[LITMUS^RT] RT-litmus behaviour under real-workloads

[Björn Brandenburg]

> On 20. Feb 2018, at 05:46, Ashraf E. Suyyagh <mrsuyyagh at gmail.com> wrote:
> 
> 2. As for the wrong period reported when it becomes too large, after investigating, this problem is expected in all versions including the recent ones. This has to do with the struct st_param_data because in there, the period and WCET are defined as u32 type. Since all timing is in nanoseconds, all execution times and periods exceeding the int type capacity will be wrongly represented. However, since internally litmus_rt WCET and periods are of type lt_t (long long), my expectation is that internally Litmus will work correctly. To correct the issue, a complete rework and resizing of the event record struct and size is needed if one wishes to see a correct large period/WCET displayed in st-job-stats. This might not be a priority, but worth noting.

This is a conscious tradeoff. 2^32 nanoseconds correspond to roughly 4294ms, or almost 4.3 seconds. This should be large enough a period for just about any interesting RT workload. It’s not worth increasing the trace record size, which would affect everyone, for the few edge cases with tasks with a period longer than 4 seconds.

- Björn 
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