[RFC PATCH v2 3/6] sched: pack small tasks

[Vincent Guittot]

On 13 December 2012 15:25, Alex Shi <alex.shi at intel.com> wrote:
> On 12/13/2012 06:11 PM, Vincent Guittot wrote:
>> On 13 December 2012 03:17, Alex Shi <alex.shi at intel.com> wrote:
>>> On 12/12/2012 09:31 PM, Vincent Guittot wrote:
>>>> During the creation of sched_domain, we define a pack buddy CPU for each CPU
>>>> when one is available. We want to pack at all levels where a group of CPU can
>>>> be power gated independently from others.
>>>> On a system that can't power gate a group of CPUs independently, the flag is
>>>> set at all sched_domain level and the buddy is set to -1. This is the default
>>>> behavior.
>>>> On a dual clusters / dual cores system which can power gate each core and
>>>> cluster independently, the buddy configuration will be :
>>>>
>>>>       | Cluster 0   | Cluster 1   |
>>>>       | CPU0 | CPU1 | CPU2 | CPU3 |
>>>> -----------------------------------
>>>> buddy | CPU0 | CPU0 | CPU0 | CPU2 |
>>>>
>>>> Small tasks tend to slip out of the periodic load balance so the best place
>>>> to choose to migrate them is during their wake up. The decision is in O(1) as
>>>> we only check again one buddy CPU
>>>
>>> Just have a little worry about the scalability on a big machine, like on
>>> a 4 sockets NUMA machine * 8 cores * HT machine, the buddy cpu in whole
>>> system need care 64 LCPUs. and in your case cpu0 just care 4 LCPU. That
>>> is different on task distribution decision.
>>
>> The buddy CPU should probably not be the same for all 64 LCPU it
>> depends on where it's worth packing small tasks
>
> Do you have further ideas for buddy cpu on such example?

yes, I have several ideas which were not really relevant for small
system but could be interesting for larger system

We keep the same algorithm in a socket but we could either use another
LCPU in the targeted socket (conf0) or chain the socket (conf1)
instead of packing directly in one LCPU

The scheme below tries to summaries the idea:

Socket      | socket 0 | socket 1   | socket 2   | socket 3   |
LCPU        | 0 | 1-15 | 16 | 17-31 | 32 | 33-47 | 48 | 49-63 |
buddy conf0 | 0 | 0    | 1  | 16    | 2  | 32    | 3  | 48    |
buddy conf1 | 0 | 0    | 0  | 16    | 16 | 32    | 32 | 48    |
buddy conf2 | 0 | 0    | 16 | 16    | 32 | 32    | 48 | 48    |

But, I don't know how this can interact with NUMA load balance and the
better might be to use conf3.

>>
>> Which kind of sched_domain configuration have you for such system ?
>> and how many sched_domain level have you ?
>
> it is general X86 domain configuration. with 4 levels,
> sibling/core/cpu/numa.
>>