Centralized management of high-contention cache lines in multi-processor computing environments
Cache lines in a multi-processor computing environment are configurable with a coherency mode. Cache lines in full-line coherency mode are operated or managed with full-line granularity. Cache lines in sub-line coherency mode are operated or managed as sub-cache line portions of a full cache line. Communications detected on a coherence interconnect may indicate that a cache line is associated with performance-reducing events. A high-contention cache line may be placed in sub-line coherency mode. Caches accessing the cache line are notified that the cache line is in sub-line coherency mode. The cache line may be associated with a counter in a centralized detection table that is incremented based on detecting the communications. The cache line may be a high-contention cache line when the counter satisfies a high-contention criterion, such as reaching a threshold value. The cache line may be returned to full-line coherency mode when a reset criterion is satisfied.
1. A method for cache management in a multi-processor computing environment, the computing environment including a plurality of caches configured to hold a plurality of cache lines, each cache line configurable with a coherency mode, the plurality of caches associated with a coherence interconnect, the method comprising:
monitoring, with a centralized detector, communications on the coherence interconnect;
detecting a first communication associated with a first cache line in the monitored communications, the first communication indicating an access of the first cache line resulting in a first performance-reducing event, the first cache line in a full-line coherency mode, wherein cache control logic associated with the first cache line is configured to operate or manage the first cache line in the full-line coherency mode with a full-line granularity;
based on the detecting, determining that the first cache line is a high-contention cache line;
based on the determining, placing the first cache line in a sub-line coherency mode, wherein the cache control logic is configured to operate or manage a plurality of sub-cache line portions of the first cache line in the sub-line coherency mode; and
notifying caches accessing the first cache line that the first cache line is in the sub-line coherency mode.
2. The method of claim 1 , wherein the first communication is a signal on the coherence interconnect, the signal raised in response to detecting a conflicting access of the first cache line while executing a transaction in the computing environment, wherein the performance-reducing event is a transactional abort.
3. The method of claim 1 , wherein the first communication is a message on the coherence interconnect, the message sent in response to detecting an access of the first cache line, the first cache line previously invalidated by a coherence event, and wherein the performance-reducing event is a coherence request.
4. The method of claim 1 , wherein the notifying comprises:
observing a request for the first cache line on the coherence interconnect; and
raising a signal on the coherence interconnect in response to the observing, the signal indicating that the first cache line is in the sub-line coherency mode.
5. The method of claim 1 , wherein the notifying comprises:
observing a request for the first cache line on the coherence interconnect; and
modifying the request for the first cache line to indicate that the first cache line is in the sub-line coherency mode.
6. The method of claim 1 , wherein the determining that the first cache line is the high-contention cache line comprises:
incrementing a counter in an entry in a detection table, the entry associating the counter with the first cache line; and
determining that the incremented counter satisfies a high-contention criterion.
7. A computer system for cache management in a multi-processor computing environment, the computing environment including a plurality of caches configured to hold a plurality of cache lines, each cache line configurable with a coherency mode, the plurality of caches associated with a coherence interconnect, the computer system comprising:
a memory; and
a processor in communication with the memory, wherein the computer system is configured to perform a method, the method comprising:
monitoring, with a centralized detector, communications on the coherence interconnect;
detecting a first communication associated with a first cache line in the monitored communications, the first communication indicating an access of the first cache line resulting in a first performance-reducing event, the first cache line in a full-line coherency mode, wherein cache control logic associated with the first cache line is configured to operate or manage the first cache line in the full-line coherency mode with a full-line granularity;
based on the detecting, determining that the first cache line is a high-contention cache line;
based on the determining, placing the first cache line in a sub-line coherency mode, wherein the cache control logic is configured to operate or manage a plurality of sub-cache line portions of the first cache line in the sub-line coherency mode; and
notifying caches accessing the first cache line that the first cache line is in the sub-line coherency mode.
8. The computer system of claim 7 , wherein the determining that the first cache line is the high-contention cache line comprises:
incrementing a counter in an entry in a detection table, the entry associating the counter with the first cache line; and
determining that the incremented counter satisfies a high-contention criterion.
9. The computer system of claim 8 , wherein the determining that the incremented counter satisfies the high-contention criterion comprises:
determining that the incremented counter has reached a threshold value.
10. The computer system of claim 8 , wherein the determining that the incremented counter satisfies the high-contention criterion comprises:
calculating a percentage, the percentage based on the incremented counter and a total number of accesses of cache lines resulting in performance-reducing events in the computing environment; and
determining that the percentage has reached a threshold value.
11. The computer system of claim 8 , wherein the method further comprises:
determining that a reset criterion associated with the counter is satisfied;
resetting the counter based on the determining that the reset criterion is satisfied; and
placing the first cache line in the full-line coherency mode based on the determining that the reset criterion is satisfied.
12. The computer system of claim 11 , wherein the reset criterion is selected from the group consisting of:
a period of time elapses;
a number of transactions complete;
a number of instructions complete; and
a cache line associated with the counter is cast out of the cache.
13. The computer system of claim 8 , wherein the entry further associates the counter with at least one cache line adjacent to the first cache line.
14. The computer system of claim 8 , wherein the entry further associates the counter with a hash value of a memory address associated with the first cache line.
15. A computer program product for cache management in a multi-processor computing environment, the computing environment including a plurality of caches configured to hold a plurality of cache lines, each cache line configurable with a coherency mode, the plurality of caches associated with a coherence interconnect, the computer program product comprising:
a non-transitory computer readable storage medium readable by a processing circuit and storing instructions for execution by the processing circuit for performing a method, the method comprising:
monitoring, with a centralized detector, communications on the coherence interconnect;
detecting a first communication associated with a first cache line in the monitored communications, the first communication indicating an access of the first cache line resulting in a first performance-reducing event, the first cache line in a full-line coherency mode, wherein cache control logic associated with the first cache line is configured to operate or manage the first cache line in the full-line coherency mode with a full-line granularity;
based on the detecting, determining that the first cache line is a high-contention cache line;
based on the determining, placing the first cache line in a sub-line coherency mode, wherein the cache control logic is configured to operate or manage a plurality of sub-cache line portions of the first cache line in the sub-line coherency mode; and
notifying caches accessing the first cache line that the first cache line is in the sub-line coherency mode.
16. The computer program product of claim 15 , wherein the first communication is a signal on the coherence interconnect, the signal raised in response to detecting a conflicting access of the first cache line while executing a transaction in the computing environment, wherein the performance-reducing event is a transactional abort.
17. The computer program product of claim 15 , wherein the first communication is a message on the coherence interconnect, the message sent in response to detecting an access of the first cache line, the first cache line previously invalidated by a coherence event, and wherein the performance-reducing event is a coherence request.
18. The computer program product of claim 15 , wherein the determining that the first cache line is the high-contention cache line comprises:
incrementing a counter in an entry in a detection table, the entry associating the counter with the first cache line; and
determining that the incremented counter satisfies a high-contention criterion.
19. The computer program product of claim 18 , wherein the entry further associates the counter with at least one cache line adjacent to the first cache line.
20. The computer program product of claim 18 , wherein the entry further associates the counter with a hash value of a memory address associated with the first cache line.