What are SMT Siblings?

Great question — this dives into how Simultaneous Multithreading (SMT) works on modern CPUs and why disabling both SMT siblings is necessary when one core is faulty.

🧠 What Are SMT Siblings?

Simultaneous Multithreading (SMT) — known as Hyper-Threading on Intel CPUs — allows a single physical CPU core to appear as two logical CPUs to the operating system. These two logical CPUs are called SMT siblings.

For example:

  • CPU18 and CPU38 are SMT siblings.
  • They share the same physical core, but each has its own architectural state (registers, program counter, etc.).
  • They share execution units, L1/L2 caches, and other core-level resources.

🧨 Why Disabling Only One SMT Sibling Isn’t Enough

If CPU38 is faulty, and you only disable it:

  • CPU18 (its sibling) still runs on the same physical core.
  • If the fault is in shared hardware (e.g., execution units, cache, or internal buses), CPU18 will also be affected.
  • This can lead to unpredictable behaviour, kernel crashes, or silent data corruption.

✅ Why This Command Is Used

echo 0 > /sys/devices/system/cpu/cpu18/online
echo 0 > /sys/devices/system/cpu/cpu38/online

This disables both logical CPUs (CPU18 and CPU38), effectively taking the entire physical core offline.

  • echo 0 → disables the CPU
  • /sys/devices/system/cpu/cpuX/online → sysfs interface to control CPU hotplugging

This ensures that no part of the faulty core is used, eliminating the risk of further corruption or crashes.

🧩 How to Identify SMT Siblings

You can find SMT sibling relationships with:

cat /sys/devices/system/cpu/cpu38/topology/thread_siblings_list

This might return something like:

18,38

Which confirms that CPU18 and CPU38 are SMT siblings — i.e., they share the same physical core.

🛠️ Summary

Term Meaning
SMT Siblings Logical CPUs sharing the same physical core
CPU18 & CPU38 SMT siblings — same physical core
Why disable both? Fault in shared hardware affects both siblings
Command effect Fully disables the faulty physical core

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