Cuda atomic other thread

Cuda atomic other thread. Meanwhile other threads in other blocks may be writing to it. Using those is not going to achieve what you want. In other words, if I write C code z=x will the write be atomic if x and z are 8-bit (unsigned char), 16-bit (unsigned short), 32-bit (unsigned long), or 64-bit (unsigned long long). \nIt has the same semantics as cuda::std::atomic_thread_fence. Atomic operations imply serialization of all memory transactions performed on a given address, and they return the previous state of an address when a thread Aug 14, 2017 · I'm trying to do an atomic read and add in CUDA 8. Asking for help, clarification, or responding to other answers. That means it is safe to be used when all threads in a block reach the barrier. In fact, the concept of blocks in CUDA is that some may be launched only after some other blocks already ended its work, for example, if the GPU it is running on is too weak to process them all in parallel. At most one thread can grab the lock, all others have to spin in the loop. 66 Core 2 Duo, while the graphics card is a GTX 280 Here, each of the N threads that execute VecAdd() performs one pair-wise addition. Nov 18, 2023 · void __threadfence_block(); is equivalent to cuda::atomic_thread_fence(cuda::memory_order_seq_cst, cuda::thread_scope_block) and ensures that: All writes to all memory made by the calling thread before the call to __threadfence_block() are observed by all threads in the block of the calling thread as occurring before all writes to all memory Dec 15, 2023 · Atomic operations. Thread Hierarchy . The thread will then free an unallocated memory area and my program crashes. I’m relatively new to CUDA programming. However, since all threads of a warp execute in lockstep, the thread that owns the lock cannot proceed to release the lock until all other threads do as well, which never happens. I guess I could do an atomic add with zero or something. To avoid it I’m trying to use atomicAdd during the accumulation. The driver must invalidate global L1 cache lines between dependent grids of parallel threads. Jul 19, 2019 · atomics operate either (logically) across the threads belonging to a single threadblock or else across all threads in the grid/kernel. Atomic functions do not act as memory fences and do not imply synchronization or ordering constraints for memory operations (see Memory Fence Functions Mar 14, 2019 · Asking for help, clarification, or responding to other answers. Dec 21, 2017 · I’m trying to sum a vector down to one element using multiple threads which results in a race condition. On the CUDA architecture, these groups are called warps, each warp has 32 threads, and this execution model is referred to as SIMT (Single Instruction Multiple Threads) (see [ 3 ] for Mar 5, 2023 · The __syncthreads() command is a block level synchronization barrier. What I need to do is to allow only one thread to write each index in this array, i. jl 3. In the meanwhile such a thread performs the mentioned operations, all the other threads of all the other blocks having threadIdx. In the first code, if I run the kernel as myadd<<<600, 600>>>(Hdt); It runs without any problem. Unfortunately, this did not work well. The SM can coalesce 32bit regular loads from several threads into one big load. In the while loop all threads within a warp will enter the while loop. Apr 13, 2014 · I have a piece of CUDA code in which threads are performing atomic operations on shared memory. I am seeking help to understand why my code using shared memory and atomic operations is not working. 0 开始，有两个 API 可用于实现这一点：Cooperative Groups，用于管理协作线程组的 CUDA 编程模型的扩展，以及 warp 同步原语函数。 完成 warp-aggregated 原子操作之后，每个线程负责将其值写入其在 dst 数组中的位置。下面将详细介绍下每个步骤。 Step 1: Leader Election Oct 16, 2016 · In addition to using volatile as recommended in the other answer, using __threadfence appropriately is also required to get an atomic load with safe memory ordering. Example: compare-and-swap. Oct 13, 2022 · A thread scope specifies the kind of threads that can synchronize with each other using a primitive such as an atomic or a barrier. 2. 0 is now the default fallback, and support for memory pools other than the CUDA stream-ordered one has been removed. 2 days ago · cuda::atomic_ref<T> and cuda::std::atomic_ref<T> may only be instantiated with a T that are either 4 or 8 bytes. old is a thread local variable. An atomic operation is capable of reading, modifying, and writing a value back to memory without the interference of any other threads, which guarentees that a race condition won’t occur. The second kernel accumulates all per-block histograms into the final histogram stored in global memory. Of course, all atomic operations involve competition between threads for access to a particular location. It is also possible to use __syncthreads() in conditional code but only when all threads evaluate identically such code otherwise the execution is likely to hang or produce unintended side effects . Contention (i. Threads with the same value of the CUDA built-in variable blockIdx are part of the same thread block group. Apr 5, 2016 · I included the info about the images just for completeness but the question is more about CUDA atomic and might be very elementary. It just helps to slow down the