[Qemu-devel] Re: [PATCH 02/10] Add buffered_file_internal constant

[Juan Quintela]

Anthony Liguori <address@hidden> wrote:
> On 11/30/2010 10:32 AM, Juan Quintela wrote:
>> "Michael S. Tsirkin"<address@hidden>  wrote:
>>    
>>> On Tue, Nov 30, 2010 at 04:40:41PM +0100, Juan Quintela wrote:
>>>      
>>>> Basically our bitmap handling code is "exponential" on memory size,
>>>>        
>>> I didn't realize this. What makes it exponential?
>>>      
>> Well, 1st of all, it is "exponential" as you measure it.
>>
>> stalls by default are:
>>
>> 1-2GB: milliseconds
>> 2-4GB: 100-200ms
>> 4-8GB: 1s
>> 64GB: 59s
>> 400GB: 24m (yes, minutes)
>>
>> That sounds really exponential.
>>    
>
> How are you measuring stalls btw?

At the end of the ram_save_live().  This was the reason that I put the
information there.

for the 24mins stall (I don't have that machine anymore) I had less
"exact" measurements.  It was the amount that it "decided" to sent in
the last non-live part of memory migration.  With the stalls & zero page
account, we just got to the point where we had basically infinity speed.

Later, Juan.

> Regards,
>
> Anthony Liguori
>
>> Now the other thing is the cache size.
>>
>> with 64GB of RAM, we basically have a 16MB bitmap size, i.e. we blow the
>> cache each time that we run ram_save_live().
>>
>> This is one of the reasons why I don't want to walk the bitmap on
>> ram_save_remaining().
>>
>> ram_save_remaining() is linear with memory size (previous my changes).
>> ram_save_live is also linear on the memory size, so we are in a worse
>> case of n^n  (notice that this is the worst case, we normally do much
>> better n^2, n^3 or so).
>>
>> Add to this that we are blowing the caches with big amounts of memory
>> (we don't do witch smaller sizes), and you can see that our behaviour is
>> clearly exponential.
>>
>> As I need to fix them, I will work on them today/tomorrow.
>>
>> Later, Juan.
>>