Re: [Qemu-devel] [PATCH 2/2] migration: Dynamic cpu throttling for auto-converge

[Juan Quintela <quintela@redhat.com>]

"Jason J. Herne" <jjherne@linux.vnet.ibm.com> wrote:
> On 06/02/2015 09:58 AM, Dr. David Alan Gilbert wrote:
>> * Jason J. Herne (jjherne@linux.vnet.ibm.com) wrote:
>>> On 06/01/2015 11:32 AM, Dr. David Alan Gilbert wrote:
>>>> * Jason J. Herne (jjherne@linux.vnet.ibm.com) 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 <jjherne@linux.vnet.ibm.com>
>>>>> Reviewed-by: Matthew Rosato <mjrosato@linux.vnet.ibm.com>
>>>>> ---
>>>>>   arch_init.c           | 95 +++++++++++++++++----------------------------------
>>>>>   migration/migration.c |  9 +++++
>>>>>   2 files changed, 41 insertions(+), 63 deletions(-)
>>>>>
>>>>> diff --git a/arch_init.c b/arch_init.c
>>>>> index 23d3feb..73ae494 100644
>>>>> --- a/arch_init.c
>>>>> +++ b/arch_init.c
>>>>> @@ -111,9 +111,7 @@ int graphic_depth = 32;
>>>>>   #endif
>>>>>
>>>>>   const uint32_t arch_type = QEMU_ARCH;
>>>>> -static bool mig_throttle_on;
>>>>>   static int dirty_rate_high_cnt;
>>>>> -static void check_guest_throttling(void);
>>>>>
>>>>>   static uint64_t bitmap_sync_count;
>>>>>
>>>>> @@ -487,6 +485,31 @@ static size_t save_page_header(QEMUFile *f, RAMBlock *block, ram_addr_t offset)
>>>>>       return size;
>>>>>   }
>>>>>
>>>>> +/* Reduce amount of guest cpu execution to hopefully slow down memory writes.
>>>>> + * If guest dirty memory rate is reduced below the rate at which we can
>>>>> + * transfer pages to the destination then we should be able to complete
>>>>> + * migration. Some workloads dirty memory way too fast and will not effectively
>>>>> + * converge, even with auto-converge. For these workloads we will continue to
>>>>> + * increase throttling until the guest is paused long enough to complete the
>>>>> + * migration. This essentially becomes a non-live migration.
>>>>> + */
>>>>> +static void mig_throttle_guest_down(void)
>>>>> +{
>>>>> +    CPUState *cpu;
>>>>> +
>>>>> +    CPU_FOREACH(cpu) {
>>>>> +        /* We have not started throttling yet. Lets start it.*/
>>>>> +        if (!cpu_throttle_active(cpu)) {
>>>>> +            cpu_throttle_start(cpu, 0.2);
>>>>> +        }
>>>>> +
>>>>> +        /* Throttling is already in place. Just increase the throttling rate */
>>>>> +        else {
>>>>> +            cpu_throttle_start(cpu, cpu_throttle_get_ratio(cpu) * 2);
>>>>> +        }
>>>>
>>>> Now that migration has migrate_parameters, it would be best to replace
>>>> the magic numbers (the 0.2, the *2 - anything else?)  by parameters that can
>>>> change the starting throttling and increase rate.  It would probably also be
>>>> good to make the current throttling rate visible in info somewhere; maybe
>>>> info migrate?
>>>>
>>>
>>> I did consider all of this. However, I don't think that the controls
>>> this patch provides are an ideal throttling mechanism. I suspect someone
>>> with
>>> vcpu/scheduling experience could whip up something more user friendly and
>>> cleaner.
>>> I merely propose this because it seems better than what we have today for
>>> auto-converge.
>>>
>>> Also, I'm not sure how useful the information really is to the user. The
>>> fact that it is a ratio instead of a percentage might be confusing. Also,
>>> I suspect it is not
>>> truly very accurate. Again, I was going for "make it better", not "make it
>>> perfect".
>>>
>>> Lastly, if we define this external interface we are kind of stuck with it,
>>> yes?
>>
>> Well, one thing you can do is add a parameter with a name starting with x-
>> which means it's not a fixed interface (so things like libvirt wont use it).
>> And the reason I was interested in seeing the information was otherwise
>> we don't really have any way of knowing how well the code is working;
>> is it already throttling down more and more?
>>
>
> Fair point. Can we add a qmp/hmp command like "info x-cpu-throttle"? Or a
> new line in "info migrate" output, "x-throttle-ration:  1.0" perhaps?
> Would this mess up libvirt's parsing of the hmp/qmp data?

info migrate with extra field, please.  Name it with x-throotle-ratio or
whatever.

I would also preffer to ha a percentage (or per thousand) that is an
integer, but that is just a prefference and I don't really care too
much.

Later, Juan.