Refer to the CUDA Toolkit Release Notes for details for the minimum driver version and the version of the driver shipped with the toolkit. Optimizations can be applied at various levels, from overlapping data transfers with computation all the way down to fine-tuning floating-point operation sequences. 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 execute code on devices of specific compute capability, an application must load binary or PTX code that is compatible with this compute capability. Note that no bank conflict occurs if only one memory location per bank is accessed by a half warp of threads. Then, as shown in the figure below, we specify that the accesses to the first freqSize * sizeof(int) bytes of the memory region are persistent. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Devices of compute capability 2.0 and later support a special addressing mode called Unified Virtual Addressing (UVA) on 64-bit Linux and Windows. Each floating-point arithmetic operation involves a certain amount of rounding. Figure 6 illustrates such a situation; in this case, threads within a warp access words in memory with a stride of 2. Handling New CUDA Features and Driver APIs, 15.4.1.4. 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. 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. As a particular example, to evaluate the sine function in degrees instead of radians, use sinpi(x/180.0). (The exceptions to this are kernel launches, which return void, and cudaGetErrorString(), which returns a character string describing the cudaError_t code that was passed into it.) Furthermore, if accesses by the threads of the warp had been permuted within or accross the four segments, still only four 32-byte transactions would have been performed by a device with compute capability 6.0 or higher. (BFloat16 only supports FP32 as accumulator), unsigned char/signed char (8-bit precision). The cudaMemcpyAsync() function is a non-blocking variant of cudaMemcpy() in which control is returned immediately to the host thread. Avoid long sequences of diverged execution by threads within the same warp. Transfers between NVLink-connected endpoints are automatically routed through NVLink, rather than PCIe. Clear single-bit and double-bit ECC error counts. If there are differences, then those differences will be seen early and can be understood in the context of a simple function. For some applications the problem size will remain constant and hence only strong scaling is applicable. Production code should, however, systematically check the error code returned by each API call and check for failures in kernel launches by calling cudaGetLastError(). To subscribe to this RSS feed, copy and paste this URL into your RSS reader. This does not apply to the NVIDIA Driver; the end user must still download and install an NVIDIA Driver appropriate to their GPU(s) and operating system. If x is the coordinate and N is the number of texels for a one-dimensional texture, then with clamp, x is replaced by 0 if x < 0 and by 1-1/N if 1 >> syntax, which does not return any error code, the return code of cudaGetLastError() should be checked immediately after the kernel launch. The two kernels are very similar, differing only in how the shared memory arrays are declared and how the kernels are invoked. An additional set of Perl and Python bindings are provided for the NVML API. Asynchronous Copy from Global Memory to Shared Memory CUDA 11.0 introduces an async-copy feature that can be used within device code . Prefer shared memory access where possible. When accessing uncached local or global memory, there are hundreds of clock cycles of memory latency. Shared memory has the lifetime of a block. CUDA Binary (cubin) Compatibility, 15.4. For more details on the new Tensor Core operations refer to the Warp Matrix Multiply section in the CUDA C++ Programming Guide. Texture references that are bound to CUDA arrays can be written to via surface-write operations by binding a surface to the same underlying CUDA array storage). 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. Increased Memory Capacity and High Bandwidth Memory, 1.4.2.2. 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. This recommendation is subject to resource availability; therefore, it should be determined in the context of the second execution parameter - the number of threads per block, or block size - as well as shared memory usage. Devices of compute capability 3.x allow a third setting of 32KB shared memory / 32KB L1 cache which can be obtained using the optioncudaFuncCachePreferEqual. It is easy and informative to explore the ramifications of misaligned accesses using a simple copy kernel, such as the one in A copy kernel that illustrates misaligned accesses. The following example is based on gprof, which is an open-source profiler for Linux platforms from the GNU Binutils collection. 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 following examples use the cuBLAS library from CUDA Toolkit 5.5 as an illustration: In a shared library on Linux, there is a string field called the SONAME that indicates the binary compatibility level of the library. But this technique is still useful for other access patterns, as Ill show in the next post.). Two types of runtime math operations are supported. While the details of how to apply these strategies to a particular application is a complex and problem-specific topic, the general themes listed here apply regardless of whether we are parallelizing code to run on for multicore CPUs or for use on CUDA GPUs. A kernel to illustrate non-unit stride data copy. Therefore, it is important to be sure to compare values of like precision and to express the results within a certain tolerance rather than expecting them to be exact. . 