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Local memory is so named because its scope is local to the thread, not because of its physical location. For devices of compute capability 6.0 or higher, the requirements can be summarized quite easily: the concurrent accesses of the threads of a warp will coalesce into a number of transactions equal to the number of 32-byte transactions necessary to service all of the threads of the warp. The combined L1 cache capacity for GPUs with compute capability 8.6 is 128 KB. Scattered accesses increase ECC memory transfer overhead, especially when writing data to global memory. The NVIDIA Ampere GPU architecture includes new Third Generation Tensor Cores that are more powerful than the Tensor Cores used in Volta and Turing SMs. Performance benefits can be more readily achieved when this ratio is higher. Your code might reflect different priority factors. To learn more, see our tips on writing great answers. Non-default streams are required for this overlap because memory copy, memory set functions, and kernel calls that use the default stream begin only after all preceding calls on the device (in any stream) have completed, and no operation on the device (in any stream) commences until they are finished. The throughput of individual arithmetic operations is detailed in the CUDA C++ Programming Guide. By comparison, threads on GPUs are extremely lightweight. In addition to the calculator spreadsheet, occupancy can be determined using the NVIDIA Nsight Compute Profiler. If from any of the four 32-byte segments only a subset of the words are requested (e.g. The cudaMemcpyAsync() function is a non-blocking variant of cudaMemcpy() in which control is returned immediately to the host thread. Adjust kernel launch configuration to maximize device utilization. In such cases, and when the execution time (tE) exceeds the transfer time (tT), a rough estimate for the overall time is tE + tT/nStreams for the staged version versus tE + tT for the sequential version. No contractual obligations are formed either directly or indirectly by this document. The application should also maximize parallel execution at a higher level by explicitly exposing concurrent execution on the device through streams, as well as maximizing concurrent execution between the host and the device. For small integer powers (e.g., x2 or x3), explicit multiplication is almost certainly faster than the use of general exponentiation routines such as pow(). As for optimizing instruction usage, the use of arithmetic instructions that have low throughput should be avoided. This is called just-in-time compilation (JIT). Answer (1 of 2): Shared memory has many more channels(and bandwidth) and works with much less latency. For 32-bit applications, the file would be cublas32_55.dll. aims to make the expression of this parallelism as simple as possible, while simultaneously enabling operation on CUDA-capable GPUs designed for maximum parallel throughput. Instead, strategies can be applied incrementally as they are learned. These many-way bank conflicts are very expensive. Using these data items, the peak theoretical memory bandwidth of the NVIDIA Tesla V100 is 898 GB/s: \(\left. All of these products (nvidia-smi, NVML, and the NVML language bindings) are updated with each new CUDA release and provide roughly the same functionality. To minimize bank conflicts, it is important to understand how memory addresses map to memory banks. For portability, that is, to be able to execute code on future GPU architectures with higher compute capability (for which no binary code can be generated yet), an application must load PTX code that will be just-in-time compiled by the NVIDIA driver for these future devices. It is therefore best to redistribute the CUDA Runtime library with the application when using dynamic linking or else to statically link against the CUDA Runtime. The following documents are especially important resources: In particular, the optimization section of this guide assumes that you have already successfully downloaded and installed the CUDA Toolkit (if not, please refer to the relevant CUDA Installation Guide for your platform) and that you have a basic familiarity with the CUDA C++ programming language and environment (if not, please refer to the CUDA C++ Programming Guide). However, the device is based on a distinctly different design from the host system, and its important to understand those differences and how they determine the performance of CUDA applications in order to use CUDA effectively. // (Must be less than cudaDeviceProp::accessPolicyMaxWindowSize), // Hint for L2 cache hit ratio for persisting accesses in the num_bytes region. Note that the process used for validating numerical results can easily be extended to validate performance results as well. The context encapsulates kernel launches and memory allocations for that GPU as well as supporting constructs such as the page