Nick Piggin <[email protected]> writes:
> I made some tests of the queued spinlock code using userspace test code on
> 64-bit processors. I believe the xadd based code no longer has any theoretical
> memory ordering problems.
Linus, the background of this is that on 8 socket Opteron systems
the current spinlocks can become very unfair to the point of severe
starvation. These boxes are becomming more common.
> The threaded results also attempt to have an unfairness count, which is the
> max number of times in a row that a lock is acquired, when all other threads
> are also executing in the loop -- the reason xadd for example is not always
> 0 there is because the other threads may not have reached the lock before
> the current thread was able to get it several times (eg. if an interrupt
> comes in, this could happen).
Interesting. I was also thinking about switching the lock types
at boot time. Since all the lock calls are out of line this would
be reasonably easy.
I would say the main drawback of switchable and queued locks
would be also that they require a larger spinlock_t thus increasing
cache usage
e.g. it would probably hurt for the large spinlock tables used
by TCP, but then those should be fixed anyways to be smaller.
-Andi
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