linux-kernel - Re: [PATCH 00/10] mm: Linux VM Infrastructure to support Memory Power Management

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Message-ID: <alpine.DEB.2.02.1107071831250.8081@asgard.lang.hm>
Date:	Thu, 7 Jul 2011 18:32:17 -0700 (PDT)
From:	david@...g.hm
To:	Pekka Enberg <penberg@...nel.org>
cc:	Ankita Garg <ankita@...ibm.com>,
	linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, linux-pm@...ts.linux-foundation.org,
	svaidy@...ux.vnet.ibm.com, thomas.abraham@...aro.org,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Christoph Lameter <cl@...ux.com>
Subject: Re: [PATCH 00/10] mm: Linux VM Infrastructure to support Memory
 Power Management

On Thu, 7 Jul 2011, Pekka Enberg wrote:

> On Wed, 6 Jul 2011, Pekka Enberg wrote:
>>> Why does the allocator need to know about address boundaries? Why
>>> isn't it enough to make the page allocator and reclaim policies favor 
>>> using
>>> memory from lower addresses as aggressively as possible? That'd mean
>>> we'd favor the first memory banks and could keep the remaining ones
>>> powered off as much as possible.
>>> 
>>> IOW, why do we need to support scenarios such as this:
>>>
>>>   bank 0     bank 1   bank 2    bank3
>>> | online  | offline | online  | offline |
>> 
> On Wed, 6 Jul 2011, david@...g.hm wrote:
>> I believe that there are memory allocations that cannot be moved after they 
>> are made (think about regions allocated to DMA from hardware where the 
>> hardware has already been given the address space to DMA into)
>> 
>> As a result, you may not be able to take bank 2 offline, so your option is 
>> to either leave banks 0-2 all online, or support emptying bank 1 and taking 
>> it offline.
>
> But drivers allocate DMA memory for hardware during module load and stay 
> pinned there until the driver is unloaded, no? So in practice DMA buffers are 
> going to be in banks 0-1?

that depends on when the device was initialized. it is common for them to 
be in the beginning, but with hotplug, who knows.

David Lang
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