Ubuntu Manpage: libpmem2 - persistent memory support library

Provided by: libpmem2-dev_1.11.1-3build1_amd64

NAME

       libpmem2 - persistent memory support library

SYNOPSIS

              #include <libpmem2.h>
              cc ... -lpmem2

DESCRIPTION

       libpmem2 provides low-level persistent memory (pmem) support for applications using direct access storage
       (DAX),  which  is  storage that supports load/store access without paging blocks from a block storage de‐
       vice.  Some types of non-volatile memory DIMMs (NVDIMMs) provide this type of byte addressable access  to
       storage.  A persistent memory aware file system is typically used to expose the direct access to applica‐
       tions.   Memory  mapping a file from this type of file system results in the load/store, non-paged access
       to pmem.

       This library is for applications that use persistent  memory  directly,  without  the  help  of  any  li‐
       brary-supplied transactions or memory allocation.  Higher-level libraries that currently build on libpmem
       (previous variation of libpmem2) are available and are recommended for most applications, see:

       • libpmemobj(7), a general use persistent memory API, providing memory allocation and transactional oper‐
         ations on variable-sized objects.

       • libpmemblk(7), providing pmem-resident arrays of fixed-sized blocks with atomic updates.

       • libpmemlog(7), providing a pmem-resident log file.

       The  libpmem2  library provides a comprehensive set of functions for robust use of Persistent Memory.  It
       relies on three core concepts: struct pmem2_src source, struct pmem2_config config and  struct  pmem2_map
       map:

       • source - an object describing the data source for mapping.  The data source can be a file descriptor, a
         file handle, or an anonymous mapping.  APIs dedicated for creating source are: pmem2_source_from_fd(3),
         pmem2_source_from_handle(3), pmem2_source_from_anon(3).

       • config - an object containing parameters that are used to create a mapping from a source.  The configu‐
         ration structure must always be provided to create a mapping, but the only required parameter to set in
         the  config is granularity.  The granularity should by set using dedicated libpmem2 function pmem2_con‐
         fig_set_required_store_granularity(3) which defines a maximum permitted granularity  requested  by  the
         user.  For more information about the granularity concept read GRANULARITY section below.

       In addition to the granularity setting, libpmem2 provides multiple optional functions to configure target
       mapping,  e.g.,  pmem2_config_set_length(3)  to  set length which will be used for mapping, or pmem2_con‐
       fig_set_offset(3) which will be used to map the contents from  the  specified  location  of  the  source,
       pmem2_config_set_sharing(3) which defines the behavior and visibility of writes to the mapping's pages.

       • map  -  an  object created by pmem2_map_new(3) using source and config as an input parameters.  The map
         structure can be then used to directly operate on the created mapping through the use of its associated
         set of functions: pmem2_map_get_address(3), pmem2_map_get_size(3), pmem2_map_get_store_granularity(3) -
         for getting address, size and effective mapping granularity.

       In addition to the basic functionality of managing the virtual address mapping,  libpmem2  also  provides
       optimized  functions  for modifying the mapped data.  This includes data flushing as well as memory copy‐
       ing.

       To  get  proper  function  for  data  flushing  use:  pmem2_get_flush_fn(3),  pmem2_get_persist_fn(3)  or
       pmem2_get_drain_fn(3).   To  get  proper  function  for  copying  to  persistent memory, use map getters:
       pmem2_get_memcpy_fn(3), pmem2_get_memset_fn(3), pmem2_get_memmove_fn(3).

       The libpmem2 API also provides support for the badblock and unsafe shutdown state handling.

       To read  or  clear  badblocks,  the  following  functions  are  provided:  pmem2_badblock_context_new(3),
       pmem2_badblock_context_delete(3), pmem2_badblock_next(3) and pmem2_badblock_clear(3).

       To  handle  unsafe  shutdown  in  the application, the following functions are provided: pmem2_source_de‐
       vice_id(3), pmem2_source_device_usc(3).  More detailed information about unsafe  shutdown  detection  and
       unsafe shutdown count and can be found in the libpmem2_unsafe_shutdown(7) man page.

GRANULARITY

The libpmem2 library introduces the concept of granularity through which you may easily distinguish be‐
tween different levels of storage performance capabilities available to the application as related to
power-fail protected domain. The way data reaches this protected domain differs based on the platform
and storage device capabilities.

Traditional block storage devices (SSD, HDD) must use system API calls such as msync(), fsync() on Linux,
or FlushFileBuffers(),FlushViewOfFile() on Windows to write data reliably. Invoking these functions
flushes the data to the medium with page granularity. In the libpmem2 library, this type of flushing be‐
havior is called PMEM2_GRANULARITY_PAGE.

In systems with persistent memory support, a power-fail protected domain may cover different sets of re‐
sources: either the memory controller or the memory controller and CPU caches. For this reason, libpmem2
distinguishes two types of granularity for persistent memory: PMEM2_GRANULARITY_CACHE_LINE and
PMEM2_GRANULARITY_BYTE.

If the power-fail protected domain covers only the memory controller, the CPU appropriate cache lines
must be flushed for the data to be considered persistent. This granularity type is called PMEM2_GRANU‐
LARITY_CACHE_LINE. Depending on the architecture, there are different types of machine instructions for
flushing cache lines (e.g., CLWB, CLFLUSHOPT, CLFLUSH for Intel x86_64 architecture). Usually, to ensure
the ordering of stores, such instructions must be followed by a barrier (e.g., SFENCE).

