Re: [PATCH v1 1/7] migration: Introduce structs for background sync

[Peter Xu <peterx@redhat.com>]

On Fri, Sep 27, 2024 at 02:13:47AM +0800, Yong Huang wrote:
> On Thu, Sep 26, 2024 at 3:17 AM Peter Xu <peterx@redhat.com> wrote:
> 
> > On Fri, Sep 20, 2024 at 10:43:31AM +0800, Yong Huang wrote:
> > > Yes, invoke migration_bitmap_sync_precopy more frequently is also my
> > > first idea but it involves bitmap updating and interfere with the
> > behavior
> > > of page sending, it also affects the migration information stats and
> > > interfere other migration logic such as migration_update_rates().
> >
> > Could you elaborate?
> >
> > For example, what happens if we start to sync in ram_save_iterate() for
> > some time intervals (e.g. 5 seconds)?
> >
> 
> I didn't try to sync in ram_save_iterate but in the
> migration_bitmap_sync_precopy.
> 
> If we use the migration_bitmap_sync_precopy in the ram_save_iterate
> function,
> This approach seems to be correct. However, the bitmap will be updated as
> the
> migration thread iterates through each dirty page in the RAMBlock list.
> Compared
> to the existing implementation, this is different but still straightforward;
> I'll give it a shot soon to see if it works.

It's still serialized in the migration thread, so I'd expect it is similar
to e.g. ->state_pending_exact() calls when QEMU flushed most dirty pages in
the current bitmap.

> 
> 
> > Btw, we shouldn't have this extra sync exist if auto converge is disabled
> > no matter which way we use, because it's pure overhead when auto converge
> > is not in use.
> >
> 
> Ok, I'll add the check in the next versioni.

Let's start with simple, and if there's anything unsure we can discuss
upfront, just to avoid coding something and change direction later.  Again,
personally I think we shouldn't add too much new code to auto converge
(unless very well justfied, but I think it's just hard.. fundamentally with
any pure throttling solutions), hopefully something small can make it start
to work for huge VMs.

Thanks,

-- 
Peter Xu