linux-kernel - Re: [PATCH RFC 1/5] sched,numa: build table of node hop distance

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Date:	Tue, 14 Oct 2014 08:47:20 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	linux-kernel@...r.kernel.org, mgorman@...e.de, chegu_vinod@...com,
	mingo@...nel.org, efault@....de, vincent.guittot@...aro.org
Subject: Re: [PATCH RFC 1/5] sched,numa: build table of node hop distance

On Sun, Oct 12, 2014 at 09:28:04AM -0400, Rik van Riel wrote:
> On 10/12/2014 09:17 AM, Peter Zijlstra wrote:
> >On Wed, Oct 08, 2014 at 03:37:26PM -0400, riel@...hat.com wrote:
> >>+	sched_domains_numa_hops = kzalloc(sizeof(int) * nr_node_ids * nr_node_ids, GFP_KERNEL);
> >>+	if (!sched_domains_numa_hops)
> >>+		return;
> >
> >That's potentially a _BIG_ table (1M for a 512 node system).
> >The node_distance has magic allocations and is of u8 size, is there any
> >way we can re-use node_distance and avoid a second O(n^2) allocation?
> 
> You are right, this should be a u8 at the least.
> 
> Beyond that, I am not convinced that merging things into
> the same array is worthwhile, since (IIRC) nr_node_ids
> should be set to the actual number of nodes on the system
> by then.

The thing is, it looks like all you do is compare hop distance, and the
order of the hop distances is the exact same order as the regular numa
distance. I could not find a place where you use the actual hop value.

So if all you're interested in is the relative ordering, that should be
the same for both.
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