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

From: [email-masked] (Alex Shi)
Date: Sun, 16 Dec 2012 15:12:46 +0800

On 12/14/2012 05:33 PM, Vincent Guittot wrote:
> On 14 December 2012 02:46, Alex Shi <[email-masked]> wrote:
>> On 12/13/2012 11:48 PM, Vincent Guittot wrote:
>>> On 13 December 2012 15:53, Vincent Guittot <[email-masked]> wrote:
>>>> On 13 December 2012 15:25, Alex Shi <[email-masked]> wrote:
>>>>> On 12/13/2012 06:11 PM, Vincent Guittot wrote:
>>>>>> On 13 December 2012 03:17, Alex Shi <[email-masked]> 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.
>>>
>>> I mean conf2 not conf3
>>
>> So, it has 4 levels 0/16/32/ for socket 3 and 0 level for socket 0, it
>> is unbalanced for different socket.
> 
> That the target because we have decided to pack the small tasks in
> socket 0 when we have parsed the topology at boot.
> We don't have to loop into sched_domain or sched_group anymore to find
> the best LCPU when a small tasks wake up.

iteration on domain and group is a advantage feature for power efficient
requirement, not shortage. If some CPU are already idle before forking,
let another waking CPU check their load/util and then decide which one
is best CPU can reduce late migrations, that save both the performance
and power.

On the contrary, move task walking on each level buddies is not only bad
on performance but also bad on power. Consider the quite big latency of
waking a deep idle CPU. we lose too much..

> 
>>
>> And the ground level has just one buddy for 16 LCPUs - 8 cores, that's
>> not a good design, consider my previous examples: if there are 4 or 8
>> tasks in one socket, you just has 2 choices: spread them into all cores,
>> or pack them into one LCPU. Actually, moving them just into 2 or 4 cores
>> maybe a better solution. but the design missed this.
> 
> You speak about tasks without any notion of load. This patch only care
> of small tasks and light LCPU load, but it falls back to default
> behavior for other situation. So if there are 4 or 8 small tasks, they
> will migrate to the socket 0 after 1 or up to 3 migration (it depends
> of the conf and the LCPU they come from).

According to your patch, what your mean 'notion of load' is the
utilization of cpu, not the load weight of tasks, right?

Yes, I just talked about tasks numbers, but it naturally extends to the
task utilization on cpu. like 8 tasks with 25% util, that just can full
fill 2 CPUs. but clearly beyond the capacity of the buddy, so you need
to wake up another CPU socket while local socket has some LCPU idle...
> 
> Then, if too much small tasks wake up simultaneously on the same LCPU,
> the default load balance will spread them in the core/cluster/socket
> 
>>
>> Obviously, more and more cores is the trend on any kinds of CPU, the
>> buddy system seems hard to catch up this.
>>
>>

-- 
Thanks
    Alex