linux-kernel - Re: [PATCH] sched: Increment cache_nice

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Date:	Tue, 14 Sep 2010 10:05:52 -0700
From:	Venkatesh Pallipadi <venki@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Mike Galbraith <efault@....de>,
	Gregory Haskins <ghaskins@...ell.com>
Subject: Re: [PATCH] sched: Increment cache_nice_tries only on periodic lb

On Sat, Sep 11, 2010 at 2:32 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Fri, 2010-09-10 at 18:19 -0700, Venkatesh Pallipadi wrote:
>> scheduler uses cache_nice_tries as an indicator to do cache_hot and active
>> load balance, when normal load balance fails. Currently, this value is changed
>> on any failed load balance attempt. That ends up being not so nice to workloads
>> that enter/exit idle often, as they do more frequent new_idle balance and
>> that pretty soon results in cache hot tasks being pulled in.
>>
>> Making the cache_nice_tries ignore failed new_idle balance seems to make
>> better sense. With that only the failed load balance in periodic load balance
>> gets accounted and the rate of accumulation of cache_nice_tries will not
>> depend on idle entry/exit (short running sleep-wakeup kind of tasks). This
>> reduces movement of cache_hot tasks.
>
> Seems to make sense..
>
> I've also wondered if it would make sense to restore 0437e109e (sched:
> zap the migration init / cache-hot balancing code), esp because what the
> comment says isn't actually true anymore, we don't use the tree for
> load-balancing.
>
> But even if we were, the left size of the tree isn't the cold side, nor
> would I guess the right side be.. tricky stuff that.
>
> I know Gregory Haskins has played with restoring it, and I think he
> found some benefit from it, although he didn't pursue it, it might be
> worth seeing if it does for your workloads.
>

Yes. We often talk about that boot time migration overhead
calculations revert here, especially with bigger caches and increased
levels of domain hierarchy. I guess, at some point one size fits all
value will not work and we will need some change like what we used to
have. I did a quick resurrect of that patch and I did not see any
significant benefit (no upside no downside) with this particular
workload. I will keep an eye on this with different workloads and
platforms.

Thanks,
Venki
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