Fixed value 1.0, The performance of the sliding-window benchmark with fixed hit-ratio of 1.0. Exponentiation With Small Fractional Arguments, 14. Local memory is used only to hold automatic variables. If L1-caching is enabled on these devices, the number of required transactions is equal to the number of required 128-byte aligned segments. Adjacent threads accessing memory with a stride of 2. To measure performance accurately, it is useful to calculate theoretical and effective bandwidth. For codes continuing to make use of PTX, in order to support compiling on an older driver, your code must be first transformed into device code via the static ptxjitcompiler library or NVRTC with the option of generating code for a specific architecture (e.g. This utility allows administrators to query GPU device state and, with the appropriate privileges, permits administrators to modify GPU device state. The following example illustrates the basic technique. If textures are fetched using tex1D(),tex2D(), or tex3D() rather than tex1Dfetch(), the hardware provides other capabilities that might be useful for some applications such as image processing, as shown in Table 4. Medium Priority: To hide latency arising from register dependencies, maintain sufficient numbers of active threads per multiprocessor (i.e., sufficient occupancy). For devices with compute capability of 2.0 or greater, the Visual Profiler can be used to collect several different memory throughput measures. Testing of all parameters of each product is not necessarily performed by NVIDIA. The context encapsulates kernel launches and memory allocations for that GPU as well as supporting constructs such as the page tables. This optimization is especially important for global memory accesses, because latency of access costs hundreds of clock cycles. Minimize data transfers between the host and the device. Any CPU timer can be used to measure the elapsed time of a CUDA call or kernel execution. In this particular example, the offset memory throughput achieved is, however, approximately 9/10th, because adjacent warps reuse the cache lines their neighbors fetched. Do new devs get fired if they can't solve a certain bug? 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). CUDA driver - User-mode driver component used to run CUDA applications (e.g. Because shared memory is shared by threads in a thread block, it provides a mechanism for threads to cooperate. These results should be compared with those in Table 2. For example, to use only devices 0 and 2 from the system-wide list of devices, set CUDA_VISIBLE_DEVICES=0,2 before launching the application. As a particular example, to evaluate the sine function in degrees instead of radians, use sinpi(x/180.0). If static linking against the CUDA Runtime is impractical for some reason, then a dynamically-linked version of the CUDA Runtime library is also available. The right value for minBlocksPerMultiprocessor should be determined using a detailed per kernel analysis. Since there are many possible optimizations that can be considered, having a good understanding of the needs of the application can help to make the process as smooth as possible. A slightly related but important topic is one of application binary compatibility across GPU architectures in CUDA. Now Let's Look at Shared Memory Common Programming Pattern (5.1.2 of CUDA manual) - Load data into shared memory - Synchronize (if necessary) - Operate on data in shared memory - Synchronize (if necessary) - Write intermediate results to global memory - Repeat until done Shared memory Global memory Familiar concept?? A CUDA device has a number of different memory components that are available to programmers - register, shared memory, local memory, global memory and constant memory. Low Priority: Use zero-copy operations on integrated GPUs for CUDA Toolkit version 2.2 and later. Threads on a CPU are generally heavyweight entities. In the code in Zero-copy host code, kernel() can reference the mapped pinned host memory using the pointer a_map in exactly the same was as it would if a_map referred to a location in device memory. The following sections discuss some caveats and considerations. The ideal scenario is one in which many threads perform a substantial amount of work. Salient Features of Device Memory, Misaligned sequential addresses that fall within five 32-byte segments, Adjacent threads accessing memory with a stride of 2, /* Set aside max possible size of L2 cache for persisting accesses */, // Stream level attributes data structure. For example, we can write our CUDA kernels as a collection of many short __device__ functions rather than one large monolithic __global__ function; each device function can be tested independently before hooking them all together. As PTX is compiled by the CUDA driver, new toolchains will generate PTX that is not