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

[Denis V. Lunev]

On 10/07/2015 12:44 PM, Paolo Bonzini wrote:
> 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.

yep, but we will have to setup iovec entry for each page
but pls see below

> > 2/Dirty memory pages are processed one by one (~60msec per 4GB VM)
> That however improves the accuracy, doesn't it?
>
> Paolo
from the point of estimate we need we need amount of dirtied
page per second as a count as a result thus I do not think
that this will make a difference.

Though the approach proposed by David in the letter below
is much better from the point of overhead and the result
was presented in the original description  as (2) aka ~60 msecs
per 4 GB VM was obtained that way. Sorry that this was not
clearly exposed in the description.

Den