RE: [RFC PATCH 5/5] locking/rwsem: Remove reader optimistic spinning

[David Laight]

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

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)