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). The cudaEventElapsedTime() function returns the time elapsed between the recording of the start and stop events. I think this pretty much implies that you are going to have the place the heads of each queue in global memory. Page-locked or pinned memory transfers attain the highest bandwidth between the host and the device. We adjust the copy_count in the kernels such that each thread block copies from 512 bytes up to 48 MB. 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. The maximum number of concurrent warps per SM remains the same as in Volta (i.e., 64), and other factors influencing warp occupancy are: The register file size is 64K 32-bit registers per SM. It then explores how bandwidth affects performance metrics and how to mitigate some of the challenges it poses. For example, on devices of compute capability 7.0 each multiprocessor has 65,536 32-bit registers and can have a maximum of 2048 simultaneous threads resident (64 warps x 32 threads per warp). Shared Memory in Matrix Multiplication (C=AB), 9.2.3.3. This padding eliminates the conflicts entirely, because now the stride between threads is w+1 banks (i.e., 33 for current devices), which, due to modulo arithmetic used to compute bank indices, is equivalent to a unit stride. As compared to the lower-level CUDA Driver API, the CUDA Runtime greatly eases device management by providing implicit initialization, context management, and device code module management. This also prevents array elements being repeatedly read from global memory if the same data is required several times. Understanding the Programming Environment, 15. 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. Code that transfers data for brief use by a small number of threads will see little or no performance benefit. Like all CUDA Runtime API functions, this function will fail gracefully and return cudaErrorNoDevice to the application if there is no CUDA-capable GPU or cudaErrorInsufficientDriver if there is not an appropriate version of the NVIDIA Driver installed. Shared Memory in Matrix Multiplication (C=AAT), 9.2.3.4. In Using shared memory to improve the global memory load efficiency in matrix multiplication, each element in a tile of A is read from global memory only once, in a fully coalesced fashion (with no wasted bandwidth), to shared memory. This limitation is not specific to CUDA, but an inherent part of parallel computation on floating-point values. The following complete code (available on GitHub) illustrates various methods of using shared memory. As a result, this section discusses size but not dimension. The CUDA Runtime handles kernel loading and setting up kernel parameters and launch configuration before the kernel is launched. Data Transfer Between Host and Device, 9.1.2. Data copied from global memory to shared memory using asynchronous copy instructions can be cached in the L1 cache or the L1 cache can be optionally bypassed. 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 ( \(i\&\left( {n - 1} \right)\) ). All CUDA compute devices follow the IEEE 754 standard for binary floating-point representation, with some small exceptions. To specify an alternate path where the libraries will be distributed, use linker options similar to those below: For Linux and Mac, the -rpath option is used as before. The number of elements is multiplied by the size of each element (4 bytes for a float), multiplied by 2 (because of the read and write), divided by 109 (or 1,0243) to obtain GB of memory transferred. 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. SPWorley April 15, 2011, 6:13pm #3 If you really need to save per-block information from dynamic shared memory between kernel launchess, you could allocate global memory, equal to the block count times the dynamic shared size. On devices of compute capability 5.x or newer, each bank has a bandwidth of 32 bits every clock cycle, and successive 32-bit words are assigned to successive banks. The functions exp2(), exp2f(), exp10(), and exp10f(), on the other hand, are similar to exp() and expf() in terms of performance, and can be as much as ten times faster than their pow()/powf() equivalents. Recall that the initial assess step allowed the developer to determine an upper bound for the potential speedup attainable by accelerating given hotspots. When using CPU timers, it is critical to remember that many CUDA API functions are asynchronous; that is, they return control back to the calling CPU thread prior to completing their work. Using asynchronous copies does not use any intermediate register. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. However, since APOD is a cyclical process, we might opt to parallelize these functions in a subsequent APOD pass, thereby limiting the scope of our work in any given pass to a smaller set of incremental changes. To analyze performance, it is necessary to consider how warps access global memory in the for loop. The synchronous version for the kernel loads an element from global memory to an intermediate register and then stores the intermediate register value to shared memory. The high-priority recommendations from those guides are as follows: Find ways to parallelize sequential code. However, it is best to avoid accessing global memory whenever possible. CUDA Toolkit and Minimum Driver Versions. In this case, no warp diverges because the controlling condition is perfectly aligned with the warps. In CUDA only threads and the host can access memory. Because it is on-chip, shared memory is much faster than local and global memory. The cudaDeviceCanAccessPeer() can be used to determine if peer access is possible between any pair of GPUs. Overall, developers can expect similar occupancy as on Volta without changes to their application. 