INVOKING DEMOTE THREADS ON PROCESSORS TO DEMOTE TRACKS INDICATED IN DEMOTE READY LISTS FROM A CACHE WHEN A NUMBER OF FREE CACHE SEGMENTS IN THE CACHE IS BELOW A FREE CACHE SEGMENT THRESHOLD

• US 20170351618A1
• Filed: 06/06/2016
• Published: 12/07/2017
• Est. Priority Date: 06/06/2016
• Status: Active Grant

• Current Assignee: International Business Machines Corporation
• Original Assignee: International Business Machines Corporation
• Inventors: Ash, Kevin J., Borlick, Matthew G., Gupta, Lokesh M., Nguyen, Trung N.
• Granted Patent: US 9,952,982 B2
• Time in Patent Office: Days
• CPC Class Codes: G06F 12/0804 with main memory updating G06F12/0806 takes precedence
• CPC Class Codes: G06F 12/12 Replacement control
• CPC Class Codes: G06F 12/122 of the least frequently used [LFU] type, e.g. with individual count value
• CPC Class Codes: G06F 12/123 with age lists, e.g. queue, most recently used [MRU] list or least recently used [LRU] list
• CPC Class Codes: G06F 12/124 being minimized, e.g. non MRU
• CPC Class Codes: G06F 12/128 adapted to multidimensional cache systems, e.g. set-associative, multicache, multiset or multilevel
• CPC Class Codes: G06F 2212/1024 Latency reduction
• CPC Class Codes: G06F 2212/462 Track or segment
• CPC Class Codes: G06F 2212/70 Details relating to dynamic memory management

Abstract

Provided are a computer program product, system, and method for invoking demote threads on processors to demote tracks from a cache. A plurality of demote ready lists indicate tracks eligible to demote from the cache. In response to determining that a number of free cache segments in the cache is below a free cache segment threshold, a determination is made of a number of demote threads to invoke on processors based on the number of free cache segments and the free cache segment threshold. The determined number of demote threads are invoked to demote tracks in the cache indicated in the demote ready lists, wherein each invoked demote thread processes one of the demote ready lists to select tracks to demote from the cache to free cache segments in the cache.

Claims

1. A computer program product for managing tracks in a storage in a cache, the computer program product comprising a computer readable storage medium having computer readable program code embodied therein that when executed performs operations, the operations comprising:

providing a plurality of demote ready lists indicating tracks eligible to demote from the cache; and

in response to determining that a number of free cache segments in the cache is below a free cache segment threshold, performing;

determining a number of demote threads to invoke on processors based on the number of free cache segments and the free cache segment threshold; and

invoking the determined number of demote threads to demote tracks in the cache indicated in the demote ready lists, wherein each invoked demote thread processes one of the demote ready lists to select tracks to demote from the cache to free cache segments in the cache.

2. The computer program product of claim 1, wherein the determining the number of demote threads to invoke based on whether the number of free cache segments is below the free cache segment threshold comprises:

determining to invoke one demote thread for each of all of the demote ready lists in response to determining that the number of free cache segments in the cache is below the free cache segment threshold; and

determining to invoke one demote thread for less than all of the demote ready lists in response to determining that the number of free cache segments is greater than the free cache segment threshold.

3. The computer program product of claim 1, wherein the free cache segment threshold comprises a first free cache segment threshold, wherein the determining the number of demote threads to invoke based on the free cache segment threshold comprises:

determining to invoke one demote thread for each of all of the demote ready lists in response to determining that the number of free cache segments in the cache is below the first free cache segment threshold; and

determining to invoke a plurality of demote threads for a plurality of demote ready lists less than all of the demote ready lists in response to determining that the number of free cache segments is greater than the first free cache segment threshold and less than a second free cache segment threshold, wherein the second free cache segment threshold is greater than the first free cache segment threshold.

4. The computer program product of claim 3, wherein a number of the demote threads invoked in response to determining that the number of free cache segments is greater than the first free cache segment threshold and less than the second free cache segment threshold extends between one and a total number of demote ready lists and is a function of the number of free cache segments and the first and second free cache segment thresholds.

5. The computer program product of claim 3, wherein the determining the number of demote threads to invoke comprises:

determining to invoke one demote thread to process one of the demote ready lists in response to determining that the number of free cache segments is greater than the second free cache segment threshold and less than a third free cache segment threshold; and

determining to invoke no demote threads to demote tracks in response to determining that the number of free cache segments is greater than the third free cache segment threshold.

6. The computer program product of claim 1, wherein there is one lock for each of the demote ready lists, wherein each of the demote threads invoked to process one of the demote ready lists obtains the lock for the demote ready list to determine tracks indicated in the demote ready list to demote from the cache.

7. The computer program product of claim 1, wherein the operations further comprise:

incrementing the number of free cache segments in response to processing one of the demote ready lists to demote one of the tracks from the cache; and

decrementing the number of free cache segments in response to allocating at least one cache segment to one of the tracks added to the cache.

8. The computer program product of claim 1, wherein there is one of the demote ready lists associated with each of a plurality of processor cores, wherein each processor core includes a plurality of independently executing processors to execute the demote thread to process the demote ready list associated with the processor core, to execute at least one Input/Output (I/O) thread to process I/O requests with respect to tracks in the storage, and to execute a demote scan thread to process an active cache list associated with the processor core to move indication of unmodified tracks in the active cache list to the demote ready list.

9. The computer program product of claim 1, wherein the operations further comprise:

executing by one of the processors a demote scheduler task, wherein the demote scheduler task performs the determining whether the number of free cache segments is below the free cache segment threshold, the determining the number of demote threads to invoke, and the invoking the determined number of demote threads.