tables. Just-in-time compilation increases application load time but allows applications to benefit from latest compiler improvements. On Wednesday, February 19, 2020, NVIDIA will present part 2 of a 9-part CUDA Training Series titled "CUDA Shared Memory". It also avoids an intermediary register file access traditionally present between the global memory read and the shared memory write. compute_80). A trivial example is when the controlling condition depends only on (threadIdx / WSIZE) where WSIZE is the warp size. If A, B, and C are floating-point values, (A+B)+C is not guaranteed to equal A+(B+C) as it is in symbolic math. Two types of runtime math operations are supported. We define binary compatibility as a set of guarantees provided by the library, where an application targeting the said library will continue to work when dynamically linked against a different version of the library. However, if multiple threads requested addresses map to the same memory bank, the accesses are serialized. This guide refers to and relies on several other documents that you should have at your disposal for reference, all of which are available at no cost from the CUDA website https://docs.nvidia.com/cuda/. Built on top of these technologies are CUDA libraries, some of which are included in the CUDA Toolkit, while others such as cuDNN may be released independently of the CUDA Toolkit. The operating system must swap threads on and off CPU execution channels to provide multithreading capability. However, compared to cache based architectures, like CPUs, latency hiding architectures, like GPUs, tend to cope better with completely random memory access patterns. While compiler optimization improvements continually seek to narrow this gap, explicit multiplication (or the use of an equivalent purpose-built inline function or macro) can have a significant advantage. As described in Asynchronous and Overlapping Transfers with Computation, CUDA streams can be used to overlap kernel execution with data transfers. "After the incident", I started to be more careful not to trip over things. This also prevents array elements being repeatedly read from global memory if the same data is required several times. For example, in the standard CUDA Toolkit installation, the files libcublas.so and libcublas.so.5.5 are both symlinks pointing to a specific build of cuBLAS, which is named like libcublas.so.5.5.x, where x is the build number (e.g., libcublas.so.5.5.17). The programming guide for tuning CUDA Applications for GPUs based on the NVIDIA Ampere GPU Architecture. By understanding how applications can scale it is possible to set expectations and plan an incremental parallelization strategy. Any PTX device code loaded by an application at runtime is compiled further to binary code by the device driver. However, this approach of determining how register count affects occupancy does not take into account the register allocation granularity. Many software libraries and applications built on top of CUDA (e.g. Is a PhD visitor considered as a visiting scholar? Zero copy can be used in place of streams because kernel-originated data transfers automatically overlap kernel execution without the overhead of setting up and determining the optimal number of streams. In our experiment, we vary the size of this persistent data region from 10 MB to 60 MB to model various scenarios where data fits in or exceeds the available L2 set-aside portion of 30 MB. All rights reserved. The CUDA event API provides calls that create and destroy events, record events (including a timestamp), and convert timestamp differences into a floating-point value in milliseconds. Register storage enables threads to keep local variables nearby for low-latency access. The third generation of NVIDIAs high-speed NVLink interconnect is implemented in A100 GPUs, which significantly enhances multi-GPU scalability, performance, and reliability with more links per GPU, much faster communication bandwidth, and improved error-detection and recovery features. It is important to include the overhead of transferring data to and from the device in determining whether operations should be performed on the host or on the device. For branches including just a few instructions, warp divergence generally results in marginal performance losses. The amount of performance benefit an application will realize by running on CUDA depends entirely on the extent to which it can be parallelized. Comparing Synchronous vs Asynchronous Copy from Global Memory to Shared Memory. There are several key strategies for parallelizing sequential code. Bandwidth - the rate at which data can be transferred - is one of the most important gating factors for performance. Ensure global memory accesses are coalesced. On PCIe x16 Gen3 cards, for example, pinned memory can attain roughly 12 GB/s transfer rates. We adjust the copy_count in the kernels such that each thread block copies from 512 bytes up to 