[ofiwg] Proposal for enhancement to support additional Persistent Memory use cases (ofiwg/libfabric#5874)

[Swaro, James E]

Introduction
Libfabric requires modifications to support RMA and atomic operations targeted at remote memory registrations backed by persistent memory devices. These modifications should be made with the intent to drive support for persistent memory usage by applications that rely on communications middleware such as SHMEM in a manner that is consistent with byte-based/stream-based addressable memory formats. Existing proposals (initial proposal) support NVMe/PMoF approaches, which this approach should support flat memory, non-block addressed memory structures and devices.
Changes may be required in as many as three areas:

  *   Memory registration calls
     *   This allows a memory region to be registered as being capable of persistence. This has already been introduced into the upstream libfabric GITHUB, but should be reviewed to ensure it matches use case requirements.
  *   Completion semantics
     *   These changes allow a completion event or notification to be deferred until the referenced data has reached the persistence domain at the target. This has already been introduced into the upstream libfabric GITHUB, but should be reviewed to ensure it matches use case requirements.
  *   Consumer control of persistence
     *   As presently implemented in the upstream libfabric GITHUB, persistence is determined on a transaction-by-transaction basis. It was acknowledged at the time that this is a simplistic implementation. We need to reach consensus on the following:
        *   Should persistence be signaled on the basis of the target memory region? For example, one can imagine a scheme where data targeted at a particular memory region is automatically pushed into the persistence domain by the target, obviating the need for any sort of commit operation.
        *   Is an explicit 'commit' operation of some type required, and if so, what is the scope of that commit operation? Is there a persistence fence defined such that every operation prior to the fence is made persistent by a commit operation?
Proposal
The experimental work in the OFIWG/libfabric branch is sufficient for the needs of SHMEM, with exception to the granularity of event generation. When the current implementation generates events, it would generate commit-level completion events with every operation. That type of operation would make the delivery of completion events take longer than necessary for most operations, so SHMEM would need finer control over commit flushing behavior.
To satisfy this, the following is being proposed:

  *   A new API: fi_commit (See definitions: fi_commit)
The new API would be used to generate a commit instruction to a target peer. The instruction would be defined by a set of memory registration keys, or regions by which the target could issue a commit to persistent memory.
     *   A single request to fi_commit should generate a control message to target hardware or software emulation environment to flush the contents of memory targets. Memory targets are defined by the iov structures, and key fields – and the number of memory targets are defined by the count field. The destination address is handled by the dest_addr field. The flags field is held reserved at this time to allow for flexibility in the API design to future proof against options we might not conceive of until after the prototype is complete, and the context available for the user and returned with the completion
     *   Since this API behaves like a data transfer API, it is expected that this API would generate a completion event to the local completion queue associated with the EP from which the transaction was initiated against.
     *   At the target, this should generate an event to the target's event queue – if and only if the provider supports software emulated events. If a provider is capable of hardware level commits to persistent memory, the transaction should be consumed transparently by the hardware, and does not need to generate an event at the target. This will require an additional event definition in libfabric (See definition for fi_eq_commit_entry)
  *   A new EQ event definition (fi_eq_commit_entry) to support software-emulated persistence for devices that cannot provide hardware support
     *   The iov, and count variables mirror the original iov, and count contents of the originating request.
     *   The flags may be a diminished set of flags from the original transaction under the assumption that only some flags would have meaning at the target and sending originator-only flags to the target would have little value to the target process.
  *   Additional flags or capabilities
     *   A provider should be able to indicate whether they support software emulated notifications of fi_commit, or whether they can handle hardware requests for commits to persistent memory
        *   An additional flag should be introduced to the fi_info structure under modes: FI_COMMIT_MANUAL (or something else)
           *   This flag would indicate to the application that events may be generated to the event queue for consumption by the application. Commit events would be generated upon receipt of a commit message from a remote peer, and the application would be responsible for handling the event.
           *   Lack of the FI_COMMIT_MANUAL flag, and the presence of the FI_RMA_PMEM (or FI_PMEM) flag in the info structure should imply that the hardware is capable of handling the commit requests to persistent memory and the application does not need to read the event queue for commit events.
  *   Change of flag definition
     *   The FI_RMA_PMEM flag should be changed to FI_PMEM to indicate that the provider is PMEM aware, and supports RMA/AMO/MSG operations to and from persistent memory.
     *   There may be little value in supporting messaging interfaces, but it is something that could supported.
  *   Addition of an event handler registration for handling event queue entries within the provider context (See Definition: fi_eq_event_handler)
     *   Essentially, this becomes a registered callback for the target application to handle specific event types. We can use this mechanism with the target application to allow the provider to handle events internally using a function provided by the application. The function would contain the logic necessary to handle the event
     *   Specific to PMEM, a function handler would be used by the target application to handle commits to persistent memory as they were delivered without requiring a fi_eq_read and some form of acknowledgement around the commit action. With the handler, the commit could be handled entirely by the function provided by the application, and the return code from the application provided call-back would be sufficient for a software emulation in the provider to produce the return message to the sender that the commit transaction is fully complete. The use of a handler allows us to make the commit transaction as light-weight, or heavy-weight as necessary.
Definitions:
fi_commit
ssize_t fi_commit(struct fid_ep *ep,
                             const struct fi_rma_iov *iov,
                             size_t count,
                             fi_addr_t dest_addr,
                             uint64_t flags,
                             void *context);
fi_eq_commit_entry
struct fi_eq_commit_entry {
    fid_t                       fid;            /* fid associated with request */
    const struct fi_rma_iov    *iov;            /* iovec of memory regions to be committed to persistent memory */
    size_t                      count;          /* number of iovec/key entries */
    uint64_t                    flags;          /* operation-specific flags */
};
fi_eq_event_handler
typedef ssize_t (*fi_eq_event_handler_t)(struct fid_eq *eq,
    uint64_t event_type,
    void *event_data,
    uint64_t len,
    void *context);
ssize_t fi_eq_register_handler(struct fid_eq *eq,
    uint64_t event_type,
    fi_eq_event_handler_t handler,
    void *context);
Use cases supported by this proposal:

  *   As an application writer, I need to commit multiple previously-sent data transfers to the persistence domain
     *   Previous functionality allows for a commit for every message as is the case for FI_COMMIT_COMPLETE, or the use of FI_COMMIT on a per-transaction basis. The need in this use case is performance-oriented, to allow less strict delivery model to the NIC for most messages followed up with a 'flush' of the NIC to the persistence domain. This allows most messages targeted to the persistence domain to complete with a less strict delivery model, and provides a mechanism to ensure that those data transfers are eventually persisted.
  *   As an application writer, I would like to be able to support persistent data models with libfabric over providers that do not provide hardware support for persistent memory devices
     *   The GNI provider, and other providers won't be able to support PMEM use cases, or at least not right away. To provide the support for PMEM in prototypes, or in providers that will never have PMEM support, a software-emulated approach was suggested to bridge the gap in functionality. In order for the target to know that something needs to be flushed to the persistence domain, the new EQ event was created. In addition to the EQ event, it was discussed that it could be useful for applications to provide function handlers that could be called in the event that a EQ would be delivered to facilitate a more passive libfabric application. If a handler was provided to libfabric, then the application itself could focus on serving requests for access to the persistence domain and sharing of the persistent memory.


James Swaro Software Engineer  | Cray, a Hewlett Packard Enterprise company
2131 Lindau Lane, Suite 1000 | Bloomington, MN 55425
+1-651-605-9000  james.swaro at hpe.com<mailto:james.swaro at hpe.com>  www.cray.com<http://www.cray.com>

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