Re: [RFC][PATCH 0/2] reworking load_balance_monitor

From: Gregory Haskins
Date: Thu Feb 14 2008 - 14:23:21 EST


>>> On Thu, Feb 14, 2008 at 1:15 PM, in message
<20080214121544.941d91f1.pj@xxxxxxx>, Paul Jackson <pj@xxxxxxx> wrote:
> Peter wrote of:
>> the lack of rd->load_balance.
>
> Could you explain to me a bit what that means?
>
> Does this mean that the existing code would, by default (default being
> a single sched domain, covering the entire system's CPUs) load balance
> across the entire system, but with your rework, not so load balance
> there? That seems unlikely.
>
> In any event, from my rather cpuset-centric perspective, there are only
> two common cases to consider.
>
> 1. In the default case, build_sched_domains() gets called once,
> at init, with a cpu_map of all non-isolated CPUs, and we should
> forever after load balance across all those non-isolated CPUs.
>
> 2. In some carefully managed systems using the per-cpuset
> 'sched_load_balance' flags, we tear down that first default
> sched domain, by calling detach_destroy_domains() on it, and we
> then setup some number of sched_domains (typically in the range
> of two to ten, though I suppose we should design to scale to
> hundreds of sched domains, on systems with thousands of CPUs)
> by additional calls to build_sched_domains(), such that their
> CPUs don't overlap (pairwise disjoint) and such that the union
> of all their CPUs may, or may not, include all non-isolated CPUs
> (some CPUs might be left 'out in the cold', intentionally, as
> essentially additional isolated CPUs.) We would then expect load
> balancing within each of these pair-wise disjoint sched domains,
> but not between one of them and another.


Hi Paul,
I think it will still work as you describe. We create a new root-domain object for each pair-wise disjoint sched-domain. In your case (1) above, we would only have one instance of a root-domain which contains (of course) a single instance of the rd->load_balance object. This would, in fact operate like the global variable that Peter is suggesting it replace (IIUC). However, for case (2), we would instantiate a root-domain object per pairwise-disjoint sched-domain, and therefore each one would have its own instance of rd->load_balance.

HTH
-Greg
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