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fi_mr(3) Libfabric v1.15.1 fi_mr(3) NAME fi_mr - Memory region operations fi_mr_reg / fi_mr_regv / fi_mr_regattr Register local memory buffers for direct fabric access fi_close Deregister registered memory buffers. fi_mr_desc Return a local descriptor associated with a registered memory region fi_mr_key Return the remote key needed to access a registered memory re- gion fi_mr_raw_attr Return raw memory region attributes. fi_mr_map_raw Converts a raw memory region key into a key that is usable for data transfer operations. fi_mr_unmap_key Releases a previously mapped raw memory region key. fi_mr_bind Associate a registered memory region with a completion counter or an endpoint. fi_mr_refresh Updates the memory pages associated with a memory region. fi_mr_enable Enables a memory region for use. SYNOPSIS #include <rdma/fi_domain.h> int fi_mr_reg(struct fid_domain *domain, const void *buf, size_t len, uint64_t access, uint64_t offset, uint64_t requested_key, uint64_t flags, struct fid_mr **mr, void *context); int fi_mr_regv(struct fid_domain *domain, const struct iovec * iov, size_t count, uint64_t access, uint64_t offset, uint64_t requested_key, uint64_t flags, struct fid_mr **mr, void *context); int fi_mr_regattr(struct fid_domain *domain, const struct fi_mr_attr *attr, uint64_t flags, struct fid_mr **mr); int fi_close(struct fid *mr); void * fi_mr_desc(struct fid_mr *mr); uint64_t fi_mr_key(struct fid_mr *mr); int fi_mr_raw_attr(struct fid_mr *mr, uint64_t *base_addr, uint8_t *raw_key, size_t *key_size, uint64_t flags); int fi_mr_map_raw(struct fid_domain *domain, uint64_t base_addr, uint8_t *raw_key, size_t key_size, uint64_t *key, uint64_t flags); int fi_mr_unmap_key(struct fid_domain *domain, uint64_t key); int fi_mr_bind(struct fid_mr *mr, struct fid *bfid, uint64_t flags); int fi_mr_refresh(struct fid_mr *mr, const struct iovec *iov, size_t count, uint64_t flags); int fi_mr_enable(struct fid_mr *mr); ARGUMENTS domain Resource domain mr Memory region bfid Fabric identifier of an associated resource. context User specified context associated with the memory region. buf Memory buffer to register with the fabric hardware. len Length of memory buffer to register. Must be > 0. iov Vectored memory buffer. count Count of vectored buffer entries. access Memory access permissions associated with registration offset Optional specified offset for accessing specified registered buffers. This parameter is reserved for future use and must be 0. requested_key Requested remote key associated with registered buffers. Para- meter is ignored if FI_MR_PROV_KEY flag is set in the domain mr_mode bits. attr Memory region attributes flags Additional flags to apply to the operation. DESCRIPTION Registered memory regions associate memory buffers with permissions granted for access by fabric resources. A memory buffer must be regis- tered with a resource domain before it can be used as the target of a remote RMA or atomic data transfer. Additionally, a fabric provider may require that data buffers be registered before being used in local transfers. Memory registration restrictions are controlled using a separate set of mode bits, specified through the domain attributes (mr_mode field). Each mr_mode bit requires that an application take specific steps in order to use memory buffers with libfabric inter- faces. The following apply to memory registration. Default Memory Registration If no mr_mode bits are set, the default behaviors describe below are followed. Historically, these defaults were collectively referred to as scalable memory registration. The default re- quirements are outlined below, followed by definitions of how each mr_mode bit alters the definition. Compatibility: For library versions 1.4 and earlier, this was indicated by setting mr_mode to FI_MR_SCALABLE and the fi_info mode bit FI_LO- CAL_MR to 0. FI_MR_SCALABLE and FI_LOCAL_MR were deprecated in libfab- ric version 1.5, though they are supported for backwards compatibility purposes. For security, memory registration is required for data buffers that are accessed directly by a peer process. For example, registration is re- quired for RMA target buffers (read or written to), and those accessed by atomic or collective operations. By default, registration occurs on virtual address ranges. Because registration refers to address ranges, rather than allocated data buffers, the address ranges do not need to map to data buffers allocat- ed by the application at the time the registration call is made. That is, an application can register any range of addresses in their virtual address space, whether or not those addresses are backed by physical pages or have been allocated. Note that physical pages must back addresses prior to the addresses be- ing accessed as part of a data transfer operation, or the data trans- fers will fail. Additionally, depending on the operation, this could result in the local process receiving a segmentation fault for access- ing invalid memory. Once registered, the resulting memory regions are accessible by peers starting at a base address of 0. That is, the target address that is specified is a byte offset into the registered region. The application also selects the access key associated with the MR. The key size is restricted to a maximum of 8 bytes. With scalable registration, locally accessed data buffers are not reg- istered. This includes source buffers for all transmit operations - sends, tagged sends, RMA, and atomics - as well as buffers posted for receive and tagged receive operations. Although the default memory registration behavior is convenient for ap- plication developers, it is difficult to implement in hardware. At- tempts to hide the hardware requirements from the application often re- sults in significant and unacceptable impacts to performance. The fol- lowing mr_mode bits are provided as input into fi_getinfo. If a provider requires the behavior defined for an mr_mode bit, it will leave the bit set on output to fi_getinfo. Otherwise, the provider can clear the bit to indicate that the behavior is not needed. By setting an mr_mode bit, the application has agreed to adjust its be- havior as indicated. Importantly, applications that choose to support an mr_mode must be prepared to handle the case where the mr_mode is not required. A provider will clear an mr_mode bit if it is not needed. FI_MR_LOCAL When the FI_MR_LOCAL mode bit is set, applications must register all data buffers that will be accessed by the local hardware and provide a valid desc parameter into applicable data transfer op- erations. When FI_MR_LOCAL is zero, applications are not re- quired to register data buffers before using them for local op- erations (e.g. send and receive data buffers). The desc parame- ter into data transfer operations will be ignored in this case, unless otherwise required (e.g. se FI_MR_HMEM). It is recom- mended that applications pass in NULL for desc when not re- quired. A provider may hide local registration requirements from applications by making use of an internal registration cache or similar mechanisms. Such mechanisms, however, may negatively impact performance for some applications, notably those which manage their own network buffers. In order to support as broad range of applications as possible, without unduly affecting their performance, applications that wish to manage their own local memory registrations may do so by using the memory reg- istration calls. Note: the FI_MR_LOCAL mr_mode bit replaces the FI_LOCAL_MR fi_info mode bit. When FI_MR_LOCAL is set, FI_LOCAL_MR is ignored. FI_MR_RAW Raw memory regions are used to support providers with keys larg- er than 64-bits or require setup at the peer. When the FI_MR_RAW bit is set, applications must use fi_mr_raw_attr() lo- cally and fi_mr_map_raw() at the peer before targeting a memory region as part of any data transfer request. FI_MR_VIRT_ADDR The FI_MR_VIRT_ADDR bit indicates that the provider references memory regions by virtual address, rather than a 0-based offset. Peers that target memory regions registered with FI_MR_VIRT_ADDR specify the destination memory buffer using the target's virtual address, with any offset into the region specified as virtual address + offset. Support of this bit typically implies that peers must exchange addressing data prior to initiating any RMA or atomic operation. FI_MR_ALLOCATED When set, all registered memory regions must be backed by physi- cal memory pages at the time the registration call is made. FI_MR_PROV_KEY This memory region mode indicates that the provider does not support application requested MR keys. MR keys are returned by the provider. Applications that support FI_MR_PROV_KEY can ob- tain the provider key using fi_mr_key(), unless FI_MR_RAW is al- so set. The returned key should then be exchanged with peers prior to initiating an RMA or atomic operation. FI_MR_MMU_NOTIFY FI_MR_MMU_NOTIFY is typically set by providers that support mem- ory registration against memory regions that are not necessarily backed by allocated physical pages at the time the memory regis- tration occurs. (That is, FI_MR_ALLOCATED is typically 0). However, such providers require that applications notify the provider prior to the MR being accessed as part of a data trans- fer operation. This notification informs the provider that all necessary physical pages now back the region. The notification is necessary for providers that cannot hook directly into the operating system page tables or memory management unit. See fi_mr_refresh() for notification details. FI_MR_RMA_EVENT This mode bit indicates that the provider must configure memory regions that are associated with RMA events prior to their use. This includes all memory regions that are associated with com- pletion counters. When set, applications must indicate if a memory region will be associated with a completion counter as part of the region's creation. This is done by passing in the FI_RMA_EVENT flag to the memory registration call. Such memory regions will be created in a disabled state and must be as- sociated with all completion counters prior to being enabled. To en- able a memory region, the application must call fi_mr_enable(). After calling fi_mr_enable(), no further resource