Re: [Gems-users] Pading to get rid of false sharing

[Luke Yen <lyen@xxxxxxxxxxx>]

Ricardo,

  You can also use perfect signatures to detect false sharing. But based 
on my experience imperfect signatures allow more opportunities for false 
sharing to occur (since each processor could potentially hold on to cache 
lines longer due to NACKs).  These extra stalls also excercise corner 
cases in the programs (e.g., deadlocks due to the interactions between 
low-level locks and transactions).
   Xact NACKS by PC and stalls by PC are identical.

  Luke


On Wed, 10 Dec 2008, Ricardo Quislant del Barrio wrote:

> Hi Luke,
> You told me in other message how to pad data structures so as to avoid
> false sharing:
>
>    "... to avoid false sharing between threads. This is done by running
>    the smallest imperfect signature (e.g., 64 bit signatures) and working
>    backwards from the stall stats by PC to see where the stalls occur in
>    the code."
>
> Why should I start by running the smallest imperfect signature?
> If I try with the Perfect Signature I'll get stalls due to false sharing
> and due to actual conflicts and with Imperfect Signature I'll get those
> conflicts
> and false positives too. So, why should I start by running the smallest
> imperfect
> signature and not the Perfect one?
>
> Another question is: When you say "stall stats by PC" do you mean "xact
> Nacks by PC"?
>
> Thanks,
> Ricardo
>
>
>
>
>
>
>
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