Volatile memory cache line directory tags

A method comprising:

• reading a data cache line;
• processing data of the data cache line with a central processing unit (CPU) to alter the data to produce altered data;
• writing a data cache line in a storage class memory, the data cache line including the altered data and a directory tag indicating ownership of the altered data;
• writing, in a volatile memory, a cache line comprising the directory tag identifying ownership of the data cache line without correspondingly writing the altered data in the volatile memory, wherein the writing of the cache line in the volatile memory further comprises writing a second directory tag identifying ownership of a second data cache line stored in the storage class memory that contains second data, wherein the cache line omits the second data
• receiving a request for third data;
• determining whether a CPU cache of the CPU contains the third data;
• searching the volatile memory for a third directory tag corresponding to the third data, the third directory tag indicating ownership of the third data;
• and
• in response to the CPU cache containing the third data and the third directory tag indicating ownership of the third data by a second CPU different from the CPU, snooping the second CPU to obtain the third data.

A non-transitory computer-readable medium containing instructions to direct a first central processing unit (CPU) to:

• receive a request, from a second CPU, for data stored as part of a data cache line in a storage class memory;
• read a cache line containing multiple directory tags in a volatile memory to determine ownership of the data in the storage class memory, each of the multiple directory tags identifying ownership of a corresponding data cache line in the storage class memory;
• determine whether a CPU cache of the first CPU contains the data;
• search the volatile memory for a directory tag corresponding to the data, the third directory tag indicating the ownership of the data;
• and
• in response to the CPU cache containing the data and the directory tag indicating ownership of the data by a third CPU different from the first CPU, snoop the third CPU to obtain the data.