Re: [Qemu-devel] [PATCH v5 3/5] migration: Dynamic cpu throttling for auto-converge

[Jason J. Herne]

On 09/01/2015 05:30 PM, Eric Blake wrote:
> On 09/01/2015 08:46 AM, Jason J. Herne wrote:
> > Remove traditional auto-converge static 30ms throttling code and replace it
> > with a dynamic throttling algorithm.
> >
> > Additionally, be more aggressive when deciding when to start throttling.
> > Previously we waited until four unproductive memory passes. Now we begin
> > throttling after only two unproductive memory passes. Four seemed quite
> > arbitrary and only waiting for two passes allows us to complete the migration
> > faster.
> >
> > Signed-off-by: Jason J. Herne <address@hidden>
> > Reviewed-by: Matthew Rosato <address@hidden>
> > ---
> >   arch_init.c           | 88 ++++++++++++++++++---------------------------------
> >   migration/migration.c |  4 +++
> >   2 files changed, 34 insertions(+), 58 deletions(-)
>
> > +static void mig_throttle_guest_down(void)
> > +{
> > +    MigrationState *s = migrate_get_current();
> > +    uint64_t pct_initial =
> > +            s->parameters[MIGRATION_PARAMETER_X_CPU_THROTTLE_INITIAL];
> > +    uint64_t pct_icrement =
> > +            s->parameters[MIGRATION_PARAMETER_X_CPU_THROTTLE_INCREMENT];
> > +
> > +    /* We have not started throttling yet. Let's start it. */
> > +    if (!cpu_throttle_active()) {
> > +        cpu_throttle_set(pct_initial);
> > +    } else {
> > +        /* Throttling already on, just increase the rate */
> > +        cpu_throttle_set(cpu_throttle_get_percentage() + pct_icrement);
>
> What happens if the addition results in a percentage larger than 99?

cpu_throttle_set takes responsibility for upper/lower bounds capping.

+void cpu_throttle_set(int new_throttle_pct)
+{
+    /* Ensure throttle percentage is within valid range */
+    new_throttle_pct = MIN(new_throttle_pct, CPU_THROTTLE_PCT_MAX);
+    new_throttle_pct = MAX(new_throttle_pct, CPU_THROTTLE_PCT_MIN);
+
+    atomic_set(&throttle_percentage, new_throttle_pct);
+
+    timer_mod(throttle_timer, qemu_clock_get_ms(QEMU_CLOCK_VIRTUAL_RT) +
+                                       CPU_THROTTLE_TIMESLICE);
+}
+

--
-- Jason J. Herne (address@hidden)