48 MB. Connect and share knowledge within a single location that is structured and easy to search. THIS DOCUMENT AND ALL NVIDIA DESIGN SPECIFICATIONS, REFERENCE BOARDS, FILES, DRAWINGS, DIAGNOSTICS, LISTS, AND OTHER DOCUMENTS (TOGETHER AND SEPARATELY, MATERIALS) ARE BEING PROVIDED AS IS. NVIDIA MAKES NO WARRANTIES, EXPRESSED, IMPLIED, STATUTORY, OR OTHERWISE WITH RESPECT TO THE MATERIALS, AND EXPRESSLY DISCLAIMS ALL IMPLIED WARRANTIES OF NONINFRINGEMENT, MERCHANTABILITY, AND FITNESS FOR A PARTICULAR PURPOSE. The host system and the device each have their own distinct attached physical memories 1. There are a number of tools that can be used to generate the profile. An additional set of Perl and Python bindings are provided for the NVML API. Compiler JIT Cache Management Tools, 18.1. To scale to future devices, the number of blocks per kernel launch should be in the thousands. Code that cannot be sufficiently parallelized should run on the host, unless doing so would result in excessive transfers between the host and the device. In reality, most applications do not exhibit perfectly linear strong scaling, even if they do exhibit some degree of strong scaling. Shared memory accesses, in counterpoint, are usually worth optimizing only when there exists a high degree of bank conflicts. Shared memory is magnitudes faster to access than global memory. Developers are notified through deprecation and documentation mechanisms of any current or upcoming changes. To analyze performance, it is necessary to consider how warps access global memory in the for loop. Some calculations use 10243 instead of 109 for the final calculation. Replace sin(*) with sinpi(), cos(*) with cospi(), and sincos(*) with sincospi(). In our use case, BLOCK_SIZE + 2 * RADIUS = $1024 + 2 \times 6000$ = $13024$ and the size of an int is $4$ Byte, therefore, the shared memory required is $17024 \times 4 / 1024$ = $50.875$ KB, which is larger than the maximum static shared memory we could have. (Note that on devices of Compute Capability 1.2 or later, the memory system can fully coalesce even the reversed index stores to global memory. So, in the previous example, had the two matrices to be added already been on the device as a result of some previous calculation, or if the results of the addition would be used in some subsequent calculation, the matrix addition should be performed locally on the device. To do this, the simpleMultiply kernel (Unoptimized matrix multiplication) calculates the output elements of a tile of matrix C. In Unoptimized matrix multiplication, a, b, and c are pointers to global memory for the matrices A, B, and C, respectively; blockDim.x, blockDim.y, and TILE_DIM are all equal to w. Each thread in the wxw-thread block calculates one element in a tile of C. row and col are the row and column of the element in C being calculated by a particular thread. Zero copy is a feature that was added in version 2.2 of the CUDA Toolkit. Single-precision floats provide the best performance, and their use is highly encouraged. The most important consideration with any profiling activity is to ensure that the workload is realistic - i.e., that information gained from the test and decisions based upon that information are relevant to real data. See the CUDA C++ Programming Guide for further explanations and software requirements for UVA and P2P. See Math Libraries. On devices that have this capability, the overlap once again requires pinned host memory, and, in addition, the data transfer and kernel must use different, non-default streams (streams with non-zero stream IDs). The access requirements for coalescing depend on the compute capability of the device and are documented in the CUDA C++ Programming Guide. Misaligned sequential addresses that fall within five 32-byte segments, Memory allocated through the CUDA Runtime API, such as via cudaMalloc(), is guaranteed to be aligned to at least 256 bytes. This means that in one of these devices, for a multiprocessor to have 100% occupancy, each thread can use at most 32 registers. Concurrent copy and execute demonstrates how to overlap kernel execution with asynchronous data transfer. Delays in rolling out new NVIDIA drivers could mean that users of such systems may not have access to new features available in CUDA releases. For example, it may be desirable to use a 64x64 element shared memory array in a kernel, but because the maximum number of threads per block is 1024, it is not possible to launch a kernel with 64x64 threads per block. Before implementing lower priority recommendations, it is good practice to make sure all higher priority recommendations that are relevant have already been applied. This difference is illustrated in Figure 13. On devices that are capable of concurrent copy and compute, it is possible to overlap kernel execution on the device with data transfers between the host and the device. Mapped pinned host memory allows you to overlap CPU-GPU