bindings may be made to the memory region. FI_MR_ENDPOINT This mode bit indicates that the provider associates memory re- gions with endpoints rather than domains. Memory regions that are registered with the provider are created in a disabled state and must be bound to an endpoint prior to being enabled. To bind the MR with an endpoint, the application must use fi_mr_bind(). To enable the memory region, the application must call fi_mr_enable(). FI_MR_HMEM This mode bit is associated with the FI_HMEM capability. If FI_MR_HMEM is set, the application must register buffers that were allocated using a device call and provide a valid desc pa- rameter into applicable data transfer operations even if they are only used for local operations (e.g. send and receive data buffers). Device memory must be registered using the fi_mr_re- gattr call, with the iface and device fields filled out. If FI_MR_HMEM is set, but FI_MR_LOCAL is unset, only device buffers must be registered when used locally. In this case, the desc parameter passed into data transfer operations must either be valid or NULL. Similarly, if FI_MR_LOCAL is set, but FI_MR_HMEM is not, the desc para- meter must either be valid or NULL. FI_MR_COLLECTIVE This bit is associated with the FI_COLLECTIVE capability. When set, the provider requires that memory regions used in collec- tion operations must explicitly be registered for use with col- lective calls. This requires registering regions passed to col- lective calls using the FI_COLLECTIVE flag. Basic Memory Registration Basic memory registration was deprecated in libfabric version 1.5, but is supported for backwards compatibility. Basic memory registration is indicated by setting mr_mode equal to FI_MR_BA- SIC. FI_MR_BASIC must be set alone and not paired with mr_mode bits. Unlike other mr_mode bits, if FI_MR_BASIC is set on input to fi_getinfo(), it will not be cleared by the provider. That is, setting mr_mode equal to FI_MR_BASIC forces basic registra- tion if the provider supports it. The behavior of basic registration is equivalent to requiring the fol- lowing mr_mode bits: FI_MR_VIRT_ADDR, FI_MR_ALLOCATED, and FI_MR_PROV_KEY. Additionally, providers that support basic registra- tion usually require the (deprecated) fi_info mode bit FI_LOCAL_MR, which was incorporated into the FI_MR_LOCAL mr_mode bit. The registrations functions - fi_mr_reg, fi_mr_regv, and fi_mr_regattr - are used to register one or more memory regions with fabric re- sources. The main difference between registration functions are the number and type of parameters that they accept as input. Otherwise, they perform the same general function. By default, memory registration completes synchronously. I.e. the registration call will not return until the registration has completed. Memory registration can complete asynchronous by binding the resource domain to an event queue using the FI_REG_MR flag. See fi_domain_bind. When memory registration is asynchronous, in order to avoid a race con- dition between the registration call returning and the corresponding reading of the event from the EQ, the mr output parameter will be writ- ten before any event associated with the operation may be read by the application. An asynchronous event will not be generated unless the registration call returns success (0). fi_mr_reg The fi_mr_reg call registers the user-specified memory buffer with the resource domain. The buffer is enabled for access by the fabric hard- ware based on the provided access permissions. See the access field description for memory region attributes below. Registered memory is associated with a local memory descriptor and, op- tionally, a remote memory key. A memory descriptor is a provider spe- cific identifier associated with registered memory. Memory descriptors often map to hardware specific indices or keys associated with the mem- ory region. Remote memory keys provide limited protection against un- wanted access by a remote node. Remote accesses to a memory region must provide the key associated with the registration. Because MR keys must be provided by a remote process, an application can use the requested_key parameter to indicate that a specific key value be returned. Support for user requested keys is provider specif- ic and is determined by the FI_MR_PROV_KEY flag value in the mr_mode domain attribute. Remote RMA and atomic operations indicate the location within a regis- tered memory region by specifying an address. The location is refer- enced by adding the offset to either the base virtual address of the buffer or to 0, depending on the mr_mode. The offset parameter is reserved for future use and must be 0. For asynchronous memory registration requests, the result will be re- ported to the user through an event queue associated with the resource domain. If successful, the allocated memory region structure will be returned to the user through the mr parameter. The mr address must re- main valid until the registration operation completes. The context specified with the registration request is returned with the completion event. fi_mr_regv The fi_mr_regv call adds support for a scatter-gather