summation processes, letting the time to the other threads from the other blocks to write their results, but this is definitely not a neat way to cope with it. " Sep 28, 2022 · Introduction. nvidia. But it is possible that the other block, after seeing the flag, still reads incorrect or incomplete data. Then suppose thread B does the atomicCAS, and replaces its index. They will however find a value of d_state equal to 1, so that atomicCAS(d_state, 0, 1) will perform no update and will return 1, so leaving these threads running the while loop. I need to update a global data structure for something like 0. Sep 25, 2023 · So we have one thread that is repeatedly trying to acquire the lock (looking for an atomic with a returned value of 0). If other threads try to access A and B while your function is executing, they might see a partial execution of the function, in both cases. __shared__ unsigned int data; unsigned int old = data; unsigned int assumed; Jul 24, 2009 · An atomic operation is capable of reading, modifying, and writing a value back to memory without the interference of any other threads, which guarentees that a race condition won’t occur. com Forcing every thread to store a value in the shared memory and using a reduction algorithm after that to determine the maximum (pro: minimum divergence cons: shared memory is limited to 48Kb on 2. Only one will exit and the rest of the threads will reside inside the while loop. This is the device-counterpart to the CuArray, and implements (part of) the array interface as well as other functionality for use on the GPU: Sep 24, 2012 · Can I be sure that in this example, an atomic operation will be performed in numerical order of threads? Or how to do it differently, excluding the use of only one thread? __shared__ unsigned int The hardware ensures that no other threads can access the location until the atomic operation is complete ! Any other threads that access the location will typically be held in a queue until its turn ! All threads perform the atomic operation serially thread_block block = this_thread_block(); As with any CUDA program, every thread that executes that line has its own instance of the variable block. the CPU is a 2. Feb 21, 2016 · Don't assign fixed tasks to your threads, forcing your threads to wait until their task becomes available (which isn't possible in CUDA since threads can't block). Mar 22, 2022 · Only a small number of CUDA threads are now required to manage the full memory bandwidth of H100 using the new Tensor Memory Accelerator, while most other CUDA threads can be focused on general-purpose computations, such as pre-processing and post-processing data for the new generation of Tensor Cores. Use case: perform an arbitrary associative and commutative operation atomically on a single variable. Apr 19, 2013 · No. warp "divergence"), the other threads will just idle until the branch is complete and they all "converge" back together on a common instruction. When it is changed by another thread during the operation, the atomicCAS call must be repeated, otherwise the update doesn't occur. Conceptually my problem is as follows Jun 19, 2016 · Writing a global sync (other than the kernel launch itself) in CUDA is an exercise fraught with peril. However, you can also use atomic operations to actually manipulate the data itself, without the need for a lock variable. The only value which can change, outside of the threads control is *address. Dec 4, 2009 · With CUDA, you can effectively perform a test-and-set using the atomicInc () instruction. Feb 5, 2022 · FWIW - kernels which synchronize across blocks like this one (where IIUC each thread waits for all threads in all blocks in the grid to arrive) should generally use cudaLaunchCooperativeKernel, which will check if all threads in the grid can be running simultaneously (and therefore can communicate & synchronize with each other) docs. So that threads do not interfere, I need to know which writes are atomic in CUDA runtime 9. Feb 14, 2024 · atomic fetch and add is implemented in CUDA hardware as atomicAdd. Unified Memory (NB: this is NOT unified virtual addressing. The modified value of *address will be copied to old variable inside the atomicCAS (see behavior 1. Perhaps you could restructure your computation to use atomics hierarchically: first, accumulate into a __shared__ variable in each thread block. 0. This could only happen if, from the point of view of the atomic, a previous write of 0 was successful. From an access perspective, a CUDA atomic is uninterrruptible. Another kernel is better – Feb 6, 2021 · The size of the reduction, the size of the grid, the block size, the kernel design, and the type of GPU you are running on, and probably many other factors are going to influence the final performance outcome for a sum reduction. Nov 28, 2016 · My understanding is that the dev_sum is visible by all threads in the kernel function after atomic function, then if I use one thread on each block to copy to the shared memory but __syncthreads() before other operations by other threads in the same block, does it make sense to achieve some kind of synchronization between blocks? As I known Before CUDA 9, there was no native way to synchronise all threads from all blocks. 