[PATCH 0/1] [Yakkety] UBUNTU: [Config] Increase the NR_CPUS to 8K for amd64 to support systems with a large number of cores.

Tim Gardner tim.gardner at canonical.com
Tue Nov 29 17:03:40 UTC 2016


On 11/29/2016 09:56 AM, Seth Forshee wrote:
> On Tue, Nov 22, 2016 at 08:35:17AM -0800, Brad Figg wrote:
>> On Tue, Nov 22, 2016 at 03:49:25PM +0000, Colin Ian King wrote:
>>> On 22/11/16 15:35, Brad Figg wrote:
>>>> BugLink: http://bugs.launchpad.net/bugs/1579205
>>>>
>>>> Brad Figg (1):
>>>>   UBUNTU: [Config] Increase the NR_CPUS to 8K for amd64 to support
>>>>     systems with a     large number of cores.
>>>>
>>>>  debian.master/config/amd64/config.common.amd64 | 2 +-
>>>>  debian.master/config/annotations               | 2 +-
>>>>  2 files changed, 2 insertions(+), 2 deletions(-)
>>>>
>>>
>>> What's the overhead or impact of bumping this to 8K?
>>>
>>> Colin
>>
>> I'm not sure. However, Tim looked at this once before and found it wasn't
>> significant if I remember right. This patch is a resubmit of one that was
>> submitted, applied and the dropped mistakenly.
> 
> According to the Kconfig documentation for NR_CPUS, "each supported CPU
> adds approximately eight kilobytes to the kernel image." If we assume
> that is correct we're talking ~64MB of extra RAM. I'd be surprised if
> anyone is actually keeping that estimate up to date though.
> 
> Tim - I don't suppose you still have the number you came up with before?
> It would be nice to have an idea - cloud people may be upset if it turns
> out to be a lot.
> 

Not offhand. However, going above 512 requires CPUMASK_OFFSTACK and
MAXSMP which I have little experience with. MAXSMP has knock-on effects
on NUMA nodes shift.

-- 
Tim Gardner tim.gardner at canonical.com




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