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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Jason J. Herne (jjherne@linux.vnet.ibm.com) wrote:
> On 06/09/2015 04:06 AM, Dr. David Alan Gilbert wrote:
> >* Jason J. Herne (jjherne@linux.vnet.ibm.com) wrote:
> >>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 (jjherne@linux.vnet.ibm.com) wrote:
> >>>>>On 06/03/2015 03:56 AM, Juan Quintela wrote:
> >>>>>>"Jason J. Herne" <jjherne@linux.vnet.ibm.com> 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.
> >
> >Oh, that's yet another problem; hadn't thought about this one.
> >I don't think the suggestions I had in the previous mail would help that one
> >either;   It might work if you flipped the type to 's' and then parsed
> >that in the hmp code.
> >
> 
> I could change it to a string, and then parse the data on a case-by-case
> basis in the switch/case logic. I feel like this is making a bad situation
> worse... But I don't see an easy way around it.

I think the easiest 'solution' for this is to make the parameter an integer percentage
rather than as a float.  Not that pretty but easier than fighting the
interface code.

Dave


> 
> -- 
> -- Jason J. Herne (jjherne@linux.vnet.ibm.com)
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK