Hello, Nick.
Nick Piggin wrote:
Tejun Heo wrote:
01_brsem_implement_brsem.patch
This patch implements big reader semaphore - a rwsem with very
cheap reader-side operations and very expensive writer-side
operations. For details, please read comments at the top of
kern/brsem.c.
This thing looks pretty overengineered. It is difficult to
read with all the little wrapper functions and weird naming
schemes.
As I've said in the other reply, I'll first rip off three stage init
thing for cpucontrol. That added pretty much complexity to it. And
with the weird naming scheme, please tell me how to fix it. I have no
problem renaming things.
What would be wrong with an array of NR_CPUS rwsems? The only
tiny trick you would have to do AFAIKS is have up_read remember
what rwsem down_read took, but that could be returned from
down_read as a token.
Using array of rwsems means that every reader-side ops performs
(unnecessary) real down and up operations on the semaphore which involve
atomic memory op and probably memory barrier. These are pretty
expensive operations.
What brsem tries to do is implementing rwsem semantics while
performing only normal (as opposed to atomic/barrier) intstructions
during reader-side operations. That's why all the call_on_all_cpus
stuff is needed while write-locking. Do you think avoiding
atomic/barrier stuff would be an overkill?
I have been meaning to do something like this for mmap_sem to
see what happens to page fault scalability (though the heavy
write-side would make such a scheme unsuitable for mainline).
I agree that brsem write-locking would be too heavy for mmap_sep for
usual cases.
Thanks.
--
tejun
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