Re: Re: [PATCH 2/6] migration/multifd: Add zero pages and zero bytes counter to migration status interface.

[Jiri Denemark]

On Wed, Feb 07, 2024 at 12:37:15 +0800, Peter Xu wrote:
> On Wed, Feb 07, 2024 at 12:13:10PM +0800, Peter Xu wrote:
> > On Tue, Feb 06, 2024 at 11:19:04PM +0000, Hao Xiang wrote:
> > > This change extends the MigrationStatus interface to track zero pages
> > > and zero bytes counter.
> > > 
> > > Signed-off-by: Hao Xiang <hao.xiang@bytedance.com>
> > 
> > Reviewed-by: Peter Xu <peterx@redhat.com>
> 
> I'll need to scratch this, sorry..
> 
> The issue is I forgot we have "duplicate" which is exactly "zero
> page"s.. See:
> 
>     info->ram->duplicate = stat64_get(&mig_stats.zero_pages);
> 
> If you think the name too confusing and want a replacement, maybe it's fine
> and maybe we can do that.  Then we can keep this zero page counter
> introduced, reporting the same value as duplicates, then with a follow up
> patch to deprecate "duplicate" parameter.  See an exmaple on how to
> deprecate in 7b24d326348e1672.
> 
> One thing I'm not sure is whether Libvirt will be fine on losing
> "duplicates" after 2+ QEMU major releases.  Copy Jiri for this.  My
> understanding is that Libvirt should be keeping an eye on deprecation list
> and react, but I'd like to double check..

This should not be a big deal as we can internally map either one
(depending on what QEMU supports) to the same libvirt's field. AFAIK
there is a consensus on Cc-ing libvirt-devel on patches that deprecate
QEMU interfaces so that we can update our code in time before the
deprecated interface is dropped.

BTW, libvirt maps "duplicate" to:

/**
 * VIR_DOMAIN_JOB_MEMORY_CONSTANT:
 *
 * virDomainGetJobStats field: number of pages filled with a constant
 * byte (all bytes in a single page are identical) transferred since the
 * beginning of the migration job, as VIR_TYPED_PARAM_ULLONG.
 *
 * The most common example of such pages are zero pages, i.e., pages filled
 * with zero bytes.
 *
 * Since: 1.0.3
 */
# define VIR_DOMAIN_JOB_MEMORY_CONSTANT          "memory_constant"

Jirka