From: Waiman Long
> Sent: 19 November 2020 18:40
...
> My own testing also show not too much performance difference when
> removing reader spinning except in the lightly loaded cases.
I'm confused.
I got massive performance improvements from changing a driver
we have to use mutex instead of the old semaphores (the driver
was written a long time ago).
While these weren't 'rw' the same issue will apply.
The problem was that the semaphore/mutex was typically only held over
a few instructions (eg to add an item to a list).
But with semaphore if you got contention the process always slept.
OTOH mutex spin 'for a while' before sleeping so the code rarely slept.
So I really expect that readers need to spin (for a while) if
a rwsem (etc) is locked for writing.
Clearly you really need a CBU (Crystal Ball Unit) to work out
whether to spin or not.
David
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