14. else return *p. 2 days ago · Note, for objects of scopes other than cuda::thread_scope_system this is a data-race, and thefore also prohibited regardless of memory characteristics. You now have a max value of 110 with an index Jul 22, 2014 · Hi, All, I am trying to sum up previously calculated values in different threads within the same thread block, and then write the value to a single variable. In colonel(), replace. This post introduces warp-aggregated atomics, a useful technique to improve performance when many CUDA threads atomically update a single counter. Oct 26, 2012 · These accesses are succesfully sequentialized by the semaphore, but sometimes, a thread will remove the same head element from the list as a previous thread. Aug 13, 2021 · The latest version of CUDA. So if you only need within-warp synchronization of threads, that happens "automagically. *a d += 1; with an atomic function, atomicAdd(a d, 1); to. of atomicCAS above). Its value doesn't change unless the local thread changes it. This was then followed by a sync-threads call. It also provides host-side APIs to launch grids whose threads are all guaranteed to be executing concurrently to enable synchronization across thread blocks. Atomic operations work for both on shared and global memory. Is there simply an atomicRead option in CUDA or something equivalent. – Mar 27, 2011 · In theory, atomic operations / optimistic retries are supposed to be faster than locks/mutexes, so the "hack" solutions that use atomic operations on other data types seem better to me than using critical sections. CUDA Atomic Operations thread or block level? 0. Mar 17, 2015 · In the first phase each CUDA thread block processes a region of the image and accumulates a corresponding local histogram, storing the local histogram in global memory at the end of the phase. 2. e. g. Below is a graph of execution time it took my CPU against the amount of time it took my graphics card. While some of the comments are saying to just use a normal read because it cannot tear, that is not the same as an atomic load. e. 1% or less of the pixels in a megapixel frame, and I suspect atomics wouldt be more efficient than collecting the results using a scan operation. However, I can’t get it to work using double precision numbers. Nov 2, 2021 · According to CUDA Programming Guide, "Atomic functions are only atomic with respect to other operations performed by threads of a particular set Block-wide atomics: atomic for all CUDA threads in the current program executing in the same thread block as the current thread. Aug 6, 2015 · About Elmar Westphal Elmar Westphal has been working as a programmer and cluster architect at Forschungszentrum Juelich for more than 15 years. Then thread B does the atomicMax and replaces the 100 value with 110. All writes to all memory made by the calling thread before the call to __threadfence_block() are observed by all threads in the block of the calling thread as occurring before all writes to all memory made by the calling thread after the call to Oct 7, 2017 · Multiple threads will be computing a large array in shared memory. The hardware ensures that no other threads can access the location until the atomic operation is complete ! Any other threads that access the location will typically be held in a queue until its turn ! All threads perform the atomic operation serially (exposed in CUDA via atomic*() functions) concurrent threads (within the same block, or within different blocks) can update the same global memory locations atomically, so thread blocks can merge their results within a single CUDA kernel. The Read-Modify-Write operation is conducted without the possibility of another thread to intervene in any way. Dec 22, 2013 · The threads in a warp run physically parallel, so if one of them (called, thread X) start an atomic operation, what other will do? Wait? Is it mean, all threads will be waiting while thread X is pushed to the atomic-queue, get the access (mutex) and do some stuff with memory, which was protected with that mutex, and realese mutex after? Jan 18, 2012 · is a straight deadlock in CUDA. 148, there are no atomic operations for float. 1 day ago · A release fence A synchronizes with an acquire fence B if there exist atomic operations X and Y, both operating on some atomic object M, such that A is sequenced before X, X modifies M, Y is sequenced before B, and Y reads the value written by X or a value written by any side effect in the hypothetical release sequence X would head if it were a release operation, and each operation (A, B, X Apr 27, 2022 · CUDA memory only supports aligned accesses - whether they be regular or atomic. Dec 3, 2019 · Do[sic] CUDA guarantee that other threads will ever see the changes made by a thread with atomic operations in steps (1) and (3)? Yes, but only for other operations performed atomically. The following Aug 29, 2024 · 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. For convenience, threadIdx is a 3-component vector, so that threads can be identified using a one-dimensional, two-dimensional, or three-dimensional thread index, forming a one-dimensional, two-dimensional, or three-dimensional block of threads, called a thread block. source Device arrays. A critical section can be used to control access to a memory area, for