Re: [Qemu-devel] [PATCH 6/8] migration: implementation of hook_ram_sync

[Paolo Bonzini]

On 07/10/2015 08:20, Denis V. Lunev wrote:
> 
> All calls of this hook will be from ram_save_pending().
> 
> At the first call of this hook we need to save the initial
> size of VM memory and put the migration thread to sleep for
> decent period (downtime for example). During this period
> guest would dirty memory.
> 
> The second and the last call.
> We make our estimation of dirty bytes rate assuming that time
> between two synchronizations of dirty bitmap differs from downtime
> negligibly.
> 
> An alternative to this approach is receiving information about
> size of data “transmitted” through the transport.

This would use before_ram_iterate/after_ram_iterate, right?

> However, this
> way creates large time and memory overheads:
> 1/Transmitted guest’s memory pages are copied to QEMUFile’s buffer
>   (~8 sec per 4GB VM)

Note that they are not if you implement writev_buffer.

> 2/Dirty memory pages are processed one by one (~60msec per 4GB VM)

That however improves the accuracy, doesn't it?

Paolo