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

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

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




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