compatible with the older CUDA driver. For example, to compute the effective bandwidth of a 2048 x 2048 matrix copy, the following formula could be used: \(\text{Effective\ bandwidth} = \left( {\left( 2048^{2} \times 4 \times 2 \right) \div 10^{9}} \right) \div \text{time}\). This guide introduces the Assess, Parallelize, Optimize, Deploy(APOD) design cycle for applications with the goal of helping application developers to rapidly identify the portions of their code that would most readily benefit from GPU acceleration, rapidly realize that benefit, and begin leveraging the resulting speedups in production as early as possible. Also, because of the overhead associated with each transfer, batching many small transfers into one larger transfer performs significantly better than making each transfer separately, even if doing so requires packing non-contiguous regions of memory into a contiguous buffer and then unpacking after the transfer. These examples assume compute capability 6.0 or higher and that accesses are for 4-byte words, unless otherwise noted. New APIs can be added in minor versions. Low Medium Priority: Use signed integers rather than unsigned integers as loop counters. Before addressing specific performance tuning issues covered in this guide, refer to the NVIDIA Ampere GPU Architecture Compatibility Guide for CUDA Applications to ensure that your application is compiled in a way that is compatible with the NVIDIA Ampere GPU Architecture. The discussions in this guide all use the C++ programming language, so you should be comfortable reading C++ code. The remainder of the kernel code is identical to the staticReverse() kernel. (Factorization). Understanding the Programming Environment, 15. 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. Even though such an access requires only 1 transaction on devices of compute capability 2.0 or higher, there is wasted bandwidth in the transaction, because only one 4-byte word out of 8 words in a 32-byte cache segment is used. It presents established parallelization and optimization techniques and explains coding metaphors and idioms that can greatly simplify programming for CUDA-capable GPU architectures. Integer division and modulo operations are particularly costly and should be avoided or replaced with bitwise operations whenever possible: If \(n\) is a power of 2, ( \(i/n\) ) is equivalent to ( \(i \gg {log2}(n)\) ) and ( \(i\% n\) ) is equivalent to ( \(i\&\left( {n - 1} \right)\) ). Week5 + Week8 by AkeelMedina22 Pull Request #9 mmmovania/CUDA We define source compatibility as a set of guarantees provided by the library, where a well-formed application built against a specific version of the library (using the SDK) will continue to build and run without errors when a newer version of the SDK is installed. The CUDA Driver API has a versioned C-style ABI, which guarantees that applications that were running against an older driver (for example CUDA 3.2) will still run and function correctly against a modern driver (for example one shipped with CUDA 11.0). CUDA Shared Memory Capacity - Lei Mao's Log Book In such a case, the bandwidth would be 836.4 GiB/s. Consequently, the order in which arithmetic operations are performed is important. To achieve high memory bandwidth for concurrent accesses, shared memory is divided into equally sized memory modules (banks) that can be accessed simultaneously. The NVIDIA Ampere GPU architecture retains and extends the same CUDA programming model provided by previous NVIDIA GPU architectures such as Turing and Volta, and applications that follow the best practices for those architectures should typically see speedups on the NVIDIA A100 GPU without any code changes. vegan) just to try it, does this inconvenience the caterers and staff? As with the dynamically-linked version of the CUDA Runtime library, these libraries should be bundled with the application executable when distributing that application. However, this latency can be completely hidden by the execution of threads in other warps. Sometimes, the compiler may unroll loops or optimize out if or switch statements by using branch predication instead. For example, improving occupancy from 66 percent to 100 percent generally does not translate to a similar increase in performance. Accesses to different addresses by threads within a warp are serialized, thus the cost scales linearly with the number of unique addresses read by all threads within a warp. Functions following functionName() naming convention are slower but have higher accuracy (e.g., sinf(x) and expf(x)). An example is transposing [1209, 9] of any type and 32 tile size. Alternatively, NVRTC can generate cubins directly starting with CUDA 11.1. In calculating each of the rows of a tile of matrix C, the entire tile of B is read. Managing your GPU cluster will help achieve maximum GPU utilization