list to fi_mr_reg. Multiple memory buffers are registered as a single memory region. Otherwise, the operation is the same. fi_mr_regattr The fi_mr_regattr call is a more generic, extensible registration call that allows the user to specify the registration request using a struct fi_mr_attr (defined below). fi_close Fi_close is used to release all resources associated with a registering a memory region. Once unregistered, further access to the registered memory is not guaranteed. Active or queued operations that reference a memory region being closed may fail or result in accesses to invalid memory. Applications are responsible for ensuring that a MR is no longer needed prior to closing it. Note that accesses to a closed MR from a remote peer will result in an error at the peer. The state of the local endpoint will be unaffected. When closing the MR, there must be no opened endpoints or counters as- sociated with the MR. If resources are still associated with the MR when attempting to close, the call will return -FI_EBUSY. fi_mr_desc Obtains the local memory descriptor associated with a MR. The memory registration must have completed successfully before invoking this call. fi_mr_key Returns the remote protection key associated with a MR. The memory registration must have completed successfully before invoking this. The returned key may be used in data transfer operations at a peer. If the FI_RAW_MR mode bit has been set for the domain, then the memory key must be obtained using the fi_mr_raw_key function instead. A return value of FI_KEY_NOTAVAIL will be returned if the registration has not completed or a raw memory key is required. fi_mr_raw_attr Returns the raw, remote protection key and base address associated with a MR. The memory registration must have completed successfully before invoking this routine. Use of this call is required if the FI_RAW_MR mode bit has been set by the provider; however, it is safe to use this call with any memory region. On input, the key_size parameter should indicate the size of the raw_key buffer. If the actual key is larger than what can fit into the buffer, it will return -FI_ETOOSMALL. On output, key_size is set to the size of the buffer needed to store the key, which may be larger than the input value. The needed key_size can also be obtained through the mr_key_size domain attribute (fi_domain_attr) field. A raw key must be mapped by a peer before it can be used in data trans- fer operations. See fi_mr_map_raw below. fi_mr_map_raw Raw protection keys must be mapped to a usable key value before they can be used for data transfer operations. The mapping is done by the peer that initiates the RMA or atomic operation. The mapping function takes as input the raw key and its size, and returns the mapped key. Use of the fi_mr_map_raw function is required if the peer has the FI_RAW_MR mode bit set, but this routine may be called on any valid key. All mapped keys must be freed by calling fi_mr_unmap_key when ac- cess to the peer memory region is no longer necessary. fi_mr_unmap_key This call releases any resources that may have been allocated as part of mapping a raw memory key. All mapped keys must be freed before the corresponding domain is closed. fi_mr_bind The fi_mr_bind function associates a memory region with a counter or endpoint. Counter bindings are needed by providers that support the generation of completions based on fabric operations. Endpoint bind- ings are needed if the provider associates memory regions with end- points (see FI_MR_ENDPOINT). When binding with a counter, the type of events tracked against the memory region is based on the bitwise OR of the following flags. FI_REMOTE_WRITE Generates an event whenever a remote RMA write or atomic opera- tion modifies the memory region. Use of this flag requires that the endpoint through which the MR is accessed be created with the FI_RMA_EVENT capability. When binding the memory region to an endpoint, flags should be 0. fi_mr_refresh The use of this call is required to notify the provider of any change to the physical pages backing a registered memory region if the FI_MR_MMU_NOTIFY mode bit has been set. This call informs the provider that the page table entries associated with the region may have been modified, and the provider should verify and update the registered re- gion accordingly. The iov parameter is optional and may be used to specify which portions of the registered region requires updating. Providers are only guaranteed to update the specified address ranges. The refresh operation has the effect of disabling and re-enabling ac- cess to the registered region. Any operations from peers that attempt to access the region will fail while the refresh is occurring. Addi- tionally, attempts to access the region by the local process through libfabric APIs may result in a page fault or other fatal operation. The fi_mr_refresh call is only needed if the physical pages might have been updated after the memory region was created. fi_mr_enable The enable call is used with memory registration associated with the FI_MR_RMA_EVENT mode bit. Memory regions created in the disabled state must be