example, so as to allow un-conflicted access to that area by a single thread. See all the latest NVIDIA advances from GTC and other leading technology conferences—free. Streamlined atomic operations Jan 27, 2017 · Global variables between concurrent streams. As shown in the following code, I used a self-defined double precision atomicAdd(), as introduced in ( Speed of double precision CUDA atomic operations on Kepler K20 - CUDA Programming and Performance - NVIDIA Developer Forums ). A flexible global sync in CUDA generally requires that you launch a limited number of threadblocks (the number is specific to your specific device), so as to not exceed the instantaneous carrying capacity of the device, allowing all threads in Jul 15, 2022 · I've been reading up on atomic operations in CUDA and the update pattern for atomicInc() seems fairly arbitrary. Jul 22, 2012 · atomicAdd serializes by definition, so you should only rely on it when you predict that collisions will be sparse. Dec 21, 2021 · The code is already written in python+numpy form and I am already busy to convert it to GPU. Thread gets its own synchronization but other threads may not see it. The code is from Page 253 of \n. Here, each of the N threads that execute VecAdd() performs one pair-wise addition. I was thinking since the result of atomic operation will be visible to other threads of the block instantly anyways, it might be good to instruct the compiler to have the shared memory volatile . The other thread is writing zero to the atomic location. 0 devices) I couldn't use atomic operations because there are both a reading and a writing operation, so threads could not be synchronized by Jul 5, 2023 · atomic operations - those are always visible by other blocks; threadfence; Imagine, that one block produces some data, and then uses atomic operation to mark a flag that the data is there. A similar effect can be achieved using vector data types to perform a 64/128 bit load in a single thread. * Some content may require login to our free NVIDIA Developer Program . Oct 22, 2020 · Hi. ” According to my understanding this new implementation has yet another Oct 5, 2017 · It provides CUDA device code APIs for defining, partitioning, and synchronizing groups of threads. Even worse - suppose that CUDA was a perfect parallel machine, with all threads working in lockstep. Feb 15, 2022 · Atomic value is going to global memory but in the while-loop you read it directly and it must be coming from the cache which will not automatically synchronize between threads (cache-coherence only handled by explicit synchronizations like threadfence). or responding to other answers. These are suffixed with _block, e. In the last several year he ported simulation programs from different fields of computational physics to single- and/or multi-GPU systems and developed CUDA-based building blocks, libraries and applications mostly for Molecular Dynamics and Sep 23, 2013 · A critical section allows one thread to execute a sequence of instructions while preventing any other thread or threadblock from executing those instructions. Jan 30, 2009 · I’d also be interested in how to properly implement a semaphore/mutex with atomic instructions in CUDA. However, CUDA can simply directly use the function, atomicMax(), and not worry about a lock variable at all. 6 days ago · Establishes memory synchronization ordering of non-atomic and relaxed atomic accesses, as instructed by order, for all threads within scope without an associated atomic operation. By “atomic”, I mean that threads are guaranteed Each thread block is mapped to one or more warps When the thread block size is not a multiple of the warp size, unused threads within the last warp are disabled automatically The hardware schedules each warp independently Warps within a thread block can execute independently Warp of 32 threads Warp of 32 threads Oct 16, 2016 · Cuda atomic lock: threads in sequence. Feb 3, 2014 · Thanks a lot. jl brings several new features, from improved atomic operations to initial support for arrays with unified memory. All you have to do is add cuda/std/ to the start of your includes and cuda:: before any uses of std::: Mar 12, 2016 · The __syncthreads() function ensures that all threads in a block are at the same spot in the code. k. For cuda::atomic_ref<T> and cuda::std::atomic_ref<T> the type T must satisfy the If there is a conditional branch and it is taken by some threads in the warp but not others (a. Instead, keep a list of available tasks (using atomic operations) and have each thread grab a task from that list. create incorrect results), but when the threads are contending to do an atomic operation on a single shared memory location, the contention gives rise to serialization, exacerbating the delay associated with atomics. Dec 26, 2012 · If the counter value equals your total thread count, then you know all threads have reached this point in execution. , a cuda::atomic<int, thread_scope_block> can be used to synchronize among threads in the same block, but not by threads in different blocks. Consider the following code, where different threads run different computations. Aug 10, 2013 · The downside is that other threads might only be able to see the changed value after the fence. The threads running on a