Re: [RFC] Documentation about unaligned memory access

[dean gaudet]

On Fri, 23 Nov 2007, Arne Georg Gleditsch wrote:

> dean gaudet <dean@xxxxxxxxxx> writes:
> > on AMD x86 pre-family 10h the boundary is 8 bytes, and on fam 10h it's 16 
> > bytes.  the penalty is a mere 3 cycles if an access crosses the specified 
> > boundary.
> 
> Worth noting though, is that atomic accesses that cross cache lines on
> an Opteron system is going to lock down the Hypertransport fabric for
> you during the operation -- which is obviously not so nice.

ooh awesome, i hadn't measured that before.

on a 2 node sockF / revF with a random pointer chase running on cpu 0 / 
node 0 i see the avg load-to-load cache miss latency jump from 77ns to 
109ns when i add an unaligned lock-intensive workload on one core of node 
1.  the worst i can get the pointer chase latency to is 273ns when i add 
two threads on node 1 fighting over an unaligned lock.

on a 4 node (square) the worst case i can get seems to be an increase from 
98ns with no antagonist to 385ns with 6 antagonists fighting over an 
unaligned lock on the other 3 nodes.

cool.

-dean
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