Starting with some background…What is the kernel timer tick (aka LOC interrupt), and what does it do ?
The kernel timer tick is a interrupt triggered at a periodic interval (based on the kernel compile option CONFIG_HZ). The tick is what keeps track of kernel statistics such as CPU and memory usage and provides for scheduler fairness through it’s load balancer. It also does timekeeping, i.e. to keep gettimeofday updated.
When the tick fires (as often as every millisecond, based on value of CONFIG_NO_HZ), it will get scheduled ahead of whatever’s currently running on a CPU core. In other words, whatever was running (with all of it’s valuable data cache-hot) will be interrupted by the tick. The CPUs L1 instruction and data caches (the smallest yet fastest) are invalidated, somewhere around 1000 times a second (if the task was 100% CPU-bound which the majority are not).
This is not an All Is Lost scenario, but certain workloads might see a 1-3% hit that could be attributed to this interference. It also caused some noticeable jitter, especially since what happens inside the tick is not deterministic. The total time the tick runs is not a predictable/constant value.
That was a mouthful, so let me dissect it a bit by describing various kernel config options that control how often this tick fires.
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