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Date: Wed, 01 Dec 2010 10:33:36 -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 10:25 AM, Juan Quintela wrote:
> Anthony Liguori<anthony@codemonkey.ws>  wrote:
>    
>> 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?
>>      
> 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.

Regards,

Anthony Liguori

> Later, Juan.
>
>    
>> Regards,
>>
>> Anthony Liguori
>>