On Friday 24 February 2006 10:26, Nick Piggin wrote:
> [aside]
> Actually I have a scalability improvement for rwsems, that moves the
> actual task wakeups out from underneath the rwsem spinlock in the up()
> paths. This was useful exactly on a mixed read+write workload on mmap_sem.
>
> The difference was quite large for the "generic rwsem" algorithm because
> it uses the spinlock in fastpaths a lot more than the xadd algorithm. I
> think x86-64 uses the former, which is what I presume you're testing with?
I used the generic algorithm because Andrea originally expressed some doubts
on the correctness of the xadd algorithms and after trying to understand them
myself I wasn't sure myself. Generic was the safer choice.
But if someone can show convincing numbers that XADD rwsems are faster
for some workload we can switch. I guess they are tested well enough now
on i386.
Or would your scalability improvement remove that difference (if it really exists)?
-Andi
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