5 ms·

I thought that if one exceeded 70% utilization on a real-time system, one lost guarantees of being real-time?

jacquesm 11y ago
That depends. As long as you don't miss any interrupts and as long as you manage to fulfill your latency promises it can work, but the higher the load the closer to the abyss you'll be walking.
- deleted 11y ago
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  - jacquesm 11y ago
    > Is the thermal envelope really that tight? Not an automotive developer but I've done quite a bit of embedded stuff that was placed in/on various items including cars. Yes, it is that tight. You're in a sealed container under the hood of a car, the ambient temperatures can be quite high and you're not going to be able to dump any of the heat you generate outside of the box you're in other than through conduction. Those boxes are typically made of ABS so conduction will be terrible. Of course you could re-design the enclosures, have a heatsink mount point on the outside (some expensive trickery required to do that without breaking the sealed environment), or even an air duct that would allow you to use a fan (but fans tend to fail). So I can totally see why the constraint on power consumption for such an embedded system would be so drastic.
brandmeyer 11y ago
More detail on the "it depends". The guarantees are of the form "if the utilization is less than X%, then an arbitrary collection of periodic tasks can be scheduled without missing deadlines". The key term there is "arbitrary". For example, rate monotonic scheduling provides this guarantee up to ~70% utilization. However, manually scheduled tasks can walk up into the 90's. A UPS application I worked on scheduled all of its periodic tasks manually by ensuring that each slower rate task had a rate that was always an integer divisor of (and lower preemptable priority than) the next higher rate task. This proceeded all the way until you get down to much slower rate periodic "housekeeping" tasks that weren't even realtime.
- OopsCriticality 11y ago
  Ahh, that clears it up for me… nice example too. Thanks!

That depends. As long as you don't miss any interrupts and as long as you manage to fulfill your latency promises it can work, but the higher the load the closer to the abyss you'll be walking.

deleted 11y ago
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- jacquesm 11y ago
  > Is the thermal envelope really that tight? Not an automotive developer but I've done quite a bit of embedded stuff that was placed in/on various items including cars. Yes, it is that tight. You're in a sealed container under the hood of a car, the ambient temperatures can be quite high and you're not going to be able to dump any of the heat you generate outside of the box you're in other than through conduction. Those boxes are typically made of ABS so conduction will be terrible. Of course you could re-design the enclosures, have a heatsink mount point on the outside (some expensive trickery required to do that without breaking the sealed environment), or even an air duct that would allow you to use a fan (but fans tend to fail). So I can totally see why the constraint on power consumption for such an embedded system would be so drastic.

brandmeyer 11y ago

More detail on the "it depends". The guarantees are of the form "if the utilization is less than X%, then an arbitrary collection of periodic tasks can be scheduled without missing deadlines". The key term there is "arbitrary". For example, rate monotonic scheduling provides this guarantee up to ~70% utilization. However, manually scheduled tasks can walk up into the 90's. A UPS application I worked on scheduled all of its periodic tasks manually by ensuring that each slower rate task had a rate that was always an integer divisor of (and lower preemptable priority than) the next higher rate task. This proceeded all the way until you get down to much slower rate periodic "housekeeping" tasks that weren't even realtime.

OopsCriticality 11y ago
Ahh, that clears it up for me… nice example too. Thanks!