[cfe-dev] [llvm-dev] [RFC 0/2] Propose a new pointer trait.

[Eric Fiselier via cfe-dev]

A couple of comments/questions:

1) I don't see why `persistent_type` is tied to pointer_traits at all?
Wouldn't it work the same as a separate trait?

2) Given an arbitrary `T` what are the requirements on the "persistent type
for T" (e.g.  `pointer_traits<T*>::persistent_type`)?
Is it intended to be usable exactly like `T` itself? If so how do you plan
to achieve this and do you have an example?
If not how are implementations supposed to access the actual underlying
object?

3) Changing the types of internal node data members is somewhat
problematic, at least in terms of implementation.
__tree and __hash_table don't actually invoke the constructors for the node
types, and instead only construct the
"value" member directly using `allocator_traits::construct` as required by
[container.requirements.general].
If the "persistent" types have non-trivial constructors or destructors they
won't be called leading to UB. This isn't
an impossible problem to solve, but it would take a lot of work.

/Eric

PS. Email regarding libc++ should be sent to cfe-dev as well.

On Mon, Jan 16, 2017 at 7:25 AM, Tomasz Kapela via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi,
> I'm part of an engineering team doing research on persistent memory
> support and
> we have stumbled upon an interesting problem. The issue is, we would like
> to be
> able to use the standard library containers in a persistent memory context
> (think NVDIMM-N). What I mean is that you allocate a container from said
> memory, use it like you normally would. After the application terminates,
> expectedly or otherwise, you can place the container back into the newly
> created process's address space - it will be in the last consistent state
> in
> which it was before termination (interruption/crash/exit). The obvious way
> to
> achieve this is to substitute the allocator. The programming model is
> based on
> memory mapped files and because of that the pointer type used by the
> allocator
> is a fancy pointer, so that it can do all the offset calculations.
>
> User data consistency is provided via a transaction mechanism provided by
> the
> library. We snapshot data before modifying them to be able to roll the
> transaction back in case of an error. The usage model we are proposing is
> this
> (pseudocode, not the actual API):
>
> int main() {
>     auto handle = pool::open("path/to/file", ...);
>     using pvector = std::vector<foo, persistent_allocator<foo>>;
>     {
>         auto tx = transaction::start(handle); // transactions are per pool
> file
>         auto vec_ptr = allocate_from_persistent_memory<pvector>();
>         vec_ptr->emplace_back(foo, parameters);
>     }
> }
>
> The problem occurs when standard library containers have metadata. For
> example
> rb tree nodes carry their color, which will not be included in the
> transaction's undo log. To address this we propose to define a new type in
> the
> std::pointer_traits, which could be used to wrap the containers' metadata
> in
> a user-defined type. This should be fully backward compatible and as such
> not
> mandatory.
>
> This is a standard level change and we would like to gather feedback for
> this
> idea before we start the whole process. This feature is not persistent
> memory
> specific, even we use it as part of providing transactional data
> consistency.
>
> This is in fact a working proof of concept available at
> (https://github.com/pmem/libcxx) combined with the allocator from
> (https://github.com/pmem/nvml).
>
> I will be happy to answer any questions you might have and await your
> feedback.
>
> Thanks,
> Tom
>
>
> Tomasz Kapela (2):
>   pm: change deque typedef
>   pm: add persistency_type typedef to pointer_traits
>
>  include/__hash_table                               | 20 ++++++++------
>  include/__tree                                     |  8 +++---
>  include/deque                                      |  8 +++---
>  include/list                                       |  7 +++--
>  include/map                                        |  7 +++--
>  include/memory                                     | 31
> ++++++++++++++++++++++
>  include/set                                        |  9 +++++--
>  .../pointer.traits.types/persistency_type.pass.cpp | 25 +++++++++++++++++
>  8 files changed, 94 insertions(+), 21 deletions(-)
>  create mode 100644 test/std/utilities/memory/
> pointer.traits/pointer.traits.types/persistency_type.pass.cpp
>
> --
> 2.9.3
>
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>
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