Introducing Intel® Optane™ DC Memory Programming Model | Intel Software

Welcome. My name is Eduardo
Berrocal from Intel. In this video, I will go
over the programming model for persistent memory. Persistent memory
aware or PMEM-aware file systems are needed in
order to use Intel Optane DC persistent memory
in-application direct or off-direct mode. The main reason is
that file systems provide useful functionality,
such as the ability to store, name, and easily
find information, as well as the
ability to protect such information
between different users and applications. The difference lies in how
database is read and written. In the case of
PMEM-aware file systems, all data accesses go directly
to the persistent media, bypassing any I/O
caching done by the OS. The better usage of this feature
is in the case of memory mapped files. Once the file is memory
mapped to the address space of the application,
all loads and stores originating from the
application go directly to the persistent
media, and data accesses are done at cache
line granularity. Bypassing I/O caching,
however, does not mean bypassing CPU caching. Recent data writes might reside
on flash in the CPU caches. Unfortunately, these
are not protected against a sudden loss of power. If that were to
happen, we might end up with corrupted data structures. To avoid corruption,
programmers need to design their data structures
in such a way that temporarily turn torn-writes are
allowed and make sure that the proper flashing
instructions are issued at exactly the right time. As you can probably guess,
this way of programming is not trivial. Fortunately, Intel has developed
a Persistent Memory Developer Kit, or PMDK– an open source collection
of libraries and tools that provide low-level
primitives, as well as useful, high-level
instructions, to help persistent
memory programmers overcome these obstacles. In the code sample
that I present next, a simple double-linked list
with head and tail pointers, I use the C++ bindings
for libpmemobj. libpmemobj is the
most flexible library and provides transactions
to avoid corrupting our data structures during writes. The first thing we need in order
to use libpmemobj is a pool. Pool is the terminology
used in PMDK to refer to a file that
will be memory mapped. We can create a pool by
either using the common line tool, pmempool, or
inside our program by calling the Create
function in the pool class. In libpmemobj, we need
to define a root object. These root acts
as the entry point to anchor all the other
objects created in the pool. In the case of our
linked list, the root consists of two pointers,
corresponding to the head and the tail of the list. You can call the function
root to retrieve a pointer to the root of a
particular pool. Due to the nature
of virtual memory, the template class,
persistent_ptr, is needed to safely
store pointers to objects in persistent memory. It also helps programmers
avoid mistakes due to mixing pointers
to volatile objects with those to
persistent objects. Regular variables residing
in persistent memory need to be declared using
the template class, p. This allows the library to add
those variables automatically to transactions, thanks
to operator overloading. One way to define
transactions in libpmemobj is using lambda functions. Lambdas allows us to insert
anonymous functions in place to easily define
code regions that need to be executed atomically,
like the node insertion example presented here. In addition to atomic
update, libpmemobj sections can also be synchronized in
a multi-threaded application by passing a mutex variable
to the transaction. There might be cases where
applications might not care about the persistent
aspect of persistent memory in off-direct mode and
just want to use it as an extra pool of
available memory. For this, developers
can take advantage of the library, memkind,
also developed by Intel– a hip manager built
on top of jemalloc, which provides a unified
malloc-like interface across an heterogeneous
set of available memory pools in the system. There is much more to
learn about Intel Optane DC Persistent Memory and
persistent memory programming. I urge you to explore further
by following the links, as well as watching our other
persistent memory videos. [MUSIC PLAYING]

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