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I have a server that's primarily running a Ruby script. Because Ruby (2.7) has a GIL, it is single threaded.

My computer (server) has an Intel i3 dual core processor, but due to hyperthreading I see 4 cores. Ruby only utilizes 25% CPU under heavy load. I wanted to see if disabling hyperthreading benefits a programming language that runs on single thread.

Also, my server is running a very minimal desktop environment and it doesn't use more than 2% CPU. So I wanted to make most of the resources available to Ruby. I did a benchmark to see if I really get any performance boost by disabling hyperthreading.

Benchmark:

I wrote a simple Ruby script that runs a while loop and adds a the value of the loop counter with another variable. This program should use 100% of a CPU core:

#!/usr/bin/env ruby $-v = true LOOPS = ENV['N'].to_i.then { |x| x < 1 ? 100_000_000 : x } + 1 i, j, t = 0, 0, Time.now puts "Counting till #{LOOPS - 1} and adding values to V..." while (i += 1) < LOOPS if i % 10000 == 0 e = Time.now - t r = LOOPS.*(e)./(i).-(e).round(2) print "\e[2KN: #{i} | Done: #{i.*(100) / LOOPS}% | Elapsed: #{e.round(2)}s | Estimated Rem: #{r}s\r" end j += i end puts "\nV = #{j}\nTime: #{(Time.now).-(t).round(2)}s"
  • With Hyperthreading:
⮚ ruby p.rb Counting till 100000000 and adding values to V... N: 100000000 | Done: 99% | Elapsed: 4.55s | Estimated Rem: 0.0s V = 5000000050000000 Time: 4.55s ⮚ ruby p.rb Counting till 100000000 and adding values to V... N: 100000000 | Done: 99% | Elapsed: 4.54s | Estimated Rem: 0.0s V = 5000000050000000 Time: 4.54s ⮚ ruby p.rb Counting till 100000000 and adding values to V... N: 100000000 | Done: 99% | Elapsed: 4.67s | Estimated Rem: 0.0s V = 5000000050000000 Time: 4.67s

gnome-system-monitor reported 25% CPU usage by Ruby while the test was running.

  • Without Hyperthreading:

[ # echo 0 | tee /sys/devices/system/cpu/cpu{2,3}/online used to disable hyperthreads ]

⮚ ruby p.rb Counting till 100000000 and adding values to V... N: 100000000 | Done: 99% | Elapsed: 4.72s | Estimated Rem: 0.0s V = 5000000050000000 Time: 4.72s ⮚ ruby p.rb Counting till 100000000 and adding values to V... N: 100000000 | Done: 99% | Elapsed: 4.54s | Estimated Rem: 0.0s V = 5000000050000000 Time: 4.54s ⮚ ruby p.rb Counting till 100000000 and adding values to V... N: 100000000 | Done: 99% | Elapsed: 4.56s | Estimated Rem: 0.0s V = 5000000050000000 Time: 4.56s

gnome-system-monitor reported 50% CPU usage by Ruby while the test was running.

I have even ran the test on my laptop, which takes around twice the time it took on my computer. But the result is identical: disabling hyperthreading doesn't help the process to do better. And even worse, my laptop gets a bit slower when multitasking.

So in the non-hyperthreading mode, Ruby used 2x the CPU power compared to the hyperthreaded mode. But why did it still take the same amount of time to complete the same task?

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    "echo 0 | tee /sys/devices/system/cpu/cpu{2,3}/online used to disable hyperthreads" - Are you sure that correctly disables hyperthreading? You might have reduced yourself to a single core with HT, instead of two cores without HT. Commented Mar 5, 2020 at 10:54
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    In that case, yes, I would expect that to effectively disable HT. Although if you're benchmarking and the results are very important, I would opt for rebooting and disabling HT in the BIOS/EFI, just to be sure :) Commented Mar 5, 2020 at 11:31
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    I don't see any difference between the two tests. You have values in the range ~4.5-4.7 seconds in both cases Commented Mar 5, 2020 at 12:43
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    Do you know what HyperThreading is? Modern CPUs spend a LOT of their time waiting for data from RAM. Like, really a LOT. So what hyperthreading does is that when one thread is waiting for RAM, the core just puts it aside and starts to execute another thread. Until that too gets to wait for RAM. Then they both wait until one of them gets the data and can continue. This makes the use of a single core much more efficient, so I would be really surprised if disabling HT would somehow make something faster. Commented Mar 5, 2020 at 14:27
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    @Vilx-, the difference is surprisingly little -- even for compiling, which is pretty much the worst case for memory dependencies, I see that once the number of threads scheduled exceeds the number of cores, wall clock time does not go down as quickly as CPU time goes up. My last experiment compiling KiCad saw 63 minutes of CPU time without HT, and 100 minutes with, wall clock time went from 15.3 to 12 minutes. Commented Mar 6, 2020 at 11:04

