Re: [RFC][PATCH v4 3/3] sched: Periodically decay max cost of idle balance

[Jason Low <jason.low2@hp.com>]

On Fri, 2013-08-30 at 12:29 +0200, Peter Zijlstra wrote:
>  	rcu_read_lock();
>  	for_each_domain(cpu, sd) {
> +		/*
> +		 * Decay the newidle max times here because this is a regular
> +		 * visit to all the domains. Decay ~0.5% per second.
> +		 */
> +		if (time_after(jiffies, sd->next_decay_max_lb_cost)) {
> +			sd->max_newidle_lb_cost =
> +				(sd->max_newidle_lb_cost * 254) / 256;

I initially picked 0.5%, but after trying it out, it appears to decay very
slowing when the max is at a high value. Should we increase the decay a
little bit more? Maybe something like:

sd->max_newidle_lb_cost = (sd->max_newidle_lb_cost * 63) / 64;

> +		/*
> +		 * Stop the load balance at this level. There is another
> +		 * CPU in our sched group which is doing load balancing more
> +		 * actively.
> +		 */
> +		if (!continue_balancing) {

Is "continue_balancing" named "balance" in older kernels?

Here are the AIM7 results with the other 2 patches + this patch with the
slightly higher decay value.

----------------------------------------------------------------
workload     | % improvement   | % improvement  | % improvement
             | with patch      | with patch     | with patch
             | 1100-2000 users | 200-1000 users | 10-100 users
----------------------------------------------------------------
alltests     |  +9.2%          |  +5.2%         |  +0.3%
----------------------------------------------------------------
compute      |  +0.0%          |  -0.9%         |  +0.6%
----------------------------------------------------------------
custom       | +18.6%          | +15.3%         |  +7.0%
----------------------------------------------------------------
disk         |  +4.0%          | +16.5%         |  +7.1%
----------------------------------------------------------------
fserver      | +64.8%          | +27.5%         |  -0.6%
----------------------------------------------------------------
high_systime | +15.1%          |  +7.9%         |  +0.0%
----------------------------------------------------------------
new_fserver  | +51.0%          | +20.1%         |  -1.3%
----------------------------------------------------------------
shared       |  +6.3%          |  +8.8%         |  +2.8%
----------------------------------------------------------------