C++ bindings for libpmemobj (part 5) - make_persistent

One of the most important features of the C++ bindings to libpmemobj is the persistent_ptr smart pointer template. While using it is fairly straightforward, the allocation and object construction with the use of the C API is hard to get right. So like it’s C++ standard’s counterparts, it needed an allocation mechanism with appropriate object construction. This is exactly what this post will try to explain.

Transactional allocations

Probably the most common usage of the allocating functions is within pmemobj transactions. The easiest way to allocate an object into a persistent_ptr is by doing the following:

auto pop = pool_base::create(...);
persistent_ptr<entry> pentry;
transaction::exec_tx(pop, [&] { pentry = make_persistent<entry>(); });

Don’t be taken aback by the strange transaction syntax. I will clarify everything in one of the next blog posts. The second line just starts a transaction and allocates one entry object. The more vigilant readers might point out, that using the default constructor is arguably the most effective way of creating objects. I could easily imagine the following constructor:

entry(int a, double b);

But this is not an issue, you can just type:

auto pop = pool_base::create(...);
persistent_ptr<entry> pentry;
transaction::exec_tx(pop, [&] { pentry = make_persistent<entry>(1, 2.0); });

And it forwards the parameters to the appropriate constructor of the entry class.

Say you need an array of objects of type entry, or a 2-D array of said objects. This is also possible to do using make_persistent:

auto a = make_persistent<entry[]>(3);     /* allocate an array of three entries */
auto b = make_persistent<entry[3]>();     /* allocate an array of three entries */
auto c = make_persistent<entry[3][2]>();  /* allocate a 3 by 2 array entries */

Unfortunately the constructor arguments passing does not work with arrays of objects, so the object has to be default constructible.

When you are done with a persistent object and would like for it to be deallocated, you need to call the complementary delete_persistent function.

delete_persistent<entry>(pentry); /* delete persistent object */
delete_persistent<entry[]>(a, 3); /* delete persistent array 'a' */
delete_persistent<entry[3]>(b);   /* delete persistent array 'b' */
delete_persistent<entry[3][2]>(c); /* delete persistent array 'c' */

In case of transactional object destruction, the libpmemobj library calls the object’s destructor. This is however not the case with atomic allocations, where there is no way to atomically destroy and deallocate an object.

Transactional allocations are the most convenient way of creating persistent objects, especially if the allocation is one in a sequence of operations that have to be made atomically with respect to persistence. There is however another way of creating objects.

Atomic allocations

If you only need to allocate an object atomically, you do not have to start a transaction for that. You can do that with the C API and now, of course, the same facility is available in C++. For that you use the make_persistent_atomic function template.

auto pop = pool_base::create(...);
persistent_ptr<entry> pentry;
make_persistent_atomic<entry>(pop, pentry);

As with transactional allocations, their atomic counterparts support both parameter passing and array allocations.

auto pop = pool_base::create(...);
persistent_ptr<entry> pentry;
persistent_ptr<entry[]> pentry_array;
make_persistent_atomic<entry>(pop, pentry, 1, 2.0);
make_persistent_atomic<entry[]>(pop, pentry_array, 3);

Atomic deletions of persistent pointers is done through the delete_persistent_atomic function template, much like the transactional versions.

delete_persistent_atomic<entry>(pentry); /* delete persistent object */
delete_persistent_atomic<entry[]>(pentry_array, 3); /* delete persistent array 'a' */

An atomic allocation/deletion guarantees an object allocation and initialization/deletion that is atomic with respect to persistence. This is important enough to have a separate section to explain.

Transactions and atomic allocations

This is the thing I absolutely have to convey clearly, atomic allocations and transactions do NOT mix. So something like the following is not a good idea:

auto pop = pool_base::create(...);
persistent_ptr<entry> pentry;
transaction::exec_tx(pop, [&] {
  make_persistent_atomic<entry>(pop, pentry); /* do NOT do this */

This might look like a small issue at first, but it could baffle you once you encounter a transaction abort. Everything gets rolled-back, except the allocation. You might get a persistent leak, an inconsistent state and depending on the logic, segfaulting is also an option. Either way, don’t try this at home. It is stated in the C API, in debug builds you get a warning log, but I still feel the need to reinforce this, because there is no way to generate a compile time error.

To conclude:

  • The atomic allocations API should NOT be used inside transactions
  • In case of atomic deallocations the memory gets freed, but the object’s destructor is never called.
  • The transactional versions can only be used within transactions. If used outside of transaction scope, an exception is thrown.
auto pop = pool_base::create(...);
persistent_ptr<entry> pentry;
transaction::exec_tx(pop, [&] {
  make_persistent_atomic<entry>(pop, pentry); /* legal but dangerous */
  auto b = make_persistent<entry>(); /* OK */
  delete_persistent<entry>(b); /* call ~entry() and free memory */

make_persistent_atomic<entry>(pop, pentry); /* OK */
auto b = make_persistent<entry>(); /* throw an exception */
delete_persistent_atomic<entry>(pop, pentry); /* free memory, no call to ~entry() */

This concludes the introduction of atomic and transactional allocations. If you ever feel like looking at more code, try our examples or tests.

In the next blog post I will introduce what I think is the heart of the C++ bindings to libpmemobj - transactions.

[This entry was edited on 2017-12-11 to reflect the name change from NVML to PMDK.]
Share this Post:

Related Posts: