[LITMUS^RT] Question from experimenting with Litmus-RT Performance

[Yu-An(Victor) Chen]

Hi,

I am doing some experiment with rt-xen and litmus-rt. What I am trying to
do is to see the schedubility of real time tasks of 1vm while the other vm
is fully utilized. Both guest VMs use litmus-rt.

The setup is the following:

Using Xen 4.5.0.
1.  2vm sharing core 0-7 ( both vm can access core0-7) , with RTDS
scheduler, both has period of 4000us and budget of 2000(us)
2.  Dom0 using one core from CPU 8-15, with RTDS scheduler, period of
10000us and budget of 10000us
3.  both guest vm have ubuntu 12.04 and are using "litmus-rt-2014.2.patch"
and with Geoffrey's patch for IPI interrupt (https://github.com/LITMUS-RT
/liblitmus/pull/1/files)

the taskset is generated as followed:

a taskset is composed of a collection of real-time tasks, and each
real-time task is a sequence of jobs that are released periodically... All
jobs are periodic, where each job Ti is defined by a period (and deadline) p
i and a worse-case execution time ei, with pi ≥ ei ≥ 0 and pi, ei ∈ integers.
Each job is comprised of a number of iterations of floating point
operations during each job. This is based on the base task.c provided with
the LITMUSRT userspace library.

The period for a task is from uniform distribution (10ms,100ms)
and the utilization rate of a task is also from a uniform distribution
(0.1,0.4)

In my experiment:

Step0: disable networking and other unused services.
Step1: I loaded VM#2 with constant running task with total utilization of 4
cores.
Step2: In VM#1 I many run iterations of tasksets from total utilization
rate 0.2 cores all the way to 4.6 and record their schedulbility using
st_trace.

In my results, I do see that schedulbility do drop to zeros at either total
util rate of 4.2 or 4.4. (we use worst-case execution time for benchmarking
the base amount of computation that is why it takes more than total util
rate of 4 for schedubility to drop to 0)

What puzzle me is why are there two groups of results as shown in the
attached graph?(one group that has schedubility of 0 at total util rate of
4.2 and another group that has schedubility of 0 at total util rate of 4.4)
 ( I used " * " in the legend to indicate the groups)

Shouldn't each run should be somewhat close to each other or showing
randomness instead of seeing the two groups of performance curves?

I wonder if my base computation is wrong but that still does not explain
why they are two types of performance curves.

Any advice or suggestion on how I can go about this will be helpful!

Thank you!

Victor
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.litmus-rt.org/pipermail/litmus-dev/attachments/20160111/dcfcfd3e/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: medium-moderate-tasksets.pdf
Type: application/pdf
Size: 8894 bytes
Desc: not available
URL: <http://lists.litmus-rt.org/pipermail/litmus-dev/attachments/20160111/dcfcfd3e/attachment.pdf>