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

[Anthony Liguori <anthony@codemonkey.ws>]

On 11/30/2010 10:32 AM, Juan Quintela wrote:
> "Michael S. Tsirkin"<mst@redhat.com>  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?

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.
>