Re: [Xen-devel] [PATCH] Segment dirty log for performance

[David Lively <dlively@xxxxxxxxxxxxxxx>]

Keir Fraser wrote:
> On 24/10/07 22:00, "Ben Guthro" <bguthro@xxxxxxxxxxxxxxx> wrote:
>
>   
> > Represent dirty log as an array of bitmaps.
> > Also, when bookkeeping the valid HVM pfn ranges, breaks the PFNs
> > into two ranges  -- RAM and VGA.  This allows the dirty
> > page bitmaps to conform to these ranges and to skip the
> > (sometimes large) empty PFN range between them.
> >
> > Signed-off-by: Ben Guthro <bguthro@xxxxxxxxxxxxxx>
> > Signed-off-by: Dave Lively <dlively@xxxxxxxxxxxxxxx>
> >     
> In terms of tracking active ranges of pseudophysical space, we already have
> a data structure that can give that info (the p2m radix tree). And the
> better way of subdividing the dirty bitmap would be to make that a radix
> tree too, where the leaves are PAGE_SIZE'd bitmaps each handling 128MB of
> pseudophys address space. This would mean we only need order-0 allocations
> in the dirty bitmap logic, greatly reducing the chance of domain-migration
> failure with ENOMEM.
>
> Apart from the obvious benefit of allocating a smaller dirty bitmap, which
> is hence more likely to succeed, how much does this improve migration
> performance for a small-ish memory guest? Is it significant?
>
>  -- Keir
>   
No, the performance benefit is not noticeable.  We (Robert Phillips, 
actually) did it to make migrate more reliable.  But relying only on 
order-0 allocations is obviously even better.  I'll take a look at using 
the radix tree as you suggest.

Thanks,
Dave


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