Re: [Question] load balance move tasks not suitable ?

Re: [Question] load balance move tasks not suitable ?

[Peter Zijlstra]

On Mon, Jul 30, 2018 at 08:08:55PM +0800, weiqi (C) wrote:
> Hi Peter,
> 
> After  Commits 5d6523ebd(sched: Fix load-balance wreckage)，

That's a _6_ year old patch... and I can barely remember last week.

> The jugement whether imbalance reached changed from twice to one-half.
> 
> from
> 
> (1) if((load * 2)> rem_load_move)
>         goto next;
> 
> to
> 
> (2) if((load / 2)> env-> load_move)
>         goto next;
> 
> I'm confused about this change.
> 
> "load*2" may be more appropriate, because if a task whose load more than
> env->imbalance is moved from high load cpu to low load cpu,
> 
> will make more imbalance.

It basically goes back to what it was before (see 367456c), in that
patch I (inadvertently) replaced (load.weight >> 1) with (load * 2)
instead of (load / 2).

I'm not entirely sure I can explain that logic at this time. Doing
software archeology on it might clarify where it came from.

Re: [Question] load balance move tasks not suitable ?

[weiqi (C)]

in latest  linux kernel version,  load-balance's  detach_tasks() keeps  
"load/2"  behavior:

"""

         if (( load / 2 ) > env->imbalance)
                 goto next;
"""

It  means  a task  "p",  which  load  is 1.5 times than "env->imbalance" 
can move from "src_rq" to "dst_rq".
after this move,  imbalance between  "src_rq"  and "dst_rq" may larger 
than  before.
but if use "load*2" here,  It means we can pick a task "p" , which load 
is half of "env->imbalance"， and after move，
"src_rq" and "dst_rq"  is balanced.

Am I right?


在 2018/7/30 20:56, Peter Zijlstra 写道:
> On Mon, Jul 30, 2018 at 08:08:55PM +0800, weiqi (C) wrote:
>> Hi Peter,
>>
>> After  Commits 5d6523ebd(sched: Fix load-balance wreckage)，
> That's a _6_ year old patch... and I can barely remember last week.
>
>> The jugement whether imbalance reached changed from twice to one-half.
>>
>> from
>>
>> (1) if((load * 2)> rem_load_move)
>>          goto next;
>>
>> to
>>
>> (2) if((load / 2)> env-> load_move)
>>          goto next;
>>
>> I'm confused about this change.
>>
>> "load*2" may be more appropriate, because if a task whose load more than
>> env->imbalance is moved from high load cpu to low load cpu,
>>
>> will make more imbalance.
> It basically goes back to what it was before (see 367456c), in that
> patch I (inadvertently) replaced (load.weight >> 1) with (load * 2)
> instead of (load / 2).
>
> I'm not entirely sure I can explain that logic at this time. Doing
> software archeology on it might clarify where it came from.
> .
>

Re: [Question] load balance move tasks not suitable ?

[Peter Zijlstra]

A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?
A: Top-posting.
Q: What is the most annoying thing in e-mail?

On Mon, Jul 30, 2018 at 09:42:27PM +0800, weiqi (C) wrote:
> in latest  linux kernel version,  load-balance's  detach_tasks() keeps
> "load/2"  behavior:
> 
> """
> 
>         if (( load / 2 ) > env->imbalance)
>                 goto next;
> """
> 
> It  means  a task  "p",  which  load  is 1.5 times than "env->imbalance" can
> move from "src_rq" to "dst_rq".
> after this move,  imbalance between  "src_rq"  and "dst_rq" may larger than
> before.

imbalance is how much we should move to get in balance. So if we move 1
imbalance we'll end up being just right. If we move 1.5 we end up
slightly better then before, but imbalanced the other way.

If we move 2 imbalance, we're exactly where we were, just the other way
around.

> but if use "load*2" here,  It means we can pick a task "p" , which load is
> half of "env->imbalance"， and after move，
> "src_rq" and "dst_rq"  is balanced.

We might not move anything and not improve even if we could.