News for hackers. Stuff that matters.

A comment on Arm Announces Cortex-R82: First 64-Bit Real Time Processor. Parent comment

Re:Arm Announces Cortex-R82: First 64-Bit Real Time Processor by SAI_Peregrinus on Monday September 07, 2020 @06:57PM (#24403637)
"realtime" means "latency is bounded to some documented value". "Hard realtime" means it never exceeds that value, and has the same latency every time. "Soft realtime" means it never exceeds that value but the actual latency varies.

That value doesn't have to be anything in particular. Could be 1ms. Could be 100ms. Could be a year. It just has to exist and be documented, and never, ever be exceeded by whatever operations are covered by its guarantee.

[ Parent | on HN ]
Re:Arm Announces Cortex-R82: First 64-Bit Real Time Processor by wtallis on Monday September 07, 2020 @07:25PM (#24403782)
Your description of hard vs soft realtime doesn't match any that I've previously encountered.

Generally, hard realtime means missing a deadline in unacceptable, but does not necessarily imply that latency is exactly fixed—just bounded. Soft realtime means that the consequences of missing a deadline are less severe, so the system may allow for some missed deadlines: eg. dropped packets for a video conference or delayed frames for a video game are noticeable especially if they become common, but they don't crash the whole system and the system remains useful even after missing a deadline.

[ Parent | on HN ]
Re:Arm Announces Cortex-R82: First 64-Bit Real Time Processor by snvzz on Monday September 07, 2020 @10:39PM (#24404766)
>and has the same latency every time.

There's no such requirement.

All hard realtime needs is the guarantee that the value will never be exceeded.

[ Parent | on HN ]
Re:Arm Announces Cortex-R82: First 64-Bit Real Time Processor by SAI_Peregrinus on Tuesday September 08, 2020 @11:20AM (#24409270)
Right. Should have written "same maximum every time". Went stupid for a minute.
[ Parent | on HN ]