X86 Serializing Instructions
AMD has always in their manual described their implementation of LFENCE as a load serializing instruction. Acts as a barrier to force strong memory ordering (serialization) between load instructions preceding the LFENCE and load instructions that follow the LFENCE. The original use case for LFENCE was ordering WC memory type loads. However, after the speculative execution vulnerabilities were. The RDTSC instruction is not a serializing instruction. Thus, it does not necessarily wait until all previous instructions have been executed before reading the counter. Similarly, subsequent instructions may begin execution before the read operation is performed. This instruction was introduced into the IA-32 Architecture in the Pentium processor.
I've measured latency for 'lock cmpxchg' and 'mfence' instructions on Pentium 4 processor. I've got following results:lock cmpxchg - 100 cyclesmfence - 104 cyclesSo I conclude that they are nearly identical wrt consumed cycles.But is there some difference between them wrt system performance? Especially on modern multicore processors (Core 2 Duo, Core 2 Quad)?Is following assumption correct: Lock prefix affects bus/cache locking, so has impact on total system performance. And mfence has only local impact on current core.Or more practical: If I have 2 algorithms - one use lock prefix, and another use mfence.
Other things being equal, what I must prefer?Thanks for any advanceDmitriy V'jukov. Those two instructions do completely different things. You cannot use mfence instead of lock prefix.Description:Performs a serializing operation on all load and store instructions that were issued prior the MFENCE instruction.
This serializing operation guarantees that every load and store instruction that precedes the MFENCE instruction is globally visible before any load or store instruction that follows the MFENCE instruction. The MFENCE instruction is ordered with respect to all load and store instructions, other MFENCE instructions, any SFENCE and LFENCE instructions, and any serializing instructions (such as the CPUID instruction).Weakly ordered memory types can enable higher performance through such techniques as out-of-order issue, speculative reads, write-combining, and write-collapsing. The degree to which a consumer of data recognizes or knows that the data is weakly ordered varies among applications and may be unknown to the producer of this data. The MFENCE instruction provides a performance-efficient way of ensuring ordering between routines that produce weakly-ordered results and routines that consume this data.It should be noted that processors are free to speculatively fetch and cache data from system memory regions that are assigned a memory-type that permits speculative reads (that is, the WB, WC, and WT memory types). The PREFETCHh instruction is considered a hint to this speculative behavior. Because this speculative fetching can occur at any time and is not tied to instruction execution, the MFENCE instruction is not ordered with respect to PREFETCHh or any of the speculative fetching mechanisms (that is, data could be speculative loaded into the cache just before, during, or after the execution of an MFENCE instruction).
I do not know what you are trying to do but I can tell you this — most I ever needed was SFENCE instruction when I was using non-temporal stores to copy data.That said, I haven't noticed any performance degradation from SFENCE. If there was any, it was offset by faster transfer speed which came from using non-temporal stores. Bear in mind though, that those stores are meant to be used only for data which won't be immediately reused and that store buffers are scarce resource in some CPUs.Finally, whatever their impact may be, they are needed for coherence so it is out of the question whether you should use them or not if your code needs them. I do not know what you are trying to do.Consider for example following situation.Program use sufficiently large amount of mutexes. Every particular mutex synchronize only 2 threads.I can implement mutex with:1.
'Traditional' scheme. Based on 'lock xchg' in acquire operation and 'naked store' in release operation.2.
Peterson algorithm. Based on #StoreLoad memory barrier (mfence) in acquire operation and 'naked store' in release operation.So net difference is - LOCK vs MFENCE.The question is: Will be any difference in system performance on quad core machine?Dmitriy V'jukov. My mem barrier is ' asm volatile ('mfence'::: 'memory');'(pardon the overkill - i've used mem barriers instead of lfences/sfences - just wanted to make sure this works, before ioptimize it)You probably mis-understand semantics of x86 fences.SFENCE is of any use ONLY if you use non-temporal store instructions (e.g. MOVNTI).And LFENCE is completely useless, it's basically a no-op.MFENCE of any use ONLY is you are trying to order critical store-load sequence. As far as I see, there is no critical store-load sequences in your example, so you need no hardware fences on x86 at all. Just declare variables as volatile so that compiler preserve program order. My mem barrier is ' asm volatile ('mfence'::: 'memory');'(pardon the overkill - i've used mem barriers instead of lfences/sfences - just wanted to make sure this works, before ioptimize it)You probably mis-understand semantics of x86 fences.SFENCE is of any use ONLY if you use non-temporal store instructions (e.g.
MOVNTI).And LFENCE is completely useless, it's basically a no-op.MFENCE of any use ONLY is you are trying to order critical store-load sequence. As far as I see, there is no critical store-load sequences in your example, so you need no hardware fences on x86 at all. Just declare variables as volatile so that compiler preserve program order.
In the x86computer architecture, HLT (halt) is an assembly language instruction which halts the central processing unit (CPU) until the next external interrupt is fired.[1] Interrupts are signals sent by hardware devices to the CPU alerting it that an event occurred to which it should react. For example, hardware timers send interrupts to the CPU at regular intervals.
The HLT instruction is executed by the operating system when there is no immediate work to be done, and the system enters its idle state. In Windows NT, for example, this instruction is run in the 'System Idle Process'. Jump king mac torrent. On x86 processors, the opcode of HLT is 0xF4.
History on x86[edit]
All x86 processors from the 8086 onwards had the HLT instruction, but it was not used by MS-DOS prior to 6.0[2] and was not specifically designed to reduce power consumption until the release of the Intel DX4 processor in 1994. MS-DOS 6.0 provided a POWER.EXE that could be installed in CONFIG.SYS and in Microsoft's tests it saved 5%.[3] Some of the first 100 MHz DX chips had a buggy HLT state, prompting the developers of Linux to implement a 'no-hlt' option for use when running on those chips,[4] but this was changed in later chips.
Process[edit]
Almost every modern processor instruction set includes an instruction or sleep mode which halts the processor until more work needs to be done. In interrupt-driven processors, this instruction halts the CPU until an external interrupt is received. On most architectures, executing such an instruction allows the processor to significantly reduce its power usage and heat output, which is why it is commonly used instead of busy waiting for sleeping and idling.
Use in operating systems[edit]
Since issuing the HLT instruction requires ring 0 access, it can only be run by privileged system software such as the kernel. Because of this, it is often best practice in application programming to use the application programming interface (API)[example needed] provided for that purpose by the operating system when no more work can be done. This is referred to as 'yielding' the processor. This allows the operating system's scheduler to decide if other processes are runnable; if not, it will normally issue the HLT instruction to cut power usage.
See also[edit]
References[edit]
- ^'Intel 64 and IA-32 Architectures Software Developer's Manual: Instruction Set Reference A-Z'(PDF). Retrieved 2012-03-01.
- ^'Why does DOS use 100% CPU under Virtual PC?'. microsoft.com. Retrieved 18 November 2018.
- ^'POWER.EXE and Advanced Power Management (APM) Support'. Archived from the original on 2014-09-27. Retrieved 2015-09-27.
- ^'The Linux BootPrompt-HowTo'. www.faqs.org. Retrieved 18 November 2018.