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

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Message-ID: <20180730155933.GK2494@hirez.programming.kicks-ass.net>
Date:   Mon, 30 Jul 2018 17:59:33 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "weiqi (C)" <weiqi4@...wei.com>
Cc:     linux-kernel@...r.kernel.org,
        Zhanghailiang <zhang.zhanghailiang@...wei.com>
Subject: Re: [Question] load balance move tasks not suitable ?

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.