Multiple copy scoping bits for cache memory

Patent metadata

Field	Value
Publication number	US-11487672-B1
Application number	US-202117407228-A
Country	US
Kind code	B1
Filing date	Aug 20, 2021
Priority date	Aug 20, 2021
Publication date	Nov 1, 2022
Grant date	Nov 1, 2022

Abstract

Aspects of the invention include computer-implemented methods, systems, and computer program products that access a multi-copy scope directory state of a cache memory that indicates a scope of sharing of a cache line in a cache memory system and determine a scope of sharing of the cache line in the cache memory system based on the multi-copy scope directory state, where the multi-copy scope directory state enumerates a plurality of scopes within the cache memory system. The scope of sharing is used to reduce a number of queries to one or more cache memories having a larger scope than a shared scope identified in the scope of sharing. The multi-copy scope directory state of the cache memory is updated based on detecting a change in shared scope of the cache line within the cache memory system.

First claim

What is claimed is:

1. A computer-implemented method comprising:
accessing a multi-copy scope directory state of a cache memory that indicates a scope of sharing of a cache line in a cache memory system;
determining a scope of sharing of the cache line in the cache memory system based on the multi-copy scope directory state, wherein the multi-copy scope directory state enumerates a plurality of scopes within the cache memory system;
using the scope of sharing to reduce a number of queries to one or more cache memories having a larger scope than a shared scope identified in the scope of sharing; and
updating the multi-copy scope directory state of the cache memory based on detecting a change in shared scope of the cache line within the cache memory system.

2. The computer-implemented method of claim 1 , wherein the scope of sharing having a smaller scope is a subset of the scope of sharing having the larger scope within the cache memory system.

3. The computer-implemented method of claim 2 , wherein the scope of sharing comprises a chip scope as a smallest scope, a module scope, a drawer scope, and a system scope as a largest scope.

4. The computer-implemented method of claim 1 , further comprising:
setting a snoop scope to a smallest scope based on determining that a requesting cache has requested a shared copy of the cache line;
sending a query to one or more remote cache memories in a current snoop scope that have not already been sent a query;
determining whether an intervention master copy of the cache line is found in a remote cache memory of the current snoop scope or encountering the cache line in a forward state; and
installing a shared copy of the cache line in the requesting cache and updating a state of a directory entry holding a master copy of the cache line based on the intervention master copy of the cache line being found in a remote cache memory of the current snoop scope or encountering the cache line in the forward state.

5. The computer-implemented method of claim 4 , further comprising:
fetching the cache line from the cache memory and updating a requesting cache directory based on one or more copies of the cache line being found in a shared non-master state after not finding the cache line in an initial snoop scope; and
updating at least one remote sourcing directory of the cache line.

6. The computer-implemented method of claim 1 , further comprising:
determining whether a requesting cache has an intervention master shared copy of the cache line based on the requesting cache requesting an exclusive copy of the cache line;
requesting invalidation of the cache line in all cache memories in a smallest snoop scope that contains a directory scope; and
updating a requesting cache directory for the cache line in a requesting core to an exclusive state.

7. The computer-implemented method of claim 6 , further comprising:
setting a current snoop scope to the smallest snoop scope based on determining that the requesting cache does not have the intervention master shared copy of the cache line;
sending a query to one or more remote caches in the current snoop scope that have not already been sent a query;
updating a remote cache directory to an invalid state based on a shared, not intervention master copy of the cache line in the current snoop scope;
updating the requesting cache directory for the cache line to the exclusive state based on determining that an intervention master copy of the cache line is not found in a remote cache in the current snoop scope and a highest snoop scope has been reached; and
setting the current snoop scope to a next larger snoop scope based on determining that the intervention master copy of the cache line is not found in the remote cache in the current snoop scope and the highest snoop scope has not been reached.

8. The computer-implemented method of claim 7 , further comprising:
updating the remote cache directory for the cache line to the invalid state, and installing the exclusive copy of the cache line in the exclusive state based on finding the remote cache with the exclusive copy of the cache line in the current snoop scope and determining that the intervention master copy of the cache line is not found in the remote cache in the current snoop scope; and
updating the remote cache directory for the cache line to the invalid state, and updating the requesting cache directory for the cache line to the exclusive state based on finding the remote cache with the intervention master shared copy of the cache line in the current snoop scope, and the directory scope in the remote cache is less than or equal to the current snoop scope.

9. The computer-implemented method of claim 8 , further comprising:
updating the remote cache directory for the cache line to the invalid state, and sending a query to the remote caches in the smallest snoop scope larger than the directory scope in the remote cache that have not already been sent the query based on the directory scope in the remote cache being larger than the current snoop scope;
updating the remote cache directory for the cache line to the invalid state; and
updating the requesting cache directory for the cache line to the exclusive state.

10. The computer-implemented method of claim 7 , further comprising:
using the intervention master copy of the cache line prior to a full system state being resolved based on detecting that a primary scope does not contain the intervention master copy of the cache line, a secondary scope does contain the intervention master copy of the cache line, and a larger scope broadcast has been initiated.

11. A system comprising:
a plurality of processors;
a cache memory system; and
a cache controller configured to perform operations comprising:
accessing a multi-copy scope directory state of a cache memory that indicates a scope of sharing of a cache line in the cache memory system;
determining a scope of sharing of the cache line in the cache memory system based on the multi-copy scope directory state, wherein the multi-copy scope directory state enumerates a plurality of scopes within the cache memory system;
using the scope of sharing to reduce a number of queries to one or more cache memories having a larger scope than a shared scope identified in the scope of sharing; and
updating the multi-copy scope directory state of the cache memory based on detecting a change in shared scope of the cache line within the cache memory system.

12. The system of claim 11 , wherein the scope of sharing having a smaller scope is a subset of the scope of sharing having the larger scope within the cache memory system.

13. The system of claim 12 , wherein the scope of sharing comprises a chip scope as a smallest scope, a module scope, a drawer scope, and a system scope as a largest scope.

14. The system of claim 11 , wherein the cache controller is configured to perform operations comprising:
setting a snoop scope to a smallest scope based on determining that a requesting cache has requested a shared copy of the cache line;
sending a query to one or more remote cache memories in a current snoop scope that have not already been sent a query;
determining whether an intervention master copy of the cache line is found in a remote cache memory of the current snoop scope or encountering the cache line in a forward state; and
installing a shared copy of the cache line in the requesting cache and updating a state of a directory entry holding a master copy of the cache line based on the intervention master copy of the cache line being found in a remote cache memory of the current snoop scope or encount

Assignees

• IBM

Inventors

• Rhee Chunggeon
• Walters Craig R
• Bamdhamravuri Ram Sai Manoj
• Bronson Timothy
• Alexander Gregory William

Classifications

• G06F2212/1024

  Latency reduction · CPC title

• G06F12/0822

  Copy directories (local copy tags for implementing a bus snooping protocol G06F12/0831) · CPC title

• G06F12/0826

  Limited pointers directories; State-only directories without pointers · CPC title

• G06F12/084

  with a shared cache · CPC title

• G06F12/0813 Primary

  with a network or matrix configuration · CPC title