and help you and your users extract the best possible performance. The throughput of individual arithmetic operations is detailed in the CUDA C++ Programming Guide. compute_80). The formulas in the table below are valid for x >= 0, x != -0, that is, signbit(x) == 0. If cudaGetDeviceCount() reports an error, the application should fall back to an alternative code path. Global memory: is the memory residing graphics/accelerator card but not inside GPU chip. On parallel systems, it is possible to run into difficulties not typically found in traditional serial-oriented programming. Setting the bank size to eight bytes can help avoid shared memory bank conflicts when accessing double precision data. It supports a number of command-line parameters, of which the following are especially useful for optimization and related best practices: -maxrregcount=N specifies the maximum number of registers kernels can use at a per-file level. The CUDA driver ensures backward Binary Compatibility is maintained for compiled CUDA applications. Shared Memory and Synchronization - GPU Programming Let's say that there are m blocks. Alternatively, the nvcc command-line option -arch=sm_XX can be used as a shorthand equivalent to the following more explicit -gencode= command-line options described above: However, while the -arch=sm_XX command-line option does result in inclusion of a PTX back-end target by default (due to the code=compute_XX target it implies), it can only specify a single target cubin architecture at a time, and it is not possible to use multiple -arch= options on the same nvcc command line, which is why the examples above use -gencode= explicitly. Asynchronous copy achieves better performance in nearly all cases. Why do academics stay as adjuncts for years rather than move around? As an example, the assignment operator in the following sample code has a high throughput, but, crucially, there is a latency of hundreds of clock cycles to read data from global memory: Much of this global memory latency can be hidden by the thread scheduler if there are sufficient independent arithmetic instructions that can be issued while waiting for the global memory access to complete. (Consider what would happen to the memory addresses accessed by the second, third, and subsequent thread blocks if the thread block size was not a multiple of warp size, for example.). 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. The maximum number of registers per thread can be set manually at compilation time per-file using the -maxrregcount option or per-kernel using the __launch_bounds__ qualifier (see Register Pressure). It is recommended to use cudaMallocAsync() and cudaFreeAsync() which are stream ordered pool allocators to manage device memory. Regardless of this possibility, it is good practice to verify that no higher-priority recommendations have been overlooked before undertaking lower-priority items. These copy instructions are asynchronous, with respect to computation and allow users to explicitly control overlap of compute with data movement from global memory into the SM. On PCIe x16 Gen3 cards, for example, pinned memory can attain roughly 12 GB/s transfer rates. We cannot declare these directly, but small static allocations go . Fast, low-precision interpolation between texels, Valid only if the texture reference returns floating-point data, Can be used only with normalized texture coordinates, 1 The automatic handling of boundary cases in the bottom row of Table 4 refers to how a texture coordinate is resolved when it falls outside the valid addressing range. Mapped pinned host memory allows you to overlap CPU-GPU memory transfers with computation while avoiding the use of CUDA streams. This is because the user could only allocate the CUDA static shared memory up to 48 KB. The results are shown in the chart below, where we see good performance regardless of whether the persistent data fits in the L2 set-aside or not. Find centralized, trusted content and collaborate around the technologies you use most. Some aspects of this behavior such as cache location and maximum cache size can be controlled via the use of environment variables; see Just in Time Compilation of the CUDA C++ Programming Guide. Shared memory can also be used to avoid uncoalesced memory accesses by loading and storing data in a coalesced pattern from global memory and then reordering it in shared memory. Dynamic parallelism - passing contents of shared memory to spawned blocks? The number of registers available, the maximum number of simultaneous threads resident on each multiprocessor, and the register allocation granularity vary over different compute capabilities. For example, on IBM Newell POWER9 nodes (where the CPUs correspond to NUMA nodes 0 and 8), use: One of the keys to good performance is to keep the multiprocessors on the device as busy as