vmmalloc API

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libvmmalloc - general purpose volatile memory allocation library


$ LD_PRELOAD=libvmmalloc.so command [ args... ]


#include <stdlib.h>
#include <malloc.h>
#include <libvmmalloc.h>

$ cc [ flag... ] file... -lvmmalloc [ library... ]

void *malloc(size_t size);
void free(void *ptr);
void *calloc(size_t number, size_t size);
void *realloc(void *ptr, size_t size);

int posix_memalign(void **memptr, size_t alignment, size_t size);
void *aligned_alloc(size_t alignment, size_t size);
void *memalign(size_t alignment, size_t size);
void *valloc(size_t size);
void *pvalloc(size_t size);

size_t malloc_usable_size(const void *ptr);
void cfree(void *ptr);


libvmmalloc transparently converts all the dynamic memory allocations into Persistent Memory allocations.

The typical usage of libvmmalloc does not require any modification of the target program. It is enough to load libvmmalloc before all other libraries by setting the environment variable LD_PRELOAD. When used in that way, libvmmalloc interposes the standard system memory allocation routines, as defined in malloc(3), posix_memalign(3) and malloc_usable_size(3), and provides that all dynamic memory allocations are made from a memory pool built on memory-mapped file, instead of a system heap. The memory managed by libvmmalloc may have different attributes, depending on the file system containing the memory-mapped file. In particular, libvmmalloc is part of the Non-Volatile Memory Library because it is sometimes useful to use non-volatile memory as a volatile memory pool, leveraging its capacity, cost, or performance characteristics.

libvmmalloc may be also linked to the program, by providing the -lvmmalloc argument to the linker. Then it becomes the default memory allocator for given program.

NOTE: Due to the fact the library operates on a memory-mapped file, it may not work properly with the programs that perform fork(3) not followed by exec(3).

There are two variants of experimental fork() support available in libvmmalloc. The desired library behavior may be selected by setting VMMALLOC_FORK environment variable. By default variant #1 is enabled. See ENVIRONMENT section for more details.

libvmmalloc uses the mmap(2) system call to create a pool of volatile memory. The library is most useful when used with Direct Access storage (DAX), which is memory-addressable persistent storage that supports load/store access without being paged via the system page cache. A Persistent Memory-aware file system is typically used to provide this type of access. Memory-mapping a file from a Persistent Memory-aware file system provides the raw memory pools, and this library supplies the traditional malloc interfaces on top of those pools.

The memory pool acting as a system heap replacement is created automatically at the library initialization time. User may control its location and size by setting the environment variables described in ENVIRONMENT section. The allocated file space is reclaimed when process terminates or in case of system crash.

Under normal usage, libvmmalloc will never print messages or intentionally cause the process to exit. The library uses pthreads(7) to be fully MT-safe, but never creates or destroys threads itself. The library does not make use of any signals, networking, and never calls select() or poll().


There are two configuration variables that must be set to make libvmmalloc work properly. If any of them is not specified, or if their values are not valid, the library prints the appropriate error message and terminates the process.


Specifies a path to directory where the memory pool file should be created. The directory must exist and be writable.


Defines the desired size (in bytes) of the memory pool file. It must be not less than the minimum allowed size VMMALLOC_MIN_POOL as defined in <libvmmalloc.h>. Note that due to the fact the library adds some metadata to the memory pool, the amount of actual usable space is typically less than the size of the memory pool file.

Setting the VMMALLOC_FORK configuration variable is optional. It controls the behavior of libvmmalloc in case of fork(3), and can be set to the following values:

  1. Fork support is disabled. The behavior is undefined in such case, but most likely results in the memory pool corruption and the program crash due to segmentation fault.

  2. The memory pool file is remapped with MAP_PRIVATE flag before the fork completes. From this moment, any access to memory that modifies the heap pages, both in the parent and in the child process, will trigger creation of a copy of those pages in RAM (copy-on-write). The benefit of such approach is that it does not significantly increase the time of fork operation, and does not require additional space on the file system. However, all the subsequent memory allocations and modifications of the memory allocated before fork, will consume system memory resources instead of the memory pool.
    This is the default setting.

  3. A copy of the entire memory pool file is created for the use of the child process. This requires additional space on the file system, but both the parent and the child process may still operate on their memory pools, not consuming the system memory resources. NOTE: In case of large memory pools, creating a copy of the pool file may stall the fork operation for a quite long time.

  4. The library first attempts to create a copy of the memory pool (as for option #2), but if it fails (i.e. because of insufficient amount of free space on the file system), it will fall back to option #1.


Two versions of libvmmalloc are typically available on a development system. The normal version is optimized for performance. That version skips checks that impact performance and never logs any trace information or performs any run-time assertions. A second version, accessed when using the libraries under /usr/lib/nvml_debug, contains run-time assertions and trace points. The typical way to access the debug version is to set the environment variable LD_LIBRARY_PATH to /usr/lib/nvml_debug or /usr/lib64/nvml_debug depending on where the debug libraries are installed on the system. The trace points in the debug version of the library are enabled using the environment variable VMMALLOC_LOG_LEVEL, which can be set to the following values:

  1. Tracing is disabled. This is the default level when VMMALLOC_LOG_LEVEL is not set.

  2. Additional details on any errors detected are logged (in addition to returning the errno-based errors as usual).

  3. A trace of basic operations is logged.

  4. This level enables a very verbose amount of function call tracing in the library.

  5. This level enables voluminous tracing information about all the memory allocations and deallocations.

The environment variable VMMALLOC_LOG_FILE specifies a file name where all logging information should be written. If the last character in the name is "-", the PID of the current process will be appended to the file name when the log file is created. If VMMALLOC_LOG_FILE is not set, output goes to stderr.

Setting the environment variable VMMALLOC_LOG_LEVEL has no effect on the non-debug version of libvmmalloc.


Setting this environment variable to 1 enables logging the human-readable summary statistics at the program termination. Statistics are written only for the debug version of libvmmalloc.


Unlike the normal malloc(), which asks the system for additional memory when it runs out, libvmmalloc allocates the size it is told to and never attempts to grow or shrink that memory pool.


libvmmalloc may not work properly with the programs that perform fork(3) and do not call exec(3) immediately afterwards. See ENVIRONMENT section for more details about the experimental fork() support.

If the trace points in the debug version of the library are enabled and the process performs fork, there is no new log file created for the child process, even if the configured log file name is terminated with "-" character. All the logging information from the child process will be written to the log file owned by the parent process, which may lead to corruption or partial loss of the log data.

Malloc hooks (see malloc_hook(3)), are not supported when using libvmmalloc.


libvmmalloc depends on jemalloc, written by Jason Evans, to do the heavy lifting of managing dynamic memory allocation. See:



ld.so(8), malloc(3), posix_memalign(3), malloc_usable_size(3), malloc_hook(3), jemalloc(3), libvmem(3), libpmem(3).

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