Impact of DQMStore booking_mutex_

[makortel]

The locks visible in the stack trace in #44561 made me wonder their performance impact. I ran the same job via VTune threading profile, which shows the following CPU utilization

On the highlighted part ("steady state processing") the wait time in the DQMStore booking_mutex_ is roughly corresponds to 1/8 of the total CPU time, i.e. one core is "lost" because of the locks.

I believe this job is not really representative of the bulk of the prompt processing in Tier0 though. The dataset is /MinimumBias, and during the highlighted part the LuminosityBlocks have about 300 Events. I'd expect the impact to be smaller for more complex Events and for higher-rate datasets.

[makortel]

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A new Issue was created by @makortel.

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[makortel]

In any case the main sources of the mutex wait times were