Re: [patch] increase spinlock-debug looping timeouts (write_lock and NMI)

[Nick Piggin <nickpiggin@yahoo.com.au>]

Nick Piggin wrote:

> And either way, spinlocks are still much more costly than rwlocks,
> because they still have that first exclusive request, who's
> effectiveness deteriorates under load. That you *also* have these
> follow on shared accesses (which will need to be invalidated somehow
> later anyway), doesn't make them better than read locks.

I shouldn't say much more costly.... Much more costly when looking
at the limit case (and we traditionally rather look at the common
case in Linux, and in those cases spinlocks _can_ be faster).

However in the limit, spinlocks scale O(N), while readlocks scale
O(1), where N is the number of CPUs trying to take the lock.
AFAIKS.

But if they're causing stability problems, I have no arguments
against converting them to spinlocks. It might even result in a
worldwide net saving of CPU cycles, for what that's worth ;)

-- 
SUSE Labs, Novell Inc.

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