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

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 19/3/08 09:42, "Cui, Dexuan" <dexuan.cui@xxxxxxxxx> wrote:

> However, I still have questions:
> For the bug, I remember the slowness
> 1) Only happens to HVM guest (PV-guest has not this issue);   -- any
> difference between HVM and PV here??
> 2) Only happens to S/R and local non-live migration, but doesn't happen to
> local live migration. -- any difference between live and non-live here??
> And when we suffer from the slowness, "local live migrating" the HVM guest can
> make the performance back to normal!
> 
> Can you reproduce these in your side? If so, can you help to explain them?

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.

The fact that live migration made the bug go away can perhaps be explained
by the fact that multiple rounds of page transmission add an extra layer of
randomisation to page allocations at the receiver?

 -- Keir



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