A subsequent read from the same logical processor will see the flushed trace data, while a read from another logical processor should be preceded by a store, fence, or architecturally serializing operation on the tracing logical processor.
This is clarified by the emphasized part. TraceEn serializes all previous stores with respect to all later stores, it does not by itself ensure global observability of previous stores. One uop can be executed on port 2 or 3, which suggests that it is an STA uop, and the other one can be executed only on port 4, which suggests that it is an STD uop.
SB occurs 5 times per iteration. That is, out of the 6 cycles per iteration, 5 of them are being spent stalling at the allocation stage.
The crucial different is that SFENCE stalls allocation until all stores that have already been allocation have become globally visible. Stalling the allocator seems to be the simplest way to achieve this, but not the most efficient one. If later uops are not stores, then there is really no need to stall them for 5 cycles.
Even if there are some store uops, a mechanism could be implemented in the store buffer itself to ensure the semantics of SFENCE. This has been verified on Haswell, Skylake, and Coffee Lake.
Another important implementation detail is how much time a store needs to become globally observable. Essentially, a store becomes globally observable when it has reached a place that can be accessed by any agent.
This description is a little ambiguous, though, because it does not say whether the store is guaranteed to have reached the memory controller or performed on main memory.
These results are measured for a loop that iterates at least 10 million times. When the number of iterations is 1 million, the loop runs at 7. The loop can be made more interesting by adding a single cacheable writeback store instruction per iteration to the same cache line.
This makes the loop execute at 7 cycles per iteration, i. The allocation stall cycles on the SB become 6 cycles. The store hits in the cache and does not require a fill buffer to be allocated for it. I think this is because the retirement unit alternates between bursty retirement and partial retirement. Otherwise, if it waits only for the store to complete, then it will have to stall for only 5 cycles.
Based on these results, I think we can say that the latency of a store that hits in the L1 is 5 cycles. The description of this event explicitly states that SFENCE may cause the allocator to stall until all previous stores are committed. All hardware prefetchers are enabled the L1 prefetchers cannot prefetch for stores. All the stores are also cacheable writeback and use 8-byte strides.
The results of this test show that the loop throughput is approximately equal to:. This makes sense since the L2 streamer is able to detect the sequential pattern and prefetch the lines so that most stores hit in the L2. There are two cases:. On my Haswell test system, the main memory access latency is on average The L3 access latency is less than c. I think that the results suggest that a WC store becomes globally observable when it reaches the memory controller, which takes more time than an L3 hit but less time than a whole main memory access.
Before the line reaches the memory controller, it is ensured that if the line is cached somewhere, it is evicted from all caches before making the NT store globally observable. NT stores to the same line gets combined in the same write-combing buffer even if this may lead to changing the order in which the stores become observable.
Combining stores may occur in the store buffer, a fill buffer, or in a dedicated write-combining buffer on AMD processors. Conclusion: An uncacheable write-combining WC store becomes globally observable when it reaches the uncore probably, the memory controller. Note that this behavior is only specified by the ISA, but the implementation can actually be such that retirement means global visibility.
It does not execute until all prior instructions have completed locally, and no later instruction begins execution until LFENCE completes. This means that later memory loads and stores will not get issued until all earlier instructions retire.
This applies to memory accesses from memory regions of all types. Not really. The two most important differences are the following:. 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 by the Pentium processor. Although this does not necessarily mean that all stores have become globally visible. This is the only difference that it makes in this code. Now consider this. Interesting, right?
What do you think? Otherwise, you can just skip it. We can also do something similar so that RDTSC is ordered with respect to all later instructions with few exceptions discussed later. Read and write accesses to the APIC registers will occur in program order. WC Loads may be performed out of order with respect to all other loads.
This is also mentioned although difficult to find in the Intel manual. Load operations that reference weakly ordered memory types such as the WC memory type may not be serialized. This is what they say in that document. Maybe because they thought they might need in the future to maintain compatibility with Intel processors regarding the behavior of LFENCE. One thing not clear to me is the part regarding AMD families 0Fh and 11h processors.