The third type of granularity PMEM2_GRANULARITY_BYTE applies to platforms where power-fail protected do‐
main covers both the memory controller and CPU caches. In such cases, cache flush instructions are no
longer needed, and the platform itself guarantees the persistence of data. But barriers might still be
required for ordering.

The library declares these granularity level in pmem2_granularity enum, which the application must set in
pmem2_config to the appropriate level for a mapping a succeed. The software should set this config para‐
meter to a value that most accurately represents the target hardware characteristics and the storage pat‐
terns of the application. For example, a database storage engine that operates on large logical pages
that reside either on SSDs or PMEM should set this value to PMEM2_GRANULARITY_PAGE. The library will
create mappings where the new map granularity is lower or equal to the requested one. For example, a
mapping with PMEM2_GRANULARITY_CACHE_LINE can created for the required granularity PMEM2_GRANULARI‐
TY_PAGE, but not vice versa.

CAVEATS

       libpmem2  relies on the library destructor being called from the main thread.  For this reason, all func‐
       tions that might trigger destruction (e.g. dlclose(3)) should be called in the  main  thread.   Otherwise
       some of the resources associated with that thread might not be cleaned up properly.

ENVIRONMENT

libpmem2 can change its default behavior based on the following environment variables. These are primar‐
ily intended for testing and are generally not required.

• PMEM2_FORCE_GRANULARITY=val

Setting this environment variable to val forces libpmem2 to use persist method specific for forced granu‐
larity and skip granularity autodetecting mechanism. The concept of the granularity is described in
GRANULARITY section above. This variable is intended for use during library testing.

The val argument accepts following text values:

• BYTE - force byte granularity.

• CACHE_LINE - force cache line granularity.

• PAGE - force page granularity.

Granularity values listed above are case-insensitive.

NOTE: The value of PMEM2_FORCE_GRANULARITY is not queried (and cached) at library initialization
time, but read during each pmem2_map_new(3) call.

This means that PMEM2_FORCE_GRANULARITY may still be set or modified by the program until the first at‐
tempt to map a file.

• PMEM_NO_CLWB=1

Setting this environment variable to 1 forces libpmem2 to never issue the CLWB instruction on Intel hard‐
ware, falling back to other cache flush instructions on that hardware instead (CLFLUSHOPT or CLFLUSH).
Without this setting, libpmem2 will always use the CLWB instruction for flushing processor caches on
platforms that support this instruction. This variable is intended for use during library testing, but
may be required for some rare cases when using CLWB has a negative impact on performance.

• PMEM_NO_CLFLUSHOPT=1

Setting this environment variable to 1 forces libpmem2 to never issue the CLFLUSHOPT instruction on Intel
hardware, falling back to the CLFLUSH instructions instead. Without this environment variable, libpmem2
will always use the CLFLUSHOPT instruction for flushing processor caches on platforms that support the
instruction, but where CLWB is not available. This variable is intended for use during library testing.

• PMEM_NO_MOVNT=1

Setting this environment variable to 1 forces libpmem2 to never use the non-temporal move instructions on
Intel hardware. Without this environment variable, libpmem2 will use the non-temporal instructions for
copying larger ranges to persistent memory on platforms that support these instructions. This variable
is intended for use during library testing.

• PMEM_MOVNT_THRESHOLD=val

This environment variable allows overriding the minimum length of the pmem2_memmove_fn operations, for
which libpmem2 uses non-temporal move instructions. Setting this environment variable to 0 forces libp‐
mem2 to always use the non-temporal move instructions if available. It has no effect if PMEM_NO_MOVNT is
set to 1. This variable is intended for use during library testing.

DEBUGGING

Two versions of libpmem2 are typically available on a development system. The normal version, accessed
when a program is linked using the -lpmem2 option, 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 of libpmem2, accessed when a program uses the libraries under /usr/lib/pmdk_debug, con‐
tains 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/pmdk_debug or /usr/lib64/pmdk_debug, as appropriate.
Debugging output is controlled using the following environment variables. These variables have no effect
on the non-debug version of the library.

NOTE: On Debian/Ubuntu systems, this extra debug version of the library is shipped in the respec‐
tive -debug Debian package and placed in the /usr/lib/$ARCH/pmdk_dbg/ directory.

• PMEM2_LOG_LEVEL

The value of PMEM2_LOG_LEVEL enables trace points in the debug version of the library, as follows:

• 0 - This is the default level when PMEM2_LOG_LEVEL is not set. No log messages are emitted at this
level.

• 1 - Additional details on any errors detected are logged, in addition to returning the errno-based er‐
rors as usual. The same information may be retrieved using pmem2_errormsg().

• 2 - A trace of basic operations is logged.

• 3 - Enables a very verbose amount of function call tracing in the library.

• 4 - Enables voluminous and fairly obscure tracing information that is likely only useful to the libp‐
mem2 developers.

Unless PMEM2_LOG_FILE is set, debugging output is written to stderr.

• PMEM2_LOG_FILE

Specifies the name of a file 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 PMEM2_LOG_FILE is not set, output is written to stderr.

EXAMPLE

       The following example uses libpmem2 to flush changes made to raw, memory-mapped persistent memory.

              WARNING: There is nothing transactional about the persist  from  pmem2_get_persist_fn(3)  call  in
              this example.  Interrupting the program may result in a partial write to pmem.  Use a transaction‐
              al library such as libpmemobj(7) to avoid torn updates.

       The above example is described in detail here (https://pmem.io/pmdk/libpmem2/).

ACKNOWLEDGEMENTS

       libpmem2  builds on the persistent memory programming model recommended by the SNIA NVM Programming Tech‐
       nical Work Group: <https://snia.org/nvmp>