memory transfers with computation while avoiding the use of CUDA streams. In particular, there is no register-related reason to pack data into vector data types such as float4 or int4 types. For devices with compute capability of 2.0 or greater, the Visual Profiler can be used to collect several different memory throughput measures. Parallelizing these functions as well should increase our speedup potential. Another important concept is the management of system resources allocated for a particular task. The most straightforward approach to parallelizing an application is to leverage existing libraries that take advantage of parallel architectures on our behalf. Pinned memory is allocated using the cudaHostAlloc() functions in the Runtime API. How do you ensure that a red herring doesn't violate Chekhov's gun? The CUDA Toolkits End-User License Agreement (EULA) allows for redistribution of many of the CUDA libraries under certain terms and conditions. Prefer shared memory access where possible. The NVIDIA Ampere GPU architectures Streaming Multiprocessor (SM) provides the following improvements over Volta and Turing. Recovering from a blunder I made while emailing a professor. I think this pretty much implies that you are going to have the place the heads of each queue in global memory. Moreover, in such cases, the argument-reduction code uses local memory, which can affect performance even more because of the high latency of local memory. In fact, shared memory latency is roughly 100x lower than uncached global memory latency (provided that there are no bank conflicts between the threads, which we will examine later in this post). Last updated on Feb 27, 2023. Information published by NVIDIA regarding third-party products or services does not constitute a license from NVIDIA to use such products or services or a warranty or endorsement thereof. On the other hand, some applications designs will require some amount of refactoring to expose their inherent parallelism. In order to maintain forward compatibility to future hardware and toolkits and to ensure that at least one thread block can run on an SM, developers should include the single argument __launch_bounds__(maxThreadsPerBlock) which specifies the largest block size that the kernel will be launched with. A copy kernel that illustrates misaligned accesses. Because of this, the maximum speedup S of a program is: Another way of looking at Gustafsons Law is that it is not the problem size that remains constant as we scale up the system but rather the execution time. Best performance with synchronous copy is achieved when the copy_count parameter is a multiple of 4 for all three element sizes. Constantly recompiling with the latest CUDA Toolkit means forcing upgrades on the end-customers of an application product. The NVIDIA Nsight Visual Studio Edition for Microsoft Windows 7, Windows HPC Server 2008, Windows 8.1, and Windows 10 is available as a free plugin for Microsoft Visual Studio; see: https://developer.nvidia.com/nsight-visual-studio-edition. It allows developers to use a CUDA-enabled graphics processing unit (GPU) to accelerate processing tasks in their applications. See Version Management for details on how to query the available CUDA software API versions. The NVIDIA nvcc compiler driver converts .cu files into C++ for the host system and CUDA assembly or binary instructions for the device. A minimum of 64 threads per block should be used, and only if there are multiple concurrent blocks per multiprocessor. For some applications the problem size will remain constant and hence only strong scaling is applicable. In both cases, kernels must be compiled into binary code by nvcc (called cubins) to execute on the device. The hardware splits a conflicting memory request into as many separate conflict-free requests as necessary, decreasing the effective bandwidth by a factor equal to the number of colliding memory requests. For optimal performance, users should manually tune the NUMA characteristics of their application. Now I have some problems. Applications compiled against a CUDA Toolkit version will only run on systems with the specified minimum driver version for that toolkit version. The discussions in this guide all use the C++ programming language, so you should be comfortable reading C++ code. While processors are evolving to expose more fine-grained parallelism to the programmer, many existing applications have evolved either as serial codes or as coarse-grained parallel codes (for example, where the data is decomposed into regions processed in parallel, with sub-regions shared using MPI). On devices with GDDR memory, accessing memory in a coalesced way is even more important when ECC is turned on. After each round of application parallelization is complete, the developer can move to optimizing the implementation to improve performance. Support for Bfloat16 Tensor Core, through HMMA instructions. Please see the MSDN documentation