multiprocessor are partitioned into groups in which all threads execute the same instruction simultaneously. There are only two possible outcomes: the atomic returns a value of 0. These primitives enable new patterns of cooperative parallelism within CUDA, including 2 days ago · If you know how to use things like the <atomic> or <type_traits> headers from the C++ Standard Library, then you know how to use libcu++. Atomic functions in CUDA can greatly enhance the performance of many algorithms. In other words, no other thread can access this address until the operation is complete. ca, the second thread may get stale L1 cache data, rather than the data stored by the first thread. The code compute the thread id first and project the id into the range of 128 * 128, which means all pixels in the same line along the dimension we perform addition will have the same idx. if *p == old then assign *p←new, return old. __threadfence() delays the current thread (and only the current thread!) to ensure that any subsequent writes by given thread do actually happen later. But this is slow if other threads are also accessing the same address - consider using block shared memory to compute an intermediate value that is applied to the global result via a single atomic. , atomicAdd_block" Say that other thread modified value of *address after assumed=oldValue and oldValue = atomicCAS(). 251-254) with additional __threadfence() as “It is documented in the CUDA programming guide that GPUs implement weak memory orderings which means other threads may observe stale values if memory fence instructions are not used. When I compile my code (from Matlab) I get an error: No instance of overloaded function "atomicAdd" matches the argument list argument types are: (double Nov 19, 2018 · I am writing a CUDA program which has an array defined in the shared memory. Mar 13, 2014 · The R-M-W operation itself is atomic in the sense that no other thread can disrupt the operation (i. Aug 2, 2017 · I see the Cuda by Example - Errata Page have updated both lock and unlock implementation (p. The native random number generator introduced in CUDA. Each thread block is mapped to one or more warps When the thread block size is not a multiple of the warp size, unused threads within the last warp are disabled automatically The hardware schedules each warp independently Warps within a thread block can execute independently Warp of 32 threads Warp of 32 threads Jul 14, 2009 · Using __syncthreads is sometimes necessary to ensure that all data from all threads is valid before threads read from shared memory which is written to by other threads. My algorithm might be a classical one. Double Jul 24, 2013 · When I try with 10^5 elements, 512 threads per bloc hence 196 blocks, I get the "nan" result. a. Yes, this is the way to solve the problem. But we can implement it by mixing atomicMax and atomicMin with signed and unsigned integer casts! But we can implement it by mixing atomicMax and atomicMin with signed and unsigned integer casts! Feb 4, 2016 · atomic in the sense that it is guaranteed to be performed without interference from other threads. For details, consult the Atomic Functions section of the CUDA Programming guide . If the work is too much for one thread, then you could issue the atomic counter by block (either by syncthreads in a block, or, preferably, utilizing an atomic counter within each Sep 11, 2020 · Kernels are launched with a grid of blocks of threads that are specified at launch time - you will see in test_atomic_add3 the call cuda_atomic_add3[1, (4, 8)](ary) - the subscripting [1, (4, 8)] specifies a grid with one block, and blocks are 4 (x dimension) by 8 (y dimension) threads. Ask Question Asked 9 years, 11 months ago. I hope that you are familiar with the concept of a warp. Atomic operation: an operation that forces otherwise parallel threads into a bottleneck, executing the operation one at a time. And then have that last thread do your summation. CUDA. This may happen immediately consecutively, or there can be one or more other threads in between. x == 0 will execute the lock method. Performance advantages and penalties of atomic operations in CUDA. if multiple threads are trying to operate on the same shared memory location) will tend to degrade performance, not unlike the looping that software must perform if there's contention on the pre-Maxwell locks. In the first three installments of this series (part 1 here, part 2 here, and part 3 here), we’ve gone through most of the basics of CUDA development such as launching kernels to perform embarrassingly parallel tasks, leveraging shared memory to perform fast reductions, encapsulating reusable logic as device functions, and how to use events and streams to organize and control Mar 5, 2023 · I just found out about the libcu++ library and am trying to use the cuda::atomic variables. Advanced Search May 11, 2023 · UPDATE: Since Maxwell (the generation after Kepler), NVIDIA has included hardware support for atomic operations in shared memory. From the CUDA Programming Guide: unsigned int atomicInc(unsigned int* address, unsigned int val); Aug 17, 2016 · Suppose thread A does the atomicMax and replaces the old value with 100. I wanted to point out a related experience I had. – Sep 3, 2023 · An