I still don't get what PC exactly means.
On Wikipedia, it says 'A system exhibits Processor Consistency if the order in which other processors see the writes from any individual processor is the same as the order they were issued'. Is it the same thing as the definition given in this slide? If the answer is yes, why the PC results of example 4 in the next slide don't match that of sequential consistency?
kayvonf
Yes, they are consistent. All the following statement: "A system exhibits Processor Consistency if the order in which other processors see the writes from any individual processor is the same as the order they were issued" says is that writes are not reordered (That W->W ordering is respected)
Processor consistency relaxes read after write (W->R) ordering. See the following from Wikipedia:
"Processor Consistency (PC) relaxes the ordering between older stores and younger loads that is enforced in Sequential consistency (SC). This allows loads to be issued to the cache and potentially complete before older stores, meaning that stores can be queued in a write buffer without the need for load speculation to be implemented (the loads can continue freely)"
Unlike TSO, PC does not provide a guarantee that all processors receive notification that a write has committed at the same time. If other words, if processor P2 receiving notification of a write from P1, there is no gaurantee that P3 is aware of this write yet.
kayvonf
However, the big point of the lecture today was to get you to think about the effects of a relaxed consistency model. We used TSO and PC was two examples of relaxed consistency schemes (W->R was relaxed) in order to work through some exercises. However, I do not consider remembering the details of TSO and PC (beyond relaxing W->R ordering) the intellectually interesting part of the lecture. Even though we do not have an exam in this course, the details of specific relaxed consistency approaches would not be information that I would ever choose to put on an exam.
I still don't get what PC exactly means. On Wikipedia, it says 'A system exhibits Processor Consistency if the order in which other processors see the writes from any individual processor is the same as the order they were issued'. Is it the same thing as the definition given in this slide? If the answer is yes, why the PC results of example 4 in the next slide don't match that of sequential consistency?
Yes, they are consistent. All the following statement: "A system exhibits Processor Consistency if the order in which other processors see the writes from any individual processor is the same as the order they were issued" says is that writes are not reordered (That W->W ordering is respected)
Processor consistency relaxes read after write (W->R) ordering. See the following from Wikipedia:
"Processor Consistency (PC) relaxes the ordering between older stores and younger loads that is enforced in Sequential consistency (SC). This allows loads to be issued to the cache and potentially complete before older stores, meaning that stores can be queued in a write buffer without the need for load speculation to be implemented (the loads can continue freely)"
Unlike TSO, PC does not provide a guarantee that all processors receive notification that a write has committed at the same time. If other words, if processor P2 receiving notification of a write from P1, there is no gaurantee that P3 is aware of this write yet.
However, the big point of the lecture today was to get you to think about the effects of a relaxed consistency model. We used TSO and PC was two examples of relaxed consistency schemes (W->R was relaxed) in order to work through some exercises. However, I do not consider remembering the details of TSO and PC (beyond relaxing W->R ordering) the intellectually interesting part of the lecture. Even though we do not have an exam in this course, the details of specific relaxed consistency approaches would not be information that I would ever choose to put on an exam.