Is it true that Sequential Consistency is a stronger property than Cache Coherence?
Sorin, Daniel J; Hill, Mark D; Wood, David A: A Primer on Memory Consistency and Cache Coherence, Morgan & Claypool, 2011
sequential consistency can be described as (not formally):
Sequential consistency memory model specifies that the system must appear to execute all threads’ loads and stores to all memory locations in a total order that respects the program order of each thread. Each load gets the value of the most recent store in that total order.
In other words, system is sequentially consistent, if given memory events (loads and stores) of each thread we can order all these events such that: 1) for each thread the order of its events is preserved, and 2) the global order is serial (any load returns latest value stored).
Now they continue and describe coherence:
A definition of coherence that is analogous to the definition of Sequential Consistency is that a coherent system must appear to execute all threads’ loads and stores to a single memory location in a total order that respects the program order of each thread.
In other words, system is coherent, if given memory events of each thread for each location we can order events for that location, such that: 1) for each thread the order of its events to that location is preserved, and 2) for each location the order is serial.
Finally, they point out the difference:
This definition highlights an important distinction between coherence and consistency: coherence is specified on a per-memory location basis, whereas consistency is specified with respect to all memory locations.
So it seems the difference is that for coherent systems we need a total order on all events for each location (thus the ordering between events for particular location), while for consistent systems the total order should be defined on the all events (and thus the ordering is also between events for different locations)?
Does that mean that coherence is less strict that consistency? (which seems amusing!) Are there traces that are coherent but not consistent?