To be safe, it should be interpreted as dispatch serializing only. There are exceptions to the ordering rules of fence instructions and serializing instructions and instructions that have serializing properties. These exceptions are subtly different between Intel and AMD processors. So AMD and Intel mean slightly different things when they talk about instructions with serializing properties.
First, as already discussed, LFENCE does not prevent the processor from fetching and decoding instructions, but only from dispatching instructions. LFENCE is not ordered with SFENCE, the global visibility of earlier writes, software prefetching instructions, hardware prefetching, and page table walks, as specified in the following quotes and in other locations in the manuals. Processors are free to fetch and cache data speculatively from regions of system memory that use the WB, WC, and WT memory types.
This speculative fetching can occur at any time and is not tied to instruction execution. Speculative loads initiated by the processor, or specified explicitly using cache-prefetch instructions, can be reordered around an LFENCE. We already know this about the writes. I demand an explanation. The instruction is subject to the permission checking and faults associated with a byte load. In the future, I might write more about this. Also I might write similar articles for the other fence instructions and other related instructions.
The following relevant macros are defined in msr-index. The rmb barrier is used many times in the kernel. Note how volatile is used to define all the barriers so that they constitute compiler barriers as well. In this code, There are three threads: the main thread, a reader thread, and a writer thread. The writer thread sleeps for 2 seconds and then writes to a shared variable.
The reader thread simply iterates in an empty loop until the writer thread updates the shared variable. Sure enough, after about 2 seconds, all threads terminate. You should specify the raw events supported by your CPU. If your CPU supports hyperthreading, disable it.
On my system, I got the following results:. Most of the executed instructions would come from the reader loop. There are three instructions in the reader loop. The first one is translated to a single uop and the other two get translated to a single fused uop. The number of L1 data cache hits is close to the number of iterations of the reader loop. LFENCE prevents the logical processor from issue instances of instructions that belong to later iterations of the loop until the value of the memory load of the current iteration has been determined.
This basically has the effect of slowing down the loop, but in an intelligent manner. There is no point in rapidly issuing load requests. The number of load requests which is very close to the number of iterations of the reader loop has been reduced by more than 10x. The number of retired instructions is now much smaller than the number of retired uops.
Although the execution time has been increased by about 0. Just like before, and as expected, the number of retired instructions is about 4 times the number of L1 data cache hits number of iterations. This technique is particularly useful when hyperthreading is enabled.
LFENCE prevents the reader from unnecessarily consuming execution resources, making them available more often to the other threads. The goal here is to basically defeat the branch predictor no matter how sophisticated it is or how it works.
The random number generator has not been seeded to make sure that all runs exhibit the same branching decisions. The fact that the number of uops in the fused domain is larger than the number of uops in the unfused domain indicates that the CPU experienced a lot of branch mispredictions. For more information on the impact of LFENCE on performance and on how it is implemented in Intel processors, refer to the following Stack Overflow post: Understanding the impact of lfence on a loop with two long dependency chains, for increasing lengths.
You seem to be talking about global observability of loads from one processor to other agents? Only stores have a global effect, and loads have no side effect at all — they only return data from some globally observable store or a local store or something else depending on the memory model. Perhaps an example would help? Pretty close is 8. They apply only to ordinary loads stores and to locked read-modify-write instructions. They do not necessarily apply to any of the following: out-of-order stores for string instructions see Section 8.
Nocturnal Nocturnal 6 6 silver badges 24 24 bronze badges. You probably want to check out AMD's and Intel's manuals. Add a comment. Active Oldest Votes. Jester Jester 54k 4 4 gold badges 71 71 silver badges bronze badges. Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password. Post as a guest Name. Email Required, but never shown.
Learn more about Collectives on Stack Overflow. The Overflow Blog. Podcast Making Agile work for data science.
0コメント