for these routines for more information. This number is divided by the time in seconds to obtain GB/s. Not all threads need to participate. Functions following the __functionName() naming convention map directly to the hardware level. CUDA calls and kernel executions can be timed using either CPU or GPU timers. Applications using the new API can load the final device code directly using driver APIs cuModuleLoadData and cuModuleLoadDataEx. Note that the timings are measured on the GPU clock, so the timing resolution is operating-system-independent. In A copy kernel that illustrates misaligned accesses, data is copied from the input array idata to the output array, both of which exist in global memory. Threads on a CPU are generally heavyweight entities. (tens of kBs capacity) Global memory is main memory (GDDR,HBM, (1-32 GB)) and data is cached by L2,L1 caches. rev2023.3.3.43278. (BFloat16 only supports FP32 as accumulator), unsigned char/signed char (8-bit precision). Shared memory is extremely fast, user managed, on-chip memory that can be used to share data between threads within a thread block. In many applications, a combination of strong and weak scaling is desirable. Testing of all parameters of each product is not necessarily performed by NVIDIA. Applications already using other BLAS libraries can often quite easily switch to cuBLAS, for example, whereas applications that do little to no linear algebra will have little use for cuBLAS. We fix the num_bytes in the access window to 20 MB and tune the hitRatio such that a random 20 MB of the total persistent data is resident in the L2 set-aside cache portion. Static linking makes the executable slightly larger, but it ensures that the correct version of runtime library functions are included in the application binary without requiring separate redistribution of the CUDA Runtime library. How to time code using CUDA events illustrates their use. For example, a 64-bit application linked to cuBLAS 5.5 will look for cublas64_55.dll at runtime, so this is the file that should be redistributed with that application, even though cublas.lib is the file that the application is linked against. This is advantageous with regard to both accuracy and performance. A slightly related but important topic is one of application binary compatibility across GPU architectures in CUDA. It provides functions to handle the following: Interoperability with OpenGL and Direct3D. I'm not sure if this will fit your overall processing. High Priority: Avoid different execution paths within the same warp. Optimizing memory usage starts with minimizing data transfers between the host and the device because those transfers have much lower bandwidth than internal device data transfers. As described in Memory Optimizations of this guide, bandwidth can be dramatically affected by the choice of memory in which data is stored, how the data is laid out and the order in which it is accessed, as well as other factors. Since shared memory is shared amongst threads in a thread block, it provides a mechanism for threads to cooperate. GPUs with compute capability 8.6 support shared memory capacity of 0, 8, 16, 32, 64 or 100 KB per SM. For each iteration i of the for loop, the threads in a warp read a row of the B tile, which is a sequential and coalesced access for all compute capabilities. Modern NVIDIA GPUs can support up to 2048 active threads concurrently per multiprocessor (see Features and Specifications of the CUDA C++ Programming Guide) On GPUs with 80 multiprocessors, this leads to more than 160,000 concurrently active threads. This new feature is exposed via the pipeline API in CUDA. The compiler must on occasion insert conversion instructions, introducing additional execution cycles. This spreadsheet, shown in Figure 15, is called CUDA_Occupancy_Calculator.xls and is located in the tools subdirectory of the CUDA Toolkit installation. In this case, multiple broadcasts from different banks are coalesced into a single multicast from the requested shared memory locations to the threads. Handling New CUDA Features and Driver APIs, 15.4.1.4. cudaOccupancyMaxActiveBlocksPerMultiprocessor, to dynamically select launch configurations based on runtime parameters. This will ensure that the executable will be able to run even if the user does not have the same CUDA Toolkit installed that the application was built against. Within each iteration of the for loop, a value in shared memory is broadcast to all threads in a warp. The hitRatio parameter can be used to specify the fraction of accesses that receive the hitProp property. CUDA kernel and thread hierarchy For exponentiation using base 2 or 10, use the functions exp2() or expf2() and exp10() or expf10() rather than the functions pow() or powf(). The CUDA Toolkit libraries (cuBLAS, cuFFT, etc.) Having understood the application profile, the developer should understand how the problem size would change if the computational