Re: [Qemu-devel] [PATCH v2 1/3] cpu: Provide vcpu throttling interface

[Jason J. Herne]

On 06/03/2015 02:11 PM, Jason J. Herne wrote:
> On 06/03/2015 02:03 PM, Dr. David Alan Gilbert wrote:
> > * Jason J. Herne (address@hidden) wrote:
> > > On 06/03/2015 03:56 AM, Juan Quintela wrote:
> > > > "Jason J. Herne" <address@hidden> wrote:
> ...
> > > > We are checking for throotling on each cpu each 10ms.
> > > > But on patch 2 we can see that we only change the throotling each
> > > > time that we call migration_bitmap_sync(), that only happens each round
> > > > through all the pages. Normally auto-converge only matters for machines
> > > > with lots of memory, so this is going to happen each more than 10ms (we
> > > > change it each 4 passes).  You changed it to each 2 passes, and you add
> > > > it a 0.2.  I think  that I would preffer to just have it each single
> > > > pass, but add a 0.1 each pass?  simpler and end result would be the
> > > > same?
> > >
> > > Well, we certainly could make it run every pass but I think it would get
> > > a little too aggressive then. The reason is, we do not increment the
> > > throttle
> > > rate by adding 0.2 each time. We increment it by multiplying the current
> > > rate
> > > by 2. So by doing that every pass we are doubling the exponential growth
> > > rate. I will admit the numbers I chose are hardly scientific... I
> > > chose them
> > > because they seemed to provide a decent balance of "throttling
> > > aggression"
> > > in
> > > my workloads.
> >
> > That's the advantage of making them parameters.
>
> I see your point. Expecting the user to configure these parameters
> seems a bit much. But I guess, in theory, it is better to have the
> ability to change them and not need it, than need it and not have it
> right?
>
> So, as you stated earlier these should hook into MigrationParams
> somehow? I'll admit this is the first I've seen this construct. If
> this is the optimal location for the two controls (x-throttle-initial,
> x-throttle-multiplier?) I can add them there. Will keep defaults of
> 0.2 for initial and 2.0 for multiplier(is there a better name?)?

So I'm attempting add the initial throttle value and the multiplier to 
MigrationParameters and I've come across a problem. 
hmp_migrate_set_parameter assumes all parameters are ints. Apparently 
floating point is not allowed...

    void hmp_migrate_set_parameter(Monitor *mon, const QDict *qdict)
    {
        const char *param = qdict_get_str(qdict, "parameter");
        int value = qdict_get_int(qdict, "value");

Also from hmp-commands.hx

    {
        .name       = "migrate_set_parameter",
        .args_type  = "parameter:s,value:i",
        .params     = "parameter value",
        .help       = "Set the parameter for migration",
        .mhandler.cmd = hmp_migrate_set_parameter,
        .command_completion = migrate_set_parameter_completion,
    },

I'm hoping someone already has an idea for dealing with this problem? If 
not, I suppose this is a good add-on for Dave's discussion on 
redesigning MigrationParameters.

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