Re: [boost] [lockfree] Review

Subject: Re: [boost] [lockfree] Review
From: Grund, Holger (Holger.Grund_at_[hidden])
Date: 2011-08-05 14:54:50

Hi Helge,

> 
> keep an "even" and an "odd" copy of your data structure, keep an
> atomically
> readable "generation counter" -- on access, read the generation
> counter, read
> the data (depending on parity of generation counter), read the
> generation
> counter again
> 
> if it changed, start over. if it didn't change, you have your data; on
> modification, update generation counter as appropriate (if you are
> paranoid
> about counter overflows, you can repeat a similar trick with the
> counter
> itself)
> 
> no need for anything larger than word-sized atomics here, size of
> shared
> read-only data structure does not matter
> 
Agreed, this is not impossible, but I still tend to think we should strive for a more efficient implementation if at all possible.

> > I don't think you want misaligned data with atomics -- ever. How
> would you
> > ensure not to cross a cacheline or even a page?
> 
> you don't -- if an x86 processor detects the access to be misaligned,
> it will
> issue a bus lock instead of doing RMW in cache

Oh I see, what you mean now. As you said, that's not a terribly efficient thing to do these days, though. 

Anyway, this diverging quite a bit from Lockfree. I would certainly love to see a useful implementation of <atomic>. We care mostly about Linux here, but Windows would be quite nice, too. If there's anything I can do to get this moving let me know.

Thanks!
-hg

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