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

[Peter Xu <peterx@redhat.com>]

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?

> > 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]: 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,

-- 
Peter Xu