Introduction to libmemkind

Introduction

Memkind is the library that simplify usage of persistent memory in a volatile mode. There are NVDIMMs technologies, such as Intel Optane DCPMM, that provides persistency, byte-addressability, and also a high capacity when compared with DRAM modules. They can be used as an expansion of main memory and utilized by applications which consume a large amount of memory and do not require persistency, such as in-memory databases, caching engines and scientific simulations.

Libmemkind

According to the current persistent memory programming model, NVDIMMs are exposed by operating system as devices on which user should create file-system. This model creates a need for a way to consume memory exposed through files in applications. Libmemkind fills this gap by utilizing jemalloc on temporary files created on File System DAX created on NVDIMMs and acts as a memory allocator for applications. Libmemkind provides various memory pools called “kinds” for memories with miscellaneous characteristics: DRAM, persistent memory and High-Bandwidth Memory. This allows partitioning of the heap of an application between these kinds. On a system equipped with DRAM and NVDIMMs, it is possible to modify an application in a way to store objects that are accessed frequently and require fast access in DRAM while larger objects which are accessed less frequently can be stored on persistent memory.

Managing application heap

With libmemkind it is possible to allocate from DRAM by calling memkind_malloc function with static kind MEMKIND_DEFAULT or MEMKIND_REGULAR:

void * ptr_default = memkind_malloc(MEMKIND_DEFAULT, size);

and from persistent memory by calling the same function with dynamically created pmem kind pointing to directory on FS DAX:

struct memkind *pmem_kind = NULL;
memkind_create_pmem("/mnt/pmem", PMEM_MAX_SIZE, &pmem_kind);
void* ptr_pmem = memkind_malloc(pmem_kind, size);

Freeing allocated objects is done by calling:

memkind_free(MEMKIND_DEFAULT, ptr_default);
memkind_free(pmem_kind, ptr_pmem);

To simplify the adoption of applications to many memory pools, library provides another feature: calling memkind_free without specifying a kind that was used for the allocation. Library will automatically recognize if a given pointer belongs to DRAM or PM. This avoids tracking each allocation’s origin in the application:

memkind_free(NULL, ptr_default);
memkind_free(NULL, ptr_pmem);

Other features

Libmemkind also provides:

• the rest of malloc-style API functions,
• an automatically increased pool size,
• configurable memory policy: performance vs fragmentation,
• C++ bindings,
• alternative heap manager (TBB).

Documentation

The complete libmemkind manual can be found at memkind.github.io.