Re: [PATCH v7 1/3] migration/dirtyrate: implement vCPU dirtyrate calculation periodically

[Hyman Huang]

On 11/30/21 21:04, Peter Xu wrote:
> On Tue, Nov 30, 2021 at 06:28:11PM +0800, huangy81@chinatelecom.cn wrote:
> > +static void dirtylimit_calc_func(void)
> > +{
> > +    CPUState *cpu;
> > +    DirtyPageRecord *dirty_pages;
> > +    int64_t start_time, end_time, calc_time;
> > +    DirtyRateVcpu rate;
> > +    int i = 0;
> > +
> > +    dirty_pages = g_malloc0(sizeof(*dirty_pages) *
> > +        dirtylimit_calc_state->data.nvcpu);
> > +
> > +    dirtylimit_global_dirty_log_start();
> > +
> > +    CPU_FOREACH(cpu) {
> > +        record_dirtypages(dirty_pages, cpu, true);
> > +    }
> > +
> > +    start_time = qemu_clock_get_ms(QEMU_CLOCK_REALTIME);
> > +    g_usleep(DIRTYLIMIT_CALC_TIME_MS * 1000);
> > +    end_time = qemu_clock_get_ms(QEMU_CLOCK_REALTIME);
> > +    calc_time = end_time - start_time;
> > +
> > +    dirtylimit_global_dirty_log_stop();
>
> I haven't looked into the details, but..  I'm wondering whether we should just
> keep the dirty ring enabled during the whole process of throttling.
>
> start/stop can be expensive, especially when huge pages are used, start dirty
> tracking will start to do huge page split. While right after the "stop" all the
> huge pages will need to be rebuild again.
>
> David from Google is even proposing a kernel change to eagerly splitting huge
> pages when dirty tracking is enabled.
>
> So I think we can keep the dirty tracking enabled until all the vcpu throttles
> are stopped.
Yes, it's a good idea and i'll try this out next version.
> > +
> > +    CPU_FOREACH(cpu) {
> > +        record_dirtypages(dirty_pages, cpu, false);
> > +    }

--
Best Regards
Hyman Huang(黄勇)