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Subject: Re: [boost] [lockfree::fifo] Review
From: Tim Blechmann (tim_at_[hidden])
Date: 2009-12-11 09:17:01

>> it is also performance related, though ... on x86_64 (nehalem) my fifo
>> stress test runs about 25% faster with pointer/tag compression than with
>> cmpxchg16b ...
>> that said, the lock-free property is for me more important than the
>> throughput, since i am using it for soft real-time systems ...
> Yes, this has also been my fear that cmpxchg outperforms cmpxch8b which
> outperforms cmpxch16b.
> Tim, have you read the replies to my post on c.p.t regarding ABA bits?
> Even on this thread someone (I think Helge) argued that even 32 bits may
> not be enough. Now I'm thinking that maybe "generation counter" solution
> may not be workable as a general solution.

i just went through the replies ... (maybe i should upcase some parts in
the documentation, that the implementation focus on WORST CASE, not
AVERAGE CASE performance ... people keep complaining that the stack/fifo
may be outperformed be blocking algorithms, which is both true and
irrelevant for me, as these implementations are soft real-time safe (and
could be made hard real-time safe).

as for the aba tag ... increasing the tag is not necessary in the
enqueue operation (chris thomasson made a valid point here), but then
16bit would give 2**16 different tags. of course a tag overflow is
possible, but not very likely ...

by definition ll/sc are immune to aba problems, but implementing cas via
ll/sc, one loses this feature ... personally i would prefer to have
language support for ll/sc transactions instead of aba-prone cas ...

most cas-architectures provide dcas, while most ll/sc architectures
shouldn't use cas emulation, but ll/sc-style transactions directly, as
it is by definition aba immune ... too bad, c++0x doesn't provide
language support for ll/sc, but only for cas :/


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