Re: 2.5isms

[Nick Piggin <nickpiggin@yahoo.com.au>]

Andi Kleen wrote:
> Nick Piggin <nickpiggin@yahoo.com.au> writes:

>>even non HT CPUs possibly slightly more efficient WRT caching the stacks of
>>multiple processes?
> 
> 
> Not on x86 no because they normally have physically indexed caches
> (except for L1, but that is not really preserved over a context switch)
> HT is just a special case because two threads essentially share cache.
> 
> In theory it could help on non x86 CPUs with virtually indexed caches,
> but it is doubtful if they don't need more advanced forms of cache 
> colouring.
> 

That makes sense. I wonder if those architectures may just want to
implement it anyway. If this is such a win here, then it may be low
hanging fruit for those architectures.

But I guess there is something fundamentally a bit different when you
have two processes competing for L1 cache *at the same time*.

> 
>>Second, on what workloads does performance suffer, can you remember? I wonder
>>if natural variations in the stack pointer as the program runs would mitigate
>>the effect of this on all but micro benchmarks?
> 
> 
> iirc on lots of different workloas that run code on both virtual
> CPUs at the same time. Without it you would get L1 cache thrashing,
> which can slow things down quite a lot.
> 
> And yes it made a real difference. The P4 cache have some pecularities
> ("64K aliasing") that made the problem worse.
> 

Interesting, thanks.