From the ops side though I think it just best to assume we do not know very well how system will scale and be very cautious with guesses. Thats true. But regarding HT: it is usually not bad for web-based load lamp stack or virtualisation. The key for additional performance is the os process scheduler. It must not use the virtual cores, until there are too many running processes.
Take a look at the network stack and how they realised that at 10gbit network traffic, even interrupts that are bound to cores do matter! The deal with HyperThreading and SMT is that it make sure there are enough independet instructions in ready to be executed out-of-order. Modern CPUs are able to execute lot of thing in parallel — not only by simultameous threads but they are also able to execute multiple instruction from single thread in parallel so called instruction level paralellism or ILP.
Those instructions are then put to reorder buffer, which is basically queue of operations waiting to be exucuted. Out-of-order engine then schedule those instructions to be run on particular ports latest intels have 8 of those and tries to pick as many instructions to be run at the same time. If instructions are independent, they can be execute in parallel which is good when those instructions are loads that can stall CPU for up to few hundreds of cycles till data arives from memory.
If one instruction depend on result of another, it has to wait in queue. Modern CPUs go to extreme lengths to make sure there are at all times some instructions ready in queue, they are prefetching data, predicting branches, speculating. But there are limits how much ILP is there in single threaded code. Modern x86 are able to fetch, decode and execute up to 4 instructions every single cycle. But in most cases there are not so much ILP available in general code. So HyperThreading come to play.
It adds one extra frontend to your CPU bit that fetches and decodes instructions that supply extra instruction stream into out-of-order machinery that is hopefully able to find and dispatch more independent instruction to run on available harware. Main problem with HT is that two threads are competing for same amount of cache. Suprisingly your results could be caused by cache effects. As you run more threads in parallel, every one of them have smaller portion of cache and that might cause slowdowns.
Run code through perf and you will see. Back when HT first came out people started doing significant benchmarking and real-world experimenting with it and found decidedly mixed results. In some cases it resulted in a nice performance boost, in others it could be crippling, actually proving to be slower than not having it on at all.
If those cores are working on completely different tasks that can result in the cache being stomped all over and repeatedly purged which can be highly inefficient. This was because of Mutexes that were designed back in the single-core days. Percona did a lot to uncork the mess, and Oracle has picked up the ball. Version 5. Conclusion: See if your app really needs, and can use, more than In reality there are dozens of parallel processing units handling the execution of code.
HT in the beginning was fairly primitive. But that was era CPUs. One of original issues surrounding HT performance was the limitations and splitting of L2 caches. Turning on HT meant splitting that L2 in half. Current generation cores have a kb L2 per core, but there is now a huge L3 cache that is shared between cores. This eliminates many of the problems for cache sensitive applciations. Aurimas, we would be delighted to see a threaded and non-hyperthreaded tests with mysql benchmarks.
This would be the right place so share these results. If I remember correctly, Intel HT could not run two different contexts on the same physical CPU core at the same time also because of security reasons. MySQL is a different story though. Otherwise you will not get your statistical model right and can not interpret the results properly. There are several reasons why you should NOT be surprised at these results; and some of your expectations are skewed by a misunderstanding of several concepts.
First, CPU utilization. In other words, the OS can double the number of threads to saturate the run queues. Weather or not that actually doubles throughput is another matter completely. This distinction matters because it means CPU utilization does not reveal how many or which execution units or pipelines in a CPU are actually being used. The benefits of HT vary greatly depending on workloads; but it can provide very real benefits.
In your specific case, your diminishing gains are likely due to cache interactions with the increased threads made possible by HT. As several people have said, this is WAY more complicated that your base premise. It all comes down to keeping the instruction pipeline full, but not at the expense of having to go to a slower memory memory, i.
L2 to L3 or L3 to main memory for the instruction to process. You get pesky things like branches, instructions that depend on the result of others. The big challenge is adequately using the resources inside the CPU with all these imperfect instruction streams otherwise known as The Real World. Interesting article. I always thought it was simply Hardware Thread switching. Saving registers using the software Kernal takes a notable amount of time.
If you have hyper threading switched on, you will see two "logical processors" per core. The "physical id" will be the processor that you can touch you have two of them. Sign up to join this community. The best answers are voted up and rise to the top.
Asked 6 years, 3 months ago. Active 2 years, 2 months ago. Viewed 2k times. Improve this question. Rui F Ribeiro Hamy Hamy 1 1 silver badge 6 6 bronze badges. Active Oldest Votes. Improve this answer. Thorsten Staerk Thorsten Staerk 3, 1 1 gold badge 18 18 silver badges 26 26 bronze badges. Yes, but do you have any confirmation that these numbers are the ones used by cpu set? I would not use cpuset. It's not that I insist, it's that my entire question is about how to figure out which IDs are correct for cpu set.
You can convert that to the processor list format to check the processor numbers match. Bill Ryder Bill Ryder 89 4 4 bronze badges. Sign up or log in Sign up using Google.
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