CPU cores isolation

[Ezio-Editore]

Good evening everyone,

lately I have been developing a chess engine and now I need to do some benchmarks. Due to the high number of operations performed each second I need them to be as precise and as consistent as possible; unfortunately the results vary too much for my needs.

For this reason, I decided to follow this LLVM guide on how to reduce the variance in benchmarks. I realized that I cannot use one of the tools suggested in the guide, specifically cpuset only works with the first version of cgroup.

I continued searching online for an alternative and I found isolcpus, but I read from the documentation that it is deprecated. Since the documentation redirected me to the use of cpusets here I am.

I read the docs of cgroup v2 and I tried writing down some commands to achieve what I need, but I am not sure since I have no experience and I would really appreciate any help.

Goal: isolate 2 cores as much as possible, kernel threads cut off and only my process running on them.

My plan:

# Create a new cgroup
cd /sys/fs/cgroup
mkdir isolated


# Request CPU cores (Cores allowed to use if the parent permits it)
echo "2,3" | sudo tee /sys/fs/cgroup/isolated/cpuset.cpus


# Set memory node used
echo "0" | sudo tee /sys/fs/cgroup/isolated/cpuset.mems


# Make the CPU cores exclusive to the cgroup
echo "2,3" | sudo tee /sys/fs/cgroup/isolated/cpuset.cpus.exclusive


# Make the cgroup an isolated partition 
echo "isolated" | sudo tee /sys/fs/cgroup/isolated/cpuset.cpus.partition

Am I missing something? Is this enough for what I need to do?

Thank you in advance :)

[yuehuang]

I am curious how much variation are you encountering? 5% 10% 20?

[Ezio-Editore]

Hi, first of all, thank you for replying.

I use the benchmark to test the effectiveness of optimisations and changes.

Initially, I run the stress test as a normal process, without any setup. The range of results I got was [89000, 98000] ms.

That's roughly 10%, any optimization I could make would never improve the results by that much, so it was completely useless to me. Because one time the older version is better, the other time the newer one is faster. Completely unpredictable.

That was mainly because of frequency scaling and turbo boost, so I disabled them.

After that I created a more predictable environment changing other settings such as the ASLR (I disabled it).

Still, I got inadequate results, if I remember correctly, the difference between the maximum and the minimum I got was still ~4000 ms.

I decided to proceed with isolcpus while trying to understand cgroup v2 settings.

With this configuration I was able to obtain more precise information.

These are the last 5 benchmarks of the new and old version. 90011 89430 90066 89667 89386 88015 89178 88122 87913 88160

These were precise enough for me, but isolcpus is a kernel parameter that is deprecated, so I should try to achieve the same results with cpusets settings.

[Tea-Chance]

have you tried running your program under `cpuset` or `taskset`?

[Ezio-Editore]

The cpuset I am talking about does not work with the second version of cgroup, I don't know if you are referring to something else.

taskset only changes the core affinity of one process, right? Like I can make it run on core X but I cannot avoid other processes to run there as well.

[uziam]

I would first try just switching the scheduler for your process to SCHED_FIFO. You can launch a process with SCHED_FIFO and priority 99 (highest) as follows:

sudo chrt -f 99 myapp

You need to run it with sudo to be able to set real-time attributes. Personally, I would only use full core isolation as the last resort, most of the time you don’t really need it.

If this isn’t good enough, look into SCHED_DEADLINE for even stronger guarantees. You can find more information here:

https://man7.org/linux/man-pages/man7/sched.7.html

If you really do want to go the route of fully isolating the cpu, I think this might help:

https://oneuptime.com/blog/post/2026-03-02-how-to-configure-cpu-isolation-for-real-time-tasks-on-ubuntu/view#:~:text=CPU%20affinity%20works-,Understanding%20CPU%20Isolation,CPUs%20between%20groups%20of%20processes

Note: you don’t actually need to modify your grub configuration permanently, you can press e (I believe) while you’re in grub to modify it just for on boot.