possible. NVIDIA Ampere GPU Architecture Tuning Guide Note that no bank conflict occurs if only one memory location per bank is accessed by a half warp of threads. Loop Counters Signed vs. Unsigned, 11.1.5. For best performance, there should be some coherence in memory access by adjacent threads running on the device. Be aware that CPU-to-GPU synchronization points such as those mentioned in this section imply a stall in the GPUs processing pipeline and should thus be used sparingly to minimize their performance impact. Another common approach to parallelization of sequential codes is to make use of parallelizing compilers. The versions of the components in the toolkit are available in this table. To illustrate the effect of strided access on effective bandwidth, see the kernel strideCopy() in A kernel to illustrate non-unit stride data copy, which copies data with a stride of stride elements between threads from idata to odata. Accesses to the remaining data of the memory region (i.e., streaming data) are considered normal or streaming accesses and will thus use the remaining 10 MB of the non set-aside L2 portion (unless part of the L2 set-aside portion is unused). CUDA Shared Memory - Oak Ridge Leadership Computing Facility 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. If instead i is declared as signed, where the overflow semantics are undefined, the compiler has more leeway to use these optimizations. Coalescing concepts are illustrated in the following simple examples. Weak Scaling and Gustafsons Law describes weak scaling, where the speedup is attained by growing the problem size. The peak theoretical bandwidth between the device memory and the GPU is much higher (898 GB/s on the NVIDIA Tesla V100, for example) than the peak theoretical bandwidth between host memory and device memory (16 GB/s on the PCIe x16 Gen3). While the contents can be used as a reference manual, you should be aware that some topics are revisited in different contexts as various programming and configuration topics are explored. CUDA: Using shared memory between different kernels.. One or more compute capability versions can be specified to the nvcc compiler while building a file; compiling for the native compute capability for the target GPU(s) of the application is important to ensure that application kernels achieve the best possible performance and are able to use the features that are available on a given generation of GPU. In the kernel launch, specify the total shared memory needed, as in the following. These memory spaces include global, local, shared, texture, and registers, as shown in Figure 2. The CUDA Toolkit includes a number of such libraries that have been fine-tuned for NVIDIA CUDA GPUs, such as cuBLAS, cuFFT, and so on. As the stride increases, the effective bandwidth decreases until the point where 32 32-byte segments are loaded for the 32 threads in a warp, as indicated in Figure 7. If from any of the four 32-byte segments only a subset of the words are requested (e.g. A useful technique to determine the sensitivity of performance to occupancy is through experimentation with the amount of dynamically allocated shared memory, as specified in the third parameter of the execution configuration. Max and current clock rates are reported for several important clock domains, as well as the current GPU performance state (pstate). Shared memory can be helpful in several situations, such as helping to coalesce or eliminate redundant access to global memory. Operations in different streams can be interleaved and in some cases overlapped - a property that can be used to hide data transfers between the host and the device. This chapter examines issues that can affect the correctness of returned data and points to appropriate solutions. If a single block needs to load all queues, then all queues will need to be placed in global memory by their respective blocks. The CUDA Driver API thus is binary-compatible (the OS loader can pick up a newer version and the application continues to work) but not source-compatible (rebuilding your application against a newer SDK might require source changes). CUDA 11.0 introduces an async-copy feature that can be used within device code to explicitly manage the asynchronous copying of data from global memory to shared memory. I have locally sorted queues in different blocks of cuda. Note that Gustafsons Law assumes that the ratio of serial to parallel execution remains constant, reflecting additional cost in setting up and handling the larger problem. 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. When the persistent data region fits well into the 30 MB set-aside portion of the L2 cache, a performance increase of as much as 50% is observed. The CUDA Runtime handles kernel loading and setting up kernel parameters and launch configuration before the kernel is launched.