explicitly enabled after being fully configured by the applica- tion. Any resource bindings to the MR must be done prior to enabling the MR. MEMORY REGION ATTRIBUTES Memory regions are created using the following attributes. The struct fi_mr_attr is passed into fi_mr_regattr, but individual fields also ap- ply to other memory registration calls, with the fields passed directly into calls as function parameters. struct fi_mr_attr { const struct iovec *mr_iov; size_t iov_count; uint64_t access; uint64_t offset; uint64_t requested_key; void *context; size_t auth_key_size; uint8_t *auth_key; enum fi_hmem_iface iface; union { uint64_t reserved; int cuda; int ze } device; }; mr_iov This is an IO vector of addresses that will represent a single memory region. The number of entries in the iovec is specified by iov_count. iov_count The number of entries in the mr_iov array. The maximum number of memo- ry buffers that may be associated with a single memory region is speci- fied as the mr_iov_limit domain attribute. See fi_domain(3). access Indicates the type of operations that the local or a peer endpoint may perform on registered memory region. Supported access permissions are the bitwise OR of the following flags: FI_SEND The memory buffer may be used in outgoing message data trans- fers. This includes fi_msg and fi_tagged send operations, as well as fi_collective operations. FI_RECV The memory buffer may be used to receive inbound message trans- fers. This includes fi_msg and fi_tagged receive operations, as well as fi_collective operations. FI_READ The memory buffer may be used as the result buffer for RMA read and atomic operations on the initiator side. Note that from the viewpoint of the application, the memory buffer is being written into by the network. FI_WRITE The memory buffer may be used as the source buffer for RMA write and atomic operations on the initiator side. Note that from the viewpoint of the application, the endpoint is reading from the memory buffer and copying the data onto the network. FI_REMOTE_READ The memory buffer may be used as the source buffer of an RMA read operation on the target side. The contents of the memory buffer are not modified by such operations. FI_REMOTE_WRITE The memory buffer may be used as the target buffer of an RMA write or atomic operation. The contents of the memory buffer may be modified as a result of such operations. FI_COLLECTIVE This flag provides an explicit indication that the memory buffer may be used with collective operations. Use of this flag is re- quired if the FI_MR_COLLECTIVE mr_mode bit has been set on the domain. This flag should be paired with FI_SEND and/or FI_RECV Note that some providers may not enforce fine grained access permis- sions. For example, a memory region registered for FI_WRITE access may also behave as if FI_SEND were specified as well. Relaxed enforcement of such access is permitted, though not guaranteed, provided security is maintained. offset The offset field is reserved for future use and must be 0. requested_key An application specified access key associated with the memory region. The MR key must be provided by a remote process when performing RMA or atomic operations to a memory region. Applications can use the re- quested_key field to indicate that a specific key be used by the provider. This allows applications to use well known key values, which can avoid applications needing to exchange and store keys. Support for user requested keys is provider specific and is determined by the the FI_MR_PROV_KEY flag in the mr_mode domain attribute field. context Application context associated with asynchronous memory registration operations. This value is returned as part of any asynchronous event associated with the registration. This field is ignored for synchro- nous registration calls. auth_key_size The size of key referenced by the auth_key field in bytes, or 0 if no authorization key is given. This field is ignored unless the fabric is opened with API version 1.5 or greater. auth_key Indicates the key to associate with this memory registration. Autho- rization keys are used to limit communication between endpoints. Only peer endpoints that are programmed to use the same authorization key may access the memory region. The domain authorization key will be used if the auth_key_size provided is 0. This field is ignored unless the fabric is opened with API version 1.5 or greater. iface Indicates the software interfaces used by the application to allocate and manage the memory region. This field is ignored unless the appli- cation has requested the FI_HMEM capability. FI_HMEM_SYSTEM Uses standard operating system calls and libraries, such as mal- loc, calloc, realloc, mmap, and free. FI_HMEM_CUDA Uses Nvidia CUDA interfaces such as cuMemAlloc, cuMemAllocHost, cuMemAllocManaged, cuMemFree, cudaMalloc, cudaFree. FI_HMEM_ROCR Uses AMD ROCR interfaces such as hsa_memory_allocate and hsa_memory_free. FI_HMEM_ZE Uses oneAPI Level Zero interfaces such as zeDriverAllocShared- Mem, zeDriverFreeMem. FI_HMEM_NEURON Uses the AWS Neuron SDK to support AWS Trainium devices. device Reserved 64 bits for device identifier