Re: [PATCH] 2.6.3-rc3-mm1: sched-group-power

[Nick Piggin <piggin@cyberone.com.au>]

Rick Lindsley wrote:

>Nick, I'm not sure what capability this patch adds .. perhaps some words
>of explanation.
>
>So we have SMT/HT situations where we'd prefer to balance across cores;
>that is, if 0, 1, 2, and 3 share a core and 4, 5, 6, and 7 share a core,
>you'd like two processes to arrange themselves so one is on [0123] and
>another is on [4567].  This is what the SD_IDLE flag indicated before.
>
>With this patch, we can "weight" the load imposed by certain cpus, right?
>What advantage does this give us?  On a given machine, won't the "weight"
>of any one set of SMT siblings and cores be uniform with respect to all
>the cores and siblings anyway?
>
>

It is difficult to propogate the SD_FLAG_IDLE attribute up
multiple domains.

For example, with SMT + CPU + NODE domains you can get into
the following situation:

01, 23 are 4 siblings in 2 cores on node 0,
45, 67 are " " " on node 1.

The top level balancing domain now spans 01234567, and wants to
balance between groups 0123, and 4567. We don't want SD_FLAG_IDLE
semantics here, because that would mean if two tasks were running
on node 0, one would be migrated to node 1. We want to migrate 1
task if one node is idle, and the other has 3 processes running for
example.

Also this copes with siblings becoming much more powerful, or
some groups with SMT turned off, some on (think hotplug cpu),
different speed CPUs, etc.