Function rcudnn::cudaMemAdvise

pub unsafe extern "C" fn cudaMemAdvise(
    devPtr: *const c_void, 
    count: usize, 
    advice: cudaMemoryAdvise, 
    device: i32
) -> cudaError

\brief Advise about the usage of a given memory range

Advise the Unified Memory subsystem about the usage pattern for the memory range starting at \p devPtr with a size of \p count bytes. The start address and end address of the memory range will be rounded down and rounded up respectively to be aligned to CPU page size before the advice is applied. The memory range must refer to managed memory allocated via ::cudaMallocManaged or declared via managed variables. The memory range could also refer to system-allocated pageable memory provided it represents a valid, host-accessible region of memory and all additional constraints imposed by \p advice as outlined below are also satisfied. Specifying an invalid system-allocated pageable memory range results in an error being returned.

The \p advice parameter can take the following values:

• ::cudaMemAdviseSetReadMostly: This implies that the data is mostly going to be read from and only occasionally written to. Any read accesses from any processor to this region will create a read-only copy of at least the accessed pages in that processor’s memory. Additionally, if ::cudaMemPrefetchAsync is called on this region, it will create a read-only copy of the data on the destination processor. If any processor writes to this region, all copies of the corresponding page will be invalidated except for the one where the write occurred. The \p device argument is ignored for this advice. Note that for a page to be read-duplicated, the accessing processor must either be the CPU or a GPU that has a non-zero value for the device attribute ::cudaDevAttrConcurrentManagedAccess. Also, if a context is created on a device that does not have the device attribute ::cudaDevAttrConcurrentManagedAccess set, then read-duplication will not occur until all such contexts are destroyed. If the memory region refers to valid system-allocated pageable memory, then the accessing device must have a non-zero value for the device attribute ::cudaDevAttrPageableMemoryAccess for a read-only copy to be created on that device. Note however that if the accessing device also has a non-zero value for the device attribute ::cudaDevAttrPageableMemoryAccessUsesHostPageTables, then setting this advice will not create a read-only copy when that device accesses this memory region.

• ::cudaMemAdviceUnsetReadMostly: Undoes the effect of ::cudaMemAdviceReadMostly and also prevents the Unified Memory driver from attempting heuristic read-duplication on the memory range. Any read-duplicated copies of the data will be collapsed into a single copy. The location for the collapsed copy will be the preferred location if the page has a preferred location and one of the read-duplicated copies was resident at that location. Otherwise, the location chosen is arbitrary.

• ::cudaMemAdviseSetPreferredLocation: This advice sets the preferred location for the data to be the memory belonging to \p device. Passing in cudaCpuDeviceId for \p device sets the preferred location as host memory. If \p device is a GPU, then it must have a non-zero value for the device attribute ::cudaDevAttrConcurrentManagedAccess. Setting the preferred location does not cause data to migrate to that location immediately. Instead, it guides the migration policy when a fault occurs on that memory region. If the data is already in its preferred location and the faulting processor can establish a mapping without requiring the data to be migrated, then data migration will be avoided. On the other hand, if the data is not in its preferred location or if a direct mapping cannot be established, then it will be migrated to the processor accessing it. It is important to note that setting the preferred location does not prevent data prefetching done using ::cudaMemPrefetchAsync. Having a preferred location can override the page thrash detection and resolution logic in the Unified Memory driver. Normally, if a page is detected to be constantly thrashing between for example host and device memory, the page may eventually be pinned to host memory by the Unified Memory driver. But if the preferred location is set as device memory, then the page will continue to thrash indefinitely. If ::cudaMemAdviseSetReadMostly is also set on this memory region or any subset of it, then the policies associated with that advice will override the policies of this advice, unless read accesses from \p device will not result in a read-only copy being created on that device as outlined in description for the advice ::cudaMemAdviseSetReadMostly. If the memory region refers to valid system-allocated pageable memory, then \p device must have a non-zero value for the device attribute ::cudaDevAttrPageableMemoryAccess. Additionally, if \p device has a non-zero value for the device attribute ::cudaDevAttrPageableMemoryAccessUsesHostPageTables, then this call has no effect. Note however that this behavior may change in the future.

• ::cudaMemAdviseUnsetPreferredLocation: Undoes the effect of ::cudaMemAdviseSetPreferredLocation and changes the preferred location to none.

• ::cudaMemAdviseSetAccessedBy: This advice implies that the data will be accessed by \p device. Passing in ::cudaCpuDeviceId for \p device will set the advice for the CPU. If \p device is a GPU, then the device attribute ::cudaDevAttrConcurrentManagedAccess must be non-zero. This advice does not cause data migration and has no impact on the location of the data per se. Instead, it causes the data to always be mapped in the specified processor’s page tables, as long as the location of the data permits a mapping to be established. If the data gets migrated for any reason, the mappings are updated accordingly. This advice is recommended in scenarios where data locality is not important, but avoiding faults is. Consider for example a system containing multiple GPUs with peer-to-peer access enabled, where the data located on one GPU is occasionally accessed by peer GPUs. In such scenarios, migrating data over to the other GPUs is not as important because the accesses are infrequent and the overhead of migration may be too high. But preventing faults can still help improve performance, and so having a mapping set up in advance is useful. Note that on CPU access of this data, the data may be migrated to host memory because the CPU typically cannot access device memory directly. Any GPU that had the ::cudaMemAdviceSetAccessedBy flag set for this data will now have its mapping updated to point to the page in host memory. If ::cudaMemAdviseSetReadMostly is also set on this memory region or any subset of it, then the policies associated with that advice will override the policies of this advice. Additionally, if the preferred location of this memory region or any subset of it is also \p device, then the policies associated with ::cudaMemAdviseSetPreferredLocation will override the policies of this advice. If the memory region refers to valid system-allocated pageable memory, then \p device must have a non-zero value for the device attribute ::cudaDevAttrPageableMemoryAccess. Additionally, if \p device has a non-zero value for the device attribute ::cudaDevAttrPageableMemoryAccessUsesHostPageTables, then this call has no effect.

• ::cudaMemAdviseUnsetAccessedBy: Undoes the effect of ::cudaMemAdviseSetAccessedBy. Any mappings to the data from \p device may be removed at any time causing accesses to result in non-fatal page faults. If the memory region refers to valid system-allocated pageable memory, then \p device must have a non-zero value for the device attribute ::cudaDevAttrPageableMemoryAccess. Additionally, if \p device has a non-zero value for the device attribute ::cudaDevAttrPageableMemoryAccessUsesHostPageTables, then this call has no effect.

\param devPtr - Pointer to memory to set the advice for \param count - Size in bytes of the memory range \param advice - Advice to be applied for the specified memory range \param device - Device to apply the advice for

\return ::cudaSuccess, ::cudaErrorInvalidValue, ::cudaErrorInvalidDevice \notefnerr \note_async \note_null_stream \note_init_rt \note_callback

\sa ::cudaMemcpy, ::cudaMemcpyPeer, ::cudaMemcpyAsync, ::cudaMemcpy3DPeerAsync, ::cudaMemPrefetchAsync, ::cuMemAdvise