if using non-standard HMEM in- terface. This field is ignore unless the iface field is valid. cuda For FI_HMEM_CUDA, this is equivalent to CUdevice (int). ze For FI_HMEM_ZE, this is equivalent to the ze_device_handle_t in- dex (int). neuron For FI_HMEM_NEURON, the device identifier for AWS Trainium de- vices. NOTES Direct access to an application's memory by a remote peer requires that the application register the targeted memory buffer(s). This is typi- cally done by calling one of the fi_mr_reg* routines. For FI_MR_PROV_KEY, the provider will return a key that must be used by the peer when accessing the memory region. The application is responsible for transferring this key to the peer. If FI_MR_RAW mode has been set, the key must be retrieved using the fi_mr_raw_attr function. FI_RAW_MR allows support for providers that require more than 8-bytes for their protection keys or need additional setup before a key can be used for transfers. After a raw key has been retrieved, it must be ex- changed with the remote peer. The peer must use fi_mr_map_raw to con- vert the raw key into a usable 64-bit key. The mapping must be done even if the raw key is 64-bits or smaller. The raw key support functions are usable with all registered memory re- gions, even if FI_MR_RAW has not been set. It is recommended that portable applications target using those interfaces; however, their use does carry extra message and memory footprint overhead, making it less desirable for highly scalable apps. There may be cases where device peer to peer support should not be used or cannot be used, such as when the PCIe ACS configuration does not permit the transfer. The FI_HMEM_DISABLE_P2P environment variable can be set to notify Libfabric that peer to peer transactions should not be used. The provider may choose to perform a copy instead, or will fail support for FI_HMEM if the provider is unable to do that. FLAGS The follow flag may be specified to any memory registration call. FI_RMA_EVENT This flag indicates that the specified memory region will be as- sociated with a completion counter used to count RMA operations that access the MR. FI_RMA_PMEM This flag indicates that the underlying memory region is backed by persistent memory and will be used in RMA operations. It must be specified if persistent completion semantics or persis- tent data transfers are required when accessing the registered region. FI_HMEM_DEVICE_ONLY This flag indicates that the memory is only accessible by a de- vice. Which device is specified by the fi_mr_attr fields iface and device. This refers to memory regions that were allocated using a device API AllocDevice call (as opposed to using the host allocation or unified/shared memory allocation). FI_HMEM_HOST_ALLOC This flag indicates that the memory is owned by the host only. Whether it can be accessed by the device is implementation de- pendent. The fi_mr_attr field iface is still used to identify the device API, but the field device is ignored. This refers to memory regions that were allocated using a device API AllocHost call (as opposed to using malloc-like host allocation, uni- fied/shared memory allocation, or AllocDevice). MEMORY DOMAINS Memory domains identify the physical separation of memory which may or may not be accessible through the same virtual address space. Tradi- tionally, applications only dealt with a single memory domain, that of host memory tightly coupled with the system CPUs. With the introduc- tion of device and non-uniform memory subsystems, applications often need to be aware of which memory domain a particular virtual address maps to. As a general rule, separate physical devices can be considered to have their own memory domains. For example, a NIC may have user accessible memory, and would be considered a separate memory domain from memory on a GPU. Both the NIC and GPU memory domains are separate from host sys- tem memory. Individual GPUs or computation accelerators may have dis- tinct memory domains, or may be connected in such a way (e.g. a GPU specific fabric) that all GPUs would belong to the same memory domain. Unfortunately, identifying memory domains is specific to each system and its physical and/or virtual configuration. Understanding memory domains in heterogenous memory environments is im- portant as it can impact data ordering and visibility as viewed by an application. It is also important to understand which memory domain an application is most tightly coupled to. In most cases, applications are tightly coupled to host memory. However, an application running directly on a GPU or NIC may be more tightly coupled to memory associ- ated with those devices. Memory regions are often associated with a single memory domain. The domain is often indicated by the fi_mr_attr iface and device fields. Though it is possible for physical pages backing a virtual memory re- gion to migrate between memory domains based on access patterns. For example, the physical pages referenced by a virtual address range could migrate between host memory and GPU memory, depending on which computa- tional unit is actively using it. See the fi_endpoint(3) and fi_cq(3) man