atomic release operation X in thread A synchronizes-with an acquire fence F in thread B, if there exists an atomic read Y (with any memory order), Y reads the value written by X (or by the release sequence headed by X), Y is sequenced-before F in thread B. Feb 9, 2024 · 1. Synchronizing a thread_block group is much like calling __syncthreads(). Atomic. Some metric related to the number of active warps on a multiprocessor is therefore important in determining how effectively the hardware is kept busy. Otherwise, each block must output its own sub-histogram, and a separate final merging kernel May 7, 2021 · Based on the CUDA Toolkit Documentation v9. Thanks for the explanation. This will introduce a divergent branch making the thread that exited the while loop stall until the branch converges Jun 26, 2022 · If one thread stores to global memory via one L1 cache, and a second thread loads that address via a second L1 cache with ld. Dec 4, 2009 · Similarly, atomicDec(&myLockVariable, 0) could be used instead of Unset. Dec 26, 2023 · void __threadfence_block(); is equivalent to cuda::atomic_thread_fence(cuda::memory_order_seq_cst, cuda::thread_scope_block) and ensures that:. jl provides a primitive, lightweight array type to manage GPU data organized in an plain, dense fashion. Atomic operations in CUDA generally work for both shared memory and global memory. Under CUDA Compute Capability 6 (Pascal), an object of type atomic may not be used: Feb 4, 2016 · atomic in the sense that it is guaranteed to be performed without interference from other threads. Making statements based on opinion; back them up with references or personal experience. Feb 28, 2016 · The canonical way to handle the race condition you mention is by using an atomic compare-and-swap operation, which is supported on CUDA capable GPUs for both shared and global memory See atomicCAS in CUDA programming guide. Share Oct 18, 2018 · Atomic operations are, as the documentation says, "read-modify-write operations" in CUDA. Every stiffness matrix is calculated separately and every value is added to corresponding global matrix index at the same time, parallel. I thought it was reasonable to speculate the possibility to perform a vector atomic ops as the SM could coalesce from different threads. I wrote the following program but it is giving me unexpected results: #include &lt;atomic&gt; #include &l B. Establishes memory synchronization ordering of non-atomic and relaxed atomic\naccesses, as instructed by order, for all threads within scope without an\nassociated atomic operation. This is useful for the :opportunistic warp-level programming” technique we explain later, as well as for debugging and understanding program behavior. Atomic functions do not act as memory fences and do not imply synchronization or ordering constraints for memory operations (see Memory Fence Functions Mar 26, 2016 · I want the first thread in the block to be responsible for reading in the global dz value to shared memory so the rest of the threads can do a reduction on it. the first thread to reach this write instruction should change its value but any other threads either in the same warp or next warps should read the written value. Atomic Functions原子函数对驻留在全局内存或共享内存中的一个32位或64位单词执行读-修改-写原子操作。例如，atomicAdd()在全局或共享内存中的某个地址读取一个单词，向其中添加一个数字，然后将结果写回相… 从 CUDA 9. In other words, it shows the calling thread which threads in its warp are also executing the same __activemask(). No object or subobject of an object referenced by an atomic_­ref shall be concurrently referenced by any other atomic_­ref that has a different Scope. ) The GPU has a separate memory space from the host CPU Oct 16, 2016 · Here is a theory. The definition used for CUDA is "The operation is atomic in the sense that it is guaranteed to be performed without interference from other threads". It is impossible to use it on local variables, since they are unique for each thread and stored in registers or in a local Aug 11, 2016 · If you know beforehand that no two threads access the same cell, there is no point to use any atomic function. . The logic was pretty simple, having one thread pause until a global variable equaled the loop variable, indicating that all threads could proceed. I’ve studied the various explanations and examples around creating custom kernels and using atomic operations (here, here, here and various other explanatory sites / links I could find on SO and this forum). Another thing you should consider is that operations are not atomic. Here are some implementations based on the threads for how to implement atomicMin for char and atomicAdd for short. In this case, all non-atomic and relaxed atomic stores that are sequenced-before X in Oct 16, 2016 · In CUDA, is the result of atomic operation immediately visible to the threads of other warps in the same block as the one performing the atomic operation? In case of non-atomic operation, I know that the result may not be visible until __syncthreads() gets called. Then thread A does the atomicCAS, and replaces thread B index with thread A index. To learn more, see our tips on writing great answers . atomicCAS(p, old, new) does atomically. But I have a question about the number of grids and number of threads. veya rgjuay axxl saia bpu fwykanavo sawndcr pubdipy ebhp poe