Why does logging sometimes fix a race condition in my multi-threaded app?

[KevinAW]

I’m trying to debug a race condition in my multi-threaded application, but the issue only surfaces maybe one in a hundred test runs. I’ve added a ton of logging, but the logs themselves seem to change the timing just enough that the bug disappears when they’re enabled. Has anyone else had this kind of frustrating experience where the act of observing the system fixes the problem?

[PaulGJ]

Yep, classic Heisenbug vibes. It shows up one in a hundred runs and vanishes the moment you flip on logging or attach a debugger.

[Paisley.J]

I’ve chased that too. I added a ton of logging and the thing shifted—the race started happening in a different place or disappeared entirely. timing fog.

[Riley27]

In a past project I tried dialing down the workload and removing I/O noise, but tiny timing gaps kept poking through. started to suspect an init order problem.

[Gary62]

Could the real issue be something else, like a memory order bug or a rarely hit initialization race, rather than the obvious code path?

[JackWL]

I did a few tweaks and ran millions of trials, but the failure rate never settled into something predictable.

[JosephFM]

Sometimes the culprit is the environment: the CI box, CPU throttling, other jobs creeping in. It mattered more than the code for me, at least for a while.