pages for addition discussion on message, data, and completion ordering semantics, including the im- pact of memory domains. RETURN VALUES Returns 0 on success. On error, a negative value corresponding to fab- ric errno is returned. Fabric errno values are defined in rdma/fi_errno.h. ERRORS -FI_ENOKEY The requested_key is already in use. -FI_EKEYREJECTED The requested_key is not available. They key may be out of the range supported by the provider, or the provider may not support user-requested memory registration keys. -FI_ENOSYS Returned by fi_mr_bind if the provider does not support report- ing events based on access to registered memory regions. -FI_EBADFLAGS Returned if the specified flags are not supported by the provider. MEMORY REGISTRATION CACHE Many hardware NICs accessed by libfabric require that data buffers be registered with the hardware while the hardware accesses it. This en- sures that the virtual to physical address mappings for those buffers do not change while the transfer is occurring. The performance impact of registering memory regions can be significant. As a result, some providers make use of a registration cache, particularly when working with applications that are unable to manage their own network buffers. A registration cache avoids the overhead of registering and unregister- ing a data buffer with each transfer. If a registration cache is going to be used for host and device memory, the device must support unified virtual addressing. If the device does not support unified virtual addressing, either an additional registra- tion cache is required to track this device memory, or device memory cannot be cached. As a general rule, if hardware requires the FI_MR_LOCAL mode bit de- scribed above, but this is not supported by the application, a memory registration cache may be in use. The following environment variables may be used to configure registration caches. FI_MR_CACHE_MAX_SIZE This defines the total number of bytes for all memory regions that may be tracked by the cache. If not set, the cache has no limit on how many bytes may be registered and cached. Setting this will reduce the amount of memory that is not actively being used as part of a data transfer that is registered with a provider. By default, the cache size is unlimited. FI_MR_CACHE_MAX_COUNT This defines the total number of memory regions that may be reg- istered with the cache. If not set, a default limit is chosen. Setting this will reduce the number of regions that are regis- tered, regardless of their size, which are not actively being used as part of a data transfer. Setting this to zero will dis- able registration caching. FI_MR_CACHE_MONITOR The cache monitor is responsible for detecting system memory (FI_HMEM_SYSTEM) changes made between the virtual addresses used by an application and the underlying physical pages. Valid mon- itor options are: userfaultfd, memhooks, and disabled. Select- ing disabled will turn off the registration cache. Userfaultfd is a Linux kernel feature used to report virtual to physical ad- dress mapping changes to user space. Memhooks operates by in- tercepting relevant memory allocation and deallocation calls which may result in the mappings changing, such as malloc, mmap, free, etc. Note that memhooks operates at the elf linker layer, and does not use glibc memory hooks. FI_MR_CUDA_CACHE_MONITOR_ENABLED The CUDA cache monitor is responsible for detecting CUDA device memory (FI_HMEM_CUDA) changes made between the device virtual addresses used by an application and the underlying device phys- ical pages. Valid monitor options are: 0 or 1. Note that the CUDA memory monitor requires a CUDA toolkit version with unified virtual addressing enabled. FI_MR_ROCR_CACHE_MONITOR_ENABLED The ROCR cache monitor is responsible for detecting ROCR device memory (FI_HMEM_ROCR) changes made between the device virtual addresses used by an application and the underlying device phys- ical pages. Valid monitor options are: 0 or 1. Note that the ROCR memory monitor requires a ROCR version with unified virtual addressing enabled. FI_MR_ZE_CACHE_MONITOR_ENABLED The ZE cache monitor is responsible for detecting oneAPI Level Zero device memory (FI_HMEM_ZE) changes made between the device virtual addresses used by an application and the underlying de- vice physical pages. Valid monitor options are: 0 or 1. More direct access to the internal registration cache is possible through the fi_open() call, using the "mr_cache" service name. Once opened, custom memory monitors may be installed. A memory monitor is a component of the cache responsible for detecting changes in virtual to physical address mappings. Some level of control over the cache is possible through the above mentioned environment variables. SEE ALSO fi_getinfo(3), fi_endpoint(3), fi_domain(3), fi_rma(3), fi_msg(3), fi_atomic(3) AUTHORS OpenFabrics. Libfabric Programmer's Manual 2022-03-31 fi_mr(3)
NAME | SYNOPSIS | ARGUMENTS | DESCRIPTION | MEMORY REGION ATTRIBUTES | NOTES | FLAGS | MEMORY DOMAINS | RETURN VALUES | ERRORS | MEMORY REGISTRATION CACHE | SEE ALSO | AUTHORS
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