Previously the calculation of sleep time between wakeups was implemented within
background_thread. This resulted in some parts of decay and hpa specific
logic mixing with background thread implementation. In this change, background
thread delegates this calculation to arena and it, in turn, delegates it to PAI.
The next step is to implement the actual calculation of time until deferred work
in HPA.
Passing is_background_thread down the decay path, so that background thread
itself won't attempt inactivity_check. This fixes an issue with background
thread doing trylock on a mutex it already owns.
To avoid background threads sleeping forever with idle arenas, we eagerly check
background threads' sleep time after extents_dalloc, and signal the thread if
necessary.
Added opt.background_thread to enable background threads, which handles purging
currently. When enabled, decay ticks will not trigger purging (which will be
left to the background threads). We limit the max number of threads to NCPUs.
When percpu arena is enabled, set CPU affinity for the background threads as
well.
The sleep interval of background threads is dynamic and determined by computing
number of pages to purge in the future (based on backlog).
