Design Choices

Table 1 describes the current design limits within the OpenFAM reference implementation. Since the maximum FAM size in the reference implementation is bound by the amount of memory available at the memory server, these parameters should not pose undue limitations on memory size, while enabling developers to build reasonable sized applications using the reference implementation.

Table 1: Data set limits per memory server in the reference implementation
Item	Limit
Maximum FAM size	128 TB
Minimum region size	8 GB
Maximum region size	1 TB
Maximum number of regions	16 K regions
Minimum data Item size	128 bytes
Maximum data Item size	512 GB
Maximum number of data items within a region	~8 B data Items

As mentioned in The OpenFAM API: A Programming Model for Disaggregated Persistent Memory, the reference implementation uses libfabric to handle the underlying memory traffic. The implementation supports the complete API with exceptions highlighted in Table 2.

Table 2: OpenFAM APIs currently not available in the reference implementation
API	Memory Server	Shared Memory	Comment
fam_map	NO	YES	Libfabric currently does not support mmap APIs
fam_unmap	NO	YES	Libfabric currently does not support mmap APIs
fam_fence	YES (only with sockets provider)	NO	Fence operation is not supported by CXI, PSM2 and Verbs libfabric provider. Support for other providers and Shared Memory model will be added in future

Since the underlying libfabric providers do not support 128-bit compare and swap (CAS) operations, CAS operations have been implemented in software.