Re: [Xen-devel] [PATCH] permute with 2MB chunk

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 20/3/08 09:05, "Ian Pratt" <Ian.Pratt@xxxxxxxxxxxxx> wrote:

>> We also tested building an HVM guest with the permuted ordering of
>> pages, versus reverse ordering, versus normal ordering. Only the
> permuted
>> ordering showed the problem. We assume that the permute() function has
> an
>> unfortunate interaction with the memory allocator in certain HVM guest
> OSes,
>> causing poor cache utilisation.
> 
> It's still very odd that the permutation fn only seems to effect Linux
> running as a HVM guest and not as a PV guest. I still think there's
> something we're not quite understanding.
> 
> Jean: have you definitely verified that building a domain with the
> permute function does not affect Linux PV guests?

Dexuan has also claimed in private email that the 2MB permute function
speeds up kernel builds in save-restored HVM guests from 62s to 58s. We
don't know the confidence intervals for those figures though. Still, there's
presumably something rather fragile underlying all this...

 -- Keir



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