10. A system for managing tracks in a storage, comprising:

a memory including a cache storing tracks from the storage; and

a plurality of processors, wherein each of the processors maintains a demote ready list indicating tracks eligible to demote from the cache, and wherein one of the processors executes a demote scheduler task to perform operations, the operations comprising;

in response to determining that a number of free cache segments in the cache is below a free cache segment threshold, performing;

determining a number of demote threads to invoke on at least one of the processors based on the number of free cache segments and the free cache segment threshold; and

invoking the determined number of demote threads to demote tracks in the cache indicated in the demote ready lists, wherein each invoked demote thread processes one of the demote ready lists to select tracks to demote from the cache to free cache segments in the cache.

11. The system of claim 10, wherein the determining the number of demote threads to invoke based on whether the number of free cache segments is below the free cache segment threshold comprises:

determining to invoke one demote thread for each of all of the demote ready lists in response to determining that the number of free cache segments in the cache is below the free cache segment threshold; and

determining to invoke one demote thread for less than all of the demote ready lists in response to determining that the number of free cache segments is greater than the free cache segment threshold.

12. The system of claim 10, wherein the free cache segment threshold comprises a first free cache segment threshold, wherein the determining the number of demote threads to invoke based on the free cache segment threshold comprises:

determining to invoke one demote thread for each of all of the demote ready lists in response to determining that the number of free cache segments in the cache is below the first free cache segment threshold; and

determining to invoke a plurality of demote threads for a plurality of demote ready lists less than all of the demote ready lists in response to determining that the number of free cache segments is greater than the first free cache segment threshold and less than a second free cache segment threshold, wherein the second free cache segment threshold is greater than the first free cache segment threshold.

13. The system of claim 12, wherein a number of the demote threads invoked in response to determining that the number of free cache segments is greater than the first free cache segment threshold and less than the second free cache segment threshold extends between one and a total number of demote ready lists and is a function of the number of free cache segments and the first and second free cache segment thresholds.

14. The system of claim 12, wherein the determining the number of demote threads to invoke comprises:

determining to invoke one demote thread to process one of the demote ready lists in response to determining that the number of free cache segments is greater than the second free cache segment threshold and less than a third free cache segment threshold; and

determining to invoke no demote threads to demote tracks in response to determining that the number of free cache segments is greater than the third free cache segment threshold.

15. The system of claim 10, wherein there is one lock for each of the demote ready lists, wherein each of the demote threads invoked to process one of the demote ready lists obtains the lock for the demote ready list to determine tracks indicated in the demote ready list to demote from the cache.

16. The system of claim 10, wherein each of the processors comprises a processor core, wherein each processor core includes a plurality of independently executing processors to execute the demote thread to process the demote ready list associated with the processor core, to execute at least one Input/Output (I/O) thread to process I/O requests with respect to tracks in the storage, and to execute a demote scan thread to process an active cache list associated with the processor core to move indication of unmodified tracks in the active cache list to the demote ready list.

17. A method for managing tracks in a storage in a cache, comprising:

providing a plurality of demote ready lists indicating tracks eligible to demote from the cache; and

in response to determining that a number of free cache segments in the cache is below a free cache segment threshold, performing;

determining a number of demote threads to invoke on processors based on the number of free cache segments and the free cache segment threshold; and

invoking the determined number of demote threads to demote tracks in the cache indicated in the demote ready lists, wherein each invoked demote thread processes one of the demote ready lists to select tracks to demote from the cache to free cache segments in the cache.

18. The method of claim 17, wherein the determining the number of demote threads to invoke based on whether the number of free cache segments is below the free cache segment threshold comprises:

determining to invoke one demote thread for each of all of the demote ready lists in response to determining that the number of free cache segments in the cache is below the free cache segment threshold; and

determining to invoke one demote thread for less than all of the demote ready lists in response to determining that the number of free cache segments is greater than the free cache segment threshold.

19. The method of claim 17, wherein the free cache segment threshold comprises a first free cache segment threshold, wherein the determining the number of demote threads to invoke based on the free cache segment threshold comprises:

determining to invoke one demote thread for each of all of the demote ready lists in response to determining that the number of free cache segments in the cache is below the first free cache segment threshold; and

determining to invoke a plurality of demote threads for a plurality of demote ready lists less than all of the demote ready lists in response to determining that the number of free cache segments is greater than the first free cache segment threshold and less than a second free cache segment threshold, wherein the second free cache segment threshold is greater than the first free cache segment threshold.

20. The method of claim 19, wherein a number of the demote threads invoked in response to determining that the number of free cache segments is greater than the first free cache segment threshold and less than the second free cache segment threshold extends between one and a total number of demote ready lists and is a function of the number of free cache segments and the first and second free cache segment thresholds.

21. The method of claim 19, wherein the determining the number of demote threads to invoke comprises:

determining to invoke one demote thread to process one of the demote ready lists in response to determining that the number of free cache segments is greater than the second free cache segment threshold and less than a third free cache segment threshold; and

determining to invoke no demote threads to demote tracks in response to determining that the number of free cache segments is greater than the third free cache segment threshold.

22. The method of claim 17, wherein there is one lock for each of the demote ready lists, wherein each of the demote threads invoked to process one of the demote ready lists obtains the lock for the demote ready list to determine tracks indicated in the demote ready list to demote from the cache.

23. The method of claim 17, wherein there is one of the demote ready lists associated with each of a plurality of processor cores, wherein each processor core includes a plurality of independently executing processors to execute the demote thread to process the demote ready list associated with the processor core, to execute at least one Input/Output (I/O) thread to process I/O requests with respect to tracks in the storage, and to execute a demote scan thread to process an active cache list associated with the processor core to move indication of unmodified tracks in the active cache list to the demote ready list.