memory-fences

What is a memory fence?

由 扶醉桌前 提交于 2019-11-27 02:27:14
What is meant by using an explicit memory fence? Gwaredd For performance gains modern CPUs often execute instructions out of order to make maximum use of the available silicon (including memory read/writes). Because the hardware enforces instructions integrity you never notice this in a single thread of execution. However for multiple threads or environments with volatile memory (memory mapped I/O for example) this can lead to unpredictable behavior. A memory fence/barrier is a class of instructions that mean memory read/writes occur in the order you expect. For example a 'full fence' means

Why is (or isn't?) SFENCE + LFENCE equivalent to MFENCE?

由 陌路散爱 提交于 2019-11-27 01:47:12
As we know from a previous answer to Does it make any sense instruction LFENCE in processors x86/x86_64? that we can not use SFENCE instead of MFENCE for Sequential Consistency. An answer there suggests that MFENCE = SFENCE + LFENCE , i.e. that LFENCE does something without which we can not provide Sequential Consistency. LFENCE makes impossible to reordering: SFENCE LFENCE MOV reg, [addr] -- To --> MOV reg, [addr] SFENCE LFENCE For example reordering of MOV [addr], reg LFENCE --> LFENCE MOV [addr], reg provided by mechanism - Store Buffer , which reorders Store - Loads for performance

Java 8 Unsafe: xxxFence() instructions

由 笑着哭i 提交于 2019-11-27 00:06:15
问题 In Java 8 three memory barrier instructions were added to Unsafe class (source): /** * Ensures lack of reordering of loads before the fence * with loads or stores after the fence. */ void loadFence(); /** * Ensures lack of reordering of stores before the fence * with loads or stores after the fence. */ void storeFence(); /** * Ensures lack of reordering of loads or stores before the fence * with loads or stores after the fence. */ void fullFence(); If we define memory barrier with the

When are x86 LFENCE, SFENCE and MFENCE instructions required?

由 核能气质少年 提交于 2019-11-26 23:54:34
问题 Ok, I have been reading the following Qs from SO regarding x86 CPU fences ( LFENCE , SFENCE and MFENCE ): Does it make any sense instruction LFENCE in processors x86/x86_64? What is the impact SFENCE and LFENCE to caches of neighboring cores? Is the MESI protocol enough, or are memory barriers still required? (Intel CPUs) and: http://www.puppetmastertrading.com/images/hwViewForSwHackers.pdf https://onedrive.live.com/view.aspx?resid=4E86B0CF20EF15AD!24884&app=WordPdf&authkey=!AMtj_EflYn2507c

Make previous memory stores visible to subsequent memory loads

由 北慕城南 提交于 2019-11-26 22:10:45
问题 I want to store data in a large array with _mm256_stream_si256() called in a loop. As I understood, a memory fence is then needed to make these changes visible to other threads. The description of _mm_sfence() says Perform a serializing operation on all store-to-memory instructions that were issued prior to this instruction. Guarantees that every store instruction that precedes, in program order, is globally visible before any store instruction which follows the fence in program order. But

What is a memory fence?

由 跟風遠走 提交于 2019-11-26 12:34:17
问题 What is meant by using an explicit memory fence? 回答1: For performance gains modern CPUs often execute instructions out of order to make maximum use of the available silicon (including memory read/writes). Because the hardware enforces instructions integrity you never notice this in a single thread of execution. However for multiple threads or environments with volatile memory (memory mapped I/O for example) this can lead to unpredictable behavior. A memory fence/barrier is a class of

Why is (or isn't?) SFENCE + LFENCE equivalent to MFENCE?

由 与世无争的帅哥 提交于 2019-11-26 09:46:14
问题 As we know from a previous answer to Does it make any sense instruction LFENCE in processors x86/x86_64? that we can not use SFENCE instead of MFENCE for Sequential Consistency. An answer there suggests that MFENCE = SFENCE + LFENCE , i.e. that LFENCE does something without which we can not provide Sequential Consistency. LFENCE makes impossible to reordering: SFENCE LFENCE MOV reg, [addr] -- To --> MOV reg, [addr] SFENCE LFENCE For example reordering of MOV [addr], reg LFENCE --> LFENCE MOV
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