concurrency - Example of execution which is sequentially consistent but not quiescently consistent -


in context of correctness of concurrent programs, sequential consistency stronger condition quiescent consistency according art of multiprocessor programming maurice herlihy , nir shavit(chapter 3) authors mention in 3.4.1 there sequentially consistent executions not quiescently consistent. don't understand how. throw light or provide sample execution ?

consider queue (fifo) enqueue , dequeue elements.

from this dissertation concurrency, read (p.20):

an example of scenario allowed in sequential consistency model , disallowed in quiescent consistency model shown in figure 2.1. 2 processes share concurrent queue data structure. first process enqueues x. @ non-overlapping subsequent interval, second process enqueues y. second process performs dequeue , receives y. example sequentially consistent not quiescently consistent, assuming time between enqueue operations falls outside quiescence interval.

figure 2.1:

t1:  --- enq(x) --------------------------- t2:  ------------- enq(y) ---- deq():y ----

this history permitted sequential consistency, can either permitted or forbidden quiescent consistency, , forbidden linearizable consistency.

if assume between 2 enqueue queue quiescent, t2 should see changes t1, , dequeue should return x. if assume there no quiescent interval between 2 enqueues, 2 enqueues can reorderd wish, , deq():y consistent.


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