linux-kernel - Re: [PATCH v6 00/15] memory-hotplug: hot-remove physical memory

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Date:	Thu, 10 Jan 2013 16:31:28 +0900
From:	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Glauber Costa <glommer@...allels.com>
CC:	Tang Chen <tangchen@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>, rientjes@...gle.com,
	len.brown@...el.com, benh@...nel.crashing.org, paulus@...ba.org,
	cl@...ux.com, minchan.kim@...il.com,
	kosaki.motohiro@...fujitsu.com, isimatu.yasuaki@...fujitsu.com,
	wujianguo@...wei.com, wency@...fujitsu.com, hpa@...or.com,
	linfeng@...fujitsu.com, laijs@...fujitsu.com, mgorman@...e.de,
	yinghai@...nel.org, x86@...nel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	linux-acpi@...r.kernel.org, linux-s390@...r.kernel.org,
	linux-sh@...r.kernel.org, linux-ia64@...r.kernel.org,
	cmetcalf@...era.com, sparclinux@...r.kernel.org
Subject: Re: [PATCH v6 00/15] memory-hotplug: hot-remove physical memory

(2013/01/10 16:14), Glauber Costa wrote:
> On 01/10/2013 06:17 AM, Tang Chen wrote:
>>>> Note: if the memory provided by the memory device is used by the
>>>> kernel, it
>>>> can't be offlined. It is not a bug.
>>>
>>> Right.  But how often does this happen in testing?  In other words,
>>> please provide an overall description of how well memory hot-remove is
>>> presently operating.  Is it reliable?  What is the success rate in
>>> real-world situations?
>>
>> We test the hot-remove functionality mostly with movable_online used.
>> And the memory used by kernel is not allowed to be removed.
>
> Can you try doing this using cpusets configured to hardwall ?
> It is my understanding that the object allocators will try hard not to
> allocate anything outside the walls defined by cpuset. Which means that
> if you have one process per node, and they are hardwalled, your kernel
> memory will be spread evenly among the machine. With a big enough load,
> they should eventually be present in all blocks.
>

I'm sorry I couldn't catch your point.
Do you want to confirm whether cpuset can work enough instead of ZONE_MOVABLE ?
Or Do you want to confirm whether ZONE_MOVABLE will not work if it's used with cpuset ?


> Another question I have for you: Have you considering calling
> shrink_slab to try to deplete the caches and therefore free at least
> slab memory in the nodes that can't be offlined? Is it relevant?
>

At this stage, we don't consider to call shrink_slab(). We require
nearly 100% success at offlining memory for removing DIMM.
It's my understanding.

IMHO, I don't think shrink_slab() can kill all objects in a node even
if they are some caches. We need more study for doing that.

Thanks,
-Kame


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