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. 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. Zero copy is a feature that was added in version 2.2 of the CUDA Toolkit. Shared memory is extremely fast, user managed, on-chip memory that can be used to share data between threads within a thread block. 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. Last updated on Feb 27, 2023. cudaFuncAttributePreferredSharedMemoryCarveout, 1. 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. Because the memory copy and the kernel both return control to the host immediately, the host function cpuFunction() overlaps their execution. As described in Asynchronous and Overlapping Transfers with Computation, CUDA streams can be used to overlap kernel execution with data transfers. The performance of the above kernel is shown in the chart below. The optimal NUMA tuning will depend on the characteristics and desired hardware affinities of each application and node, but in general applications computing on NVIDIA GPUs are advised to choose a policy that disables automatic NUMA balancing. Block-column matrix (A) multiplied by block-row matrix (B) with resulting product matrix (C).. In this code, two streams are created and used in the data transfer and kernel executions as specified in the last arguments of the cudaMemcpyAsync call and the kernels execution configuration. In the example above, we can clearly see that the function genTimeStep() takes one-third of the total running time of the application. Awareness of how instructions are executed often permits low-level optimizations that can be useful, especially in code that is run frequently (the so-called hot spot in a program). Starting with CUDA 11, the toolkit versions are based on an industry-standard semantic versioning scheme: .X.Y.Z, where: .X stands for the major version - APIs have changed and binary compatibility is broken. Another, more aggressive, option is -use_fast_math, which coerces every functionName() call to the equivalent __functionName() call. 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. For example, on a device of compute capability 7.0, a kernel with 128-thread blocks using 37 registers per thread results in an occupancy of 75% with 12 active 128-thread blocks per multi-processor, whereas a kernel with 320-thread blocks using the same 37 registers per thread results in an occupancy of 63% because only four 320-thread blocks can reside on a multiprocessor. By leveraging the semantic versioning, starting with CUDA 11, components in the CUDA Toolkit will remain binary compatible across the minor versions of the toolkit. Shared memory is a powerful feature for writing well optimized CUDA code. Adjust kernel launch configuration to maximize device utilization. Why do academics stay as adjuncts for years rather than move around? It is customers sole responsibility to evaluate and determine the applicability of any information contained in this document, ensure the product is suitable and fit for the application planned by customer, and perform the necessary testing for the application in order to avoid a default of the application or the product. These recommendations are categorized by priority, which is a blend of the effect of the recommendation and its scope. To get a closer match between values, set the x86 host processor to use regular double or single precision (64 bits and 32 bits, respectively). Minimize redundant accesses to global memory whenever possible. (See Data Transfer Between Host and Device.) Because separate registers are allocated to all active threads, no swapping of registers or other state need occur when switching among GPU threads. To scale to future devices, the number of blocks per kernel launch should be in the thousands. Since you don't indicate where your "locally sorted" data resides, this could indicate a copying of that much data at least (for example, if they are locally sorted and reside in shared memory). Hardware Acceleration for Split Arrive/Wait Barrier, 1.4.1.4. The way to avoid strided access is to use shared memory as before, except in this case a warp reads a row of A into a column of a shared memory tile, as shown in An optimized handling of strided accesses using coalesced reads from global memory. Users should refer to the CUDA headers and documentation for new CUDA APIs introduced in a release. Before implementing lower priority recommendations, it is good practice to make sure all higher priority recommendations that are relevant have already been applied. Therefore, the total number of links available is increased to twelve in A100, versus six in V100, yielding 600 GB/s bidirectional bandwidth versus 300 GB/s for V100. 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. Thedriver will honor the specified preference except when a kernel requires more shared memory per thread block than available in the specified configuration. //Set the attributes to a CUDA stream of type cudaStream_t, Mapping Persistent data accesses to set-aside L2 in sliding window experiment, /*Each CUDA thread accesses one element in the persistent data section.