Strict consistency vs atomic consistency

Question

I have read a couple of articles and I am confused about the difference between strict consistency (which is defined as "It can be better understood as though a global clock is present in which every write should be reflected in all processor caches by the end of that clock period.") and atomic consistency (or linearizability, which is defined as "sequential consistency with the real-time constraint"). Both definitions come from Wikipedia. The source of my confusion is the fact that the strict model provides that every process see a change immediately and atomic consistency is also said to work in real-time providing the same sequence of writes for every process.

Answer 1

The requirement for maintaining a system-wide distribution of global clock for a strict consistency case is hopefully clean enough by itself.

The atomic consistency thus needs some more warranties, in exchange to not maintaining the global clock, to still become and stay consistent system-wide.

Here comes useful the warranty from HRT-system, as it keeps the sequential consistency within its realm of deterministic, a-priori known finite time. Thus the state-change propagation planning is possible and holds throughout the whole life-cycle of the HRT-system operation.

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On "sequential consistency with the real-time constraint" :

This option ought be understood as a technically less strict, yet for maintaining the system-wide consistency-goal sufficient enough ( see determinism + known deadline below ), not having a need to guarantee a system-wide distribution of a uniform clock.

For touching what the "real-time constraint" actually is useful for, let me borrow ( incl. original typos, accents added ) from a book from Giovanni Di Sirio on Real-Time Operating System (RTOS) disambiguation :

<sub>What an RTOS is

An RTOS is an operating system whose internal processes are guaranteed to be compliant with (hard or soft) realtime requirements. The fundamental qualities of an RTOS are:

- Predictable. It is the quality of being predictable in the scheduling behavior.
- Deterministic. It is the quality of being able to consistently produce the same results under the same conditions.

RTOS are often confused with “fast” operating systems. While efficiency is a positive attribute of an RTOS, efficiency alone does not qualifies an OS as RTOS but it could separate a good RTOS from a not so good one.</sub>

A deciding factor is the (un-)certainty of completing each work-unit within a(n un-)known deadline :

“A non real time system is a system where the programmed reaction to an event will certainly happen sometime in the future”.

Whereas :

Soft Real Time.
A Soft Real Time (SRT) system is a system where not meeting a deadline can have undesirable but not catastrophic effects, a performance degradation for example. Such systems could be described as follow :

“A soft real time system is a system where the programmed reaction to an event is almost always completed within a known finite time”.

Hard Real Time.
An Hard Real Time (HRT) system is a system where not meeting a deadline can have catastrophic effects. Hard realtime systems require a much more strict definition and could be described as follow :

“An hard real time system is a system where the programmed reaction to an event is guaranteed to be completed within a known finite time”.