 2.6.24-rc2-rt1, 750B UDP datagrams
high throughput and high latency;
low throughput and low latency.
Fixed priority scheduling is not suitable =⇒ we look for
a method to assign CPU quota to tasks.

12. Analysis of
Completely Fair Scheduler interrupt latencies
in a real-time
kernel
Real-time
operating systems
Real-time Linux
CFS
Recap
implemented by Ingo Molnar;
merged in mainline since 2.6.23;
weights tasks by ratio;
group scheduling;

13. Analysis of
cfs & rt-preempt interrupt latencies
in a real-time
kernel
My work
Real-time
use CFS and modify the nice system call to set operating systems
weights (> 200 µs); Real-time Linux
CFS
use CFS and tune Control Groups (> 1000 µs).
Recap
Results
net cyc load lat (µs) Tp (Mbps)
1 1 1 2931 49.9
2 1 1 3911 67.3
2 2 1 3721 71.1
4 2 1 6117 80.9
1 1 4 2871 35.2
Table: Scheduling real-time threads with CFS

14. Analysis of
cfs & rt-preempt interrupt latencies
in a real-time
kernel
Real-time
operating systems
Real-time Linux
CFS
impact on the I/O throughput =⇒ the ratio system
Recap
works;
high latencies.
Short time possible solutions
CFS clean up and tuning (in progress);
EDF scheduling + time limit (Linux 2.6.25).

15. Analysis of
Overview of the work interrupt latencies
in a real-time
Step 1 kernel
litterature study on real-time computing;
Real-time
study of the rt-preempt patch and real-time under Linux; operating systems
Real-time Linux
Step 2 CFS
Recap
benchmark real-time performance;
study the relationship between I/O and real-time
applications;
Step 3
test the CFS scheduler;
try custom and standard solutions;
Future work
results presented at the RTAS 2008 conference
(http://www.rtas.org/ );
tuning CFS and testing new scheduling policies.

16. Analysis of
Questions interrupt latencies
in a real-time
kernel
Real-time
operating systems
Real-time Linux
CFS
Recap
Any question?