Re: [boost] Nonlocking data structures.

From: Patrick Twohig (p-twohig_at_[hidden])
Date: 2008-04-18 11:31:43

Another article talks about writing on the Xbox360, which is what I needed a
queue for: http://msdn2.microsoft.com/en-us/library/bb310595(VS.85).aspx

On Fri, Apr 18, 2008 at 3:51 AM, Giovanni Piero Deretta <gpderetta_at_[hidden]>
wrote:

> On Fri, Apr 18, 2008 at 3:09 AM, Cory Nelson <phrosty_at_[hidden]> wrote:
> > On Thu, Apr 17, 2008 at 5:24 PM, Patrick Twohig <p-twohig_at_[hidden]>
> wrote:
> >  > Theoretically, the CAS should atomically compare and swap the value
> in one
> >  >  clock cycle.  However, with multiple cores/processors/hyper
> threading where
> >  >  multiple instructions are being executed simultaneously over
> arbitrary
> >  >  numbers of clock cycles.  There can be writes pending while you want
> to read
> >  >  from memory.  As a result, when you go to read something another
> process
> >  >  will have written to but you read stale data.  To combat this, you
> enforce a
> >  >  memory barrier, which guarantees that all pending memory
> transactions before
> >  >  the barrier have completed before moving on with the program.
>  Additionally,
> >  >  some architectures (like x86) allow for unaligned access of memory.
>  When an
> >  >  unaligned value is accessed, it sets an exception then it replaces
> the
> >  >  single read/write operation with multiple bus operations which
> wreaks havoc
> >  >  on any compare/swap operations.
> >
> >  They don't happen in a single cycle, I don't think there is anything
> >  specifying that they should.  Barriers aren't needed on x86 or x64,
> >  other than compile-time only ones to make sure the compiler doesn't
> >  reorder something.
> >
>
> Well, you actually need StoreLoad memory barriers on x86. All other
> barriers are always implicit (unless you use non temporal SSE
> store/loads).
>
> StoreLoad is also implicit if you use locked operations, otherwise you
> need an explicit mfence.
>
> See, for example, http://g.oswego.edu/dl/jmm/cookbook.html
>
> --
> gpd
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