Dynamic snoop-filter allocation for large-scale memory coherent system
Aspects of the disclosed technology include techniques and mechanisms for dynamic snoop-filter allocation for large-scale memory coherent systems. A coherency manager on an interconnect manages a directory storing data that indicates a current state of each cache line stored within each caching agent on the interconnect and a number of caching agents that share a cache line. Based on receiving a request from a caching agent to access to a cache line, the coherency manager uses a capacity of the directory, such as the number of caching agents that share a cache line, to determine whether to change an operation mode of a memory from a first mode to a second mode.
1 . A method for switching between modes of memory, the method comprising:
operating the memory in a first mode, wherein the first mode maintains a current state of a cache line in a directory using indices, an index of the directory corresponding to a caching agent storing a copy of the cache line;
comparing a count of caching agents sharing the cache line to a threshold directory capacity, wherein the threshold directory capacity indicates a maximum number of caching agents that can be tracked in the directory;
determining that the count is greater than the threshold directory capacity;
based on determining that the count is greater than the threshold directory capacity, dynamically changing operation of the memory from the first mode to a second mode by generating a bitmap, wherein a bit of the bitmap indicates at least one caching agent within a group of caching agents stores a copy of the cache line; and
operating the memory in the second mode using the bitmap.
2 . The method of claim 1 , further comprising:
receiving, from a first caching agent, a request to access the cache line; and
increasing the count.
3 . The method of claim 2 , further comprising:
determining that the count is less than or equal to the threshold directory capacity; and;
based on determining that the count is less than or equal to the threshold directory capacity, querying the directory using a cache line address to identify a second index of a second caching agent storing the copy of the cache line.
4 . The method of claim 3 , further comprising transmitting a snoop to the second caching agent, the snoop comprising instructions to:
transmit the copy of the cache line to the first caching agent;
invalidate the copy of the cache line; or
change a state of the cache line.
5 . The method of claim 3 , further comprising:
determining, based on querying the directory, none of the caching agents in the directory store the copy of the cache line;
retrieving, in response to determining none of the caching agents in the directory store the copy of the cache line, the copy of the cache line from a main memory; and
transmitting the copy of the cache line to the first caching agent.
6 . The method of claim 5 , further comprising:
assigning the index to the first caching agent; and
generating a record in the directory, the record indicating that the first caching agent stores a copy of the cache line.
7 . The method of claim 1 , further comprising:
receiving, from a first caching agent, a request to remove the cache line from a memory associated with the first caching agent; and
decreasing the count.
8 . The method of claim 1 , further comprising, based on receiving a request to access the cache line, transmitting multicast snoops to one or more groups to identify the caching agent storing the copy of the cache line.
9 . A system for switching between modes of memory, the system comprising:
a first caching agent;
a second caching agent;
an interconnect configured to connect the first caching agent, the second caching agent, and a caching agent manager; and
the caching agent manager comprising one or more processors configured to:
operate the memory in a first mode, wherein the first mode maintains a current state of a cache line in a directory using indices, an index of the directory corresponding to a caching agent storing a copy of the cache line;
compare a count of caching agents sharing the cache line to a threshold directory capacity, wherein the threshold directory capacity indicates a maximum number of caching agents that can be tracked in the directory;
determine that the count is greater than the threshold directory capacity;
based on determining that the count is greater than the threshold directory capacity, dynamically change operation of the memory from the first mode to a second mode by generating a bitmap, wherein a bit of the bitmap indicates at least one caching agent within a group of caching agents stores a copy of the cache line; and
operate the memory in the second mode using the bitmap.
10 . The system of claim 9 , wherein the one or more processors are further configured to:
receive, from the first caching agent, a request to access the cache line;
increase the count;
determine that the count is less than or equal to the threshold directory capacity; and
based on determining that the count is less than or equal to the threshold directory capacity, query the directory using a cache line address to identify a second index of the second caching agent storing the copy of the cache line.
11 . The system of claim 10 , wherein the one or more processors are further configured to:
determine, based on querying the directory, none of the caching agents in the directory store the copy of the cache line;
retrieve the copy of the cache line from a main memory based on determining none of the caching agents in the directory store the copy of the cache line; and
transmit the copy of the cache line to the first caching agent.
12 . The system of claim 9 , wherein the one or more processors are further configured to, based on receiving a request to access the cache line, transmit multicast snoops to one or more groups to identify a caching agent storing the copy of the cache line.
13 . A non-transitory computer readable storage medium storing instructions that, when executed by one or more processors for switching between modes of memory, cause the one or more processors to:
operate the memory in a first mode, wherein the first mode maintains a current state of a cache line in a directory using indices, an index of the directory corresponding to a caching agent storing a copy of the cache line;
compare a count of caching agents sharing the cache line to a threshold directory capacity, wherein the threshold directory capacity indicates a maximum number of caching agents that can be tracked in the directory;
determine that the count is greater than the threshold directory capacity;
based on determining the count is greater than the threshold directory capacity, dynamically change operation of the memory from the first mode to a second mode by generating a bitmap, wherein a bit of the bitmap indicates at least one caching agent within a group of caching agents stores a copy of the cache line; and
operate the memory in the second mode using the bitmap.
14 . The non-transitory computer readable storage medium of claim 13 , wherein the one or more processors are further configured to:
receive, from a first caching agent, a request to access the cache line;
increase the count;
determine that the count is less than or equal to the threshold directory capacity; and
based on determining the count is less than or equal to the threshold directory capacity, query the directory using a cache line address to identify a second index of a second caching agent storing the copy of the cache line.
15 . The transitory computer readable storage medium of claim 14 , wherein the one or more processors are further configured to:
determine, based on querying the directory, none of the caching agents in the directory store the copy of the cache line;
retrieve the copy of the cache line from a main memory; and
transmit the copy of the cache line to the first caching agent.
16 . The non-transitory computer readable storage medium of claim 13 , wherein the one or more processors are further configured to, based on receiving a request to access the cache line, transmit multicast snoops to one or more groups to identify a caching agent storing the copy of the cache line.