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Date: Wed, 01 Dec 2010 09:55:23 -0600
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: [Qemu-devel] Re: [PATCH 10/10] Maintaing number of dirty pages
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To: Juan Quintela <quintela@redhat.com>
Cc: Avi Kivity <avi@redhat.com>, qemu-devel@nongnu.org

On 12/01/2010 09:51 AM, Juan Quintela wrote:
> Avi Kivity<avi@redhat.com>  wrote:
>    
>> On 11/30/2010 04:46 PM, Juan Quintela wrote:
>>      
>>> Anthony Liguori<anthony@codemonkey.ws>   wrote:
>>>        
>>>>   On 11/23/2010 05:03 PM, Juan Quintela wrote:
>>>>          
>>>>>   From: Juan Quintela<quintela@trasno.org>
>>>>>
>>>>>   Calculate the number of dirty pages takes a lot on hosts with lots
>>>>>   of memory.  Just maintain how many pages are dirty.  Only sync bitmaps
>>>>>   if number is small enough.
>>>>>
>>>>>            
>>>>   There needs to be numbers that justify this as part of the commit message.
>>>>          
>>> They are on patch 0/6.
>>>
>>> Additionally, with 64GB of RAM, this bitmap is HUGE, having to walk over
>>> it in each ram_save_live() call is too onerous.
>>>        
>> It's not so huge.  It's scaled down by a factor of 8 * 4096 = 32K.  So
>> it's a 2MB bitmap.  If kept as a bitmap and accessed in longs, it can
>> be read in less than a millisecond.

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?

Regards,

Anthony Liguori