performance changes and then apply either Amdahls or Gustafsons Law to determine an upper bound for the speedup. For example, many kernels have complex addressing logic for accessing memory in addition to their actual computation. Instead of a __syncthreads()synchronization barrier call, a __syncwarp() is sufficient after reading the tile of A into shared memory because only threads within the warp that write the data into shared memory read this data. This document is provided for information purposes only and shall not be regarded as a warranty of a certain functionality, condition, or quality of a product. By describing your computation in terms of these high-level abstractions you provide Thrust with the freedom to select the most efficient implementation automatically. Each component in the toolkit is recommended to be semantically versioned. High Priority: To get the maximum benefit from CUDA, focus first on finding ways to parallelize sequential code. This is of particular note with loop counters: since it is common for loop counters to have values that are always positive, it may be tempting to declare the counters as unsigned. Users should refer to the CUDA headers and documentation for new CUDA APIs introduced in a release. Some metric related to the number of active warps on a multiprocessor is therefore important in determining how effectively the hardware is kept busy. Generally, accessing a register consumes zero extra clock cycles per instruction, but delays may occur due to register read-after-write dependencies and register memory bank conflicts. My code is GPL licensed, can I issue a license to have my code be distributed in a specific MIT licensed project? Not using intermediate registers can help reduce register pressure and can increase kernel occupancy. Because L2 cache is on-chip, it potentially provides higher bandwidth and lower latency accesses to global memory. For devices of compute capability 1.x, the warp size is 32 threads and the number of banks is 16. The easiest option is to statically link against the CUDA Runtime. Because shared memory is shared by threads in a thread block, it provides a mechanism for threads to cooperate. The cudaDeviceEnablePeerAccess() API call remains necessary to enable direct transfers (over either PCIe or NVLink) between GPUs. The kernel is executed within a loop in host code that varies the parameter offset from 0 to 32. See Hardware Multithreading of the CUDA C++ Programming Guide for the register allocation formulas for devices of various compute capabilities and Features and Technical Specifications of the CUDA C++ Programming Guide for the total number of registers available on those devices. This capability (combined with thread synchronization) has a number of uses, such as user-managed data caches, high-performance cooperative parallel algorithms (parallel reductions, for example), and tofacilitate global memory coalescing in cases where it would otherwise not be possible. NVIDIA shall have no liability for the consequences or use of such information or for any infringement of patents or other rights of third parties that may result from its use. Coalesced using shared memory to store a tile of A, Using shared memory to eliminate redundant reads of a tile of B. Thread instructions are executed sequentially in CUDA, and, as a result, executing other warps when one warp is paused or stalled is the only way to hide latencies and keep the hardware busy. Therefore, an application that compiled successfully on an older version of the toolkit may require changes in order to compile against a newer version of the toolkit. The implicit driver version checking, code initialization, CUDA context management, CUDA module management (cubin to function mapping), kernel configuration, and parameter passing are all performed by the CUDA Runtime. Medium Priority: Prefer faster, more specialized math functions over slower, more general ones when possible. Within a kernel call, the texture cache is not kept coherent with respect to global memory writes, so texture fetches from addresses that have been written via global stores in the same kernel call return undefined data. Recommendations for taking advantage of minor version compatibility in your application, 16.4. How to notate a grace note at the start of a bar with lilypond? Note also that whenever sine and cosine of the same argument are computed, the sincos family of instructions should be used to optimize performance: __sincosf() for single-precision fast math (see next paragraph). Otherwise, five 32-byte segments are loaded per warp, and we would expect approximately 4/5th of the memory throughput achieved with no offsets. The new Tensor Cores use a larger base matrix size and add powerful new math modes including: Support for FP64 Tensor Core, using new DMMA instructions. The cudaEventElapsedTime() function returns the time elapsed between the recording of the start and stop events.