I solved one of the reservations that held me back on threaded udevd. I
still have some other concerns to work on, but I'm feeling much more
optimistic now.
Replacing processes with threads reduced page faults (copy-on-write) by
60-80%, but that _still_ left over 10% of udevd time in page fault overhead.
I've discovered I can reduce this overhead much further by marking
thread stacks with MADV_DONTFORK. When a thread needs to fork an
external program, it can temporarily unmark its own thread stack.
Disclaimer: I have no idea why this should reduce the number of page
faults. I could be getting something horribly wrong. But even if it's
wrong, at least it gives me a new angle on this problem.
Alan
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