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#48969 add a utility thread that would concurrently to the mutator threads
perform the cleanup work required to return pages to the OS.
During performance anlysis I saw on some benchmarks negative effects due
to syscalls causing iTLB flushes (caused by Meltdown mitigations).
For that reason and due to not wanting to oversubscribe the users system,
we by default turned the utility thread off, and require explicit enablement
through
--gcthreads=N,1
.The overall goal was to shorten the STW phase by moving the work out of the
critical section. This PR finds a middle-ground and moves the page freeing work
before finalizers are being called, e.g. outside the STW phase.
I still need to do some more performance analysis, especially since I am still
seeing iTLB flushes on the tree benchmarks.
As a note the utility thread opens up the possibility to move the finalizers to
that thread as well.
My general tuning advise is to use
--gcthreads=N,1
if your application is latencysensitive and
--gcthreads=N,0
if you are worried about oversubscription.