Messages in this thread | | | Date | Fri, 20 Feb 2004 12:37:30 +1100 | From | Nick Piggin <> | Subject | Re: [PATCH] 2.6.3-rc3-mm1: sched-group-power |
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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.
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