Re: [PATCH v3 3/3] migration/ram: Optimize ram_save_host_page()

[Kunkun Jiang]

Hi,

On 2021/3/9 5:36, Peter Xu wrote:
> On Mon, Mar 08, 2021 at 09:58:02PM +0800, Kunkun Jiang wrote:
> > Hi,
> >
> > On 2021/3/5 22:30, Peter Xu wrote:
> > > On Fri, Mar 05, 2021 at 03:50:35PM +0800, Kunkun Jiang wrote:
> > > > Starting from pss->page, ram_save_host_page() will check every page
> > > > and send the dirty pages up to the end of the current host page or
> > > > the boundary of used_length of the block. If the host page size is
> > > > a huge page, the step "check" will take a lot of time.
> > > >
> > > > This will improve performance to use migration_bitmap_find_dirty().
> > > Is there any measurement done?
> > I tested it on Kunpeng 920.  VM params: 1U 4G( page size 1G).
> > The time of ram_save_host_page() in the last round of ram saving:
> > before optimize: 9250us               after optimize: 34us
> Looks like an idle VM, but still this is a great improvement.  Would you mind
> add this into the commit message too?
Ok, I will add it in the next version.😉
> > > This looks like an optimization, but to me it seems to have changed a lot
> > > context that it doesn't need to... Do you think it'll also work to just look up
> > > dirty again and update pss->page properly if migration_bitmap_clear_dirty()
> > > returned zero?
> > >
> > > Thanks,
> > This just inverted the body of the loop, suggested by @David Edmondson.
> > Here is the v2[1]. Do you mean to change it like this?
> >
> > [1]: 
> > 20210301082132.1107-4-jiangkunkun@huawei.com/">http://patchwork.ozlabs.org/project/qemu-devel/patch/20210301082132.1107-4-jiangkunkun@huawei.com/
> I see, then it's okay - But indeed I still prefer your previous version. :)
>
> Thanks,
Both versions are fine to me. This version may make the final code 
slightly cleaner, I think.

Thanks,

Kunkun Jiang