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

[Avi Kivity <avi@redhat.com>]

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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