2 Answers 2

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Your Ruby program did not use 2x the CPU time when running with HT disabled. Rather, as it maximizes one core out of two total cores, gnome-system-monitor will report as the utilization as 50%. If, due to HT, the system reports four total cores, one core out of four would be 25%.

Disabling HT did cause more variation in your results because less resources were available: recent Intel (or AMD) cores are quite wide, so additional threads are often useful to extract 10-20% more aggregate performance. If some background process was automatically executed during the test runs, the system without HT is prone to more variance and lower total throughput.

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    It is worth noting that many CPU monitors read relative to a single core, in those you will often see a process taking >100% CPU because it is threaded and taking advantage of more than one full core worth of CPU time. If is important to know what your monitor is reading when interpreting the results. Commented Mar 6, 2020 at 15:20
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    Yep, this is how top and the likes work. Commented Mar 6, 2020 at 17:07
  • @DavidSpillett That can get a bit confusing with Windows, especially since the algorithm was changed with newer versions of Windows. Windows 8/Server 2012's default view works exactly as you describe (relative to a single core), while Windows 7/Server 2008 R2's view (as well as later versions' Details view) reads relative to the entire CPU. Commented Mar 7, 2020 at 22:27
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I wanted to see if disabling hyperthreading benefits a programming language that runs on single thread.

I don't know how cutting the number of cores would improve performance, even for a single threaded app. When hyperthreading is enabled, your cpu is running with 4 virtual cores. A single threaded app using all the cpu it can would use 25% of the available CPU. When you disabled hyperthreading, you took the number of cores down to 2. Now that single threaded app can use 50% of the available CPU.

Ruby isn't using 2x the CPU, it's that you have 1/2 the CPU available when you disable hyperthreading. If you have a large cup 1/4 full of water and pour it into a smaller cup that becomes 1/2 full of water, you still have the same amount of water.

I have even ran the test on my laptop, which takes around twice the time it took on my computer. But the result is identical: disabling hyperthreading doesn't help the process to do better. And even worse, my laptop gets a bit slower when multitasking.

Yes, you are taking away about 1/2 the power of your CPU. That can make the Ruby thread run slower also. Say you have 3 threads that want to be running at the same time in addition to your Ruby thread. If you cut the virtual cores down to 2, it's more likely that your Ruby thread will be paused at least a little to let another thread have sime time.

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    Disabling HT will improve the performance of high-priority tasks that effectively get exclusive access to a CPU, because there are no instructions from a lower-priority task interjected, which can reduce the performance of that single task by 40%. Wall clock time until "all tasks completed" is going to be lower with HT, wall clock time until "high priority tasks completed" is going to be lower without HT unless you have more high priority tasks than cores (because if everyone has high priority, then no one has). Commented Mar 6, 2020 at 10:45
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    @SimonRichter I've even actually seen 1..4-job parallel compilations speed up when I disabled HT on a 4-core i7 CPU. Commented Mar 6, 2020 at 13:15
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    @Ruslan To be fair, that's seven years old and on Linux. Linux doesn't have the best track record on keeping up with driver quality - it's very possible the kernel you were using simply didn't have any idea about the HT "virtual" cores and treated them as homogeneous, thus leading to overscheduling the already partially utilized cores. It'd be interesting to see if anything changed in the meantime, but then again, Intel no longer recommends using HT (though it's for security rather than performance reasons). Commented Mar 6, 2020 at 14:14
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    @Luaan it's unlikely to have had no idea about HT: I remember enabling CONFIG_SCHED_SMT when configuring the kernel. Commented Mar 6, 2020 at 14:26
  • And I have ran the xonotic game without HT, xonotic uses multiple cores at once. But still, I got a FPS increase and was able to enable some of the effects even without a GPU! Commented Mar 7, 2020 at 6:11

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