[Qemu-devel] Re: [PATCH 10/10] Maintaing number of dirty pages

[Avi Kivity]

On 12/01/2010 06:33 PM, Anthony Liguori wrote:
> > > BTW, by this logic, even a 1-byte dirty bitmap is only 16mb which can
> > > be read in less than 16ms so where is the reported 24 minute stall
> > > coming from?
> > a) we read the bitmap more than once
>
> Not in a single iteration which is what the "stall" would consist of.
>
> > b) the 1ms is based on "we read" it with longs and optimized, just now
> >     we have to read it by byte and inside the byte.
>
> Byte accesses verse long access doesn't turn 16ms into 24 minutes.

We need actual measurements instead of speculations.  Walking the dirty 
bitmap _did_ _not_ introduce any stalls.  It was the qemu mutex, or 
walking kvm data structures in the kernel, or something.  No amount of 
dirty bitmap optimization will fix it.

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