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

[Jianguo Wu]

On 2013/1/31 18:38, Simon Jeons wrote:

> Hi Tang,
> On Thu, 2013-01-31 at 17:44 +0800, Tang Chen wrote:
>> Hi Simon,
>>
>> On 01/31/2013 04:48 PM, Simon Jeons wrote:
>>> Hi Tang,
>>> On Thu, 2013-01-31 at 15:10 +0800, Tang Chen wrote:
>>>
>>> 1. IIUC, there is a button on machine which supports hot-remove memory,
>>> then what's the difference between press button and echo to /sys?
>>
>> No important difference, I think. Since I don't have the machine you are
>> saying, I cannot surely answer you. :)
>> AFAIK, pressing the button means trigger the hotplug from hardware, sysfs
>> is just another entrance. At last, they will run into the same code.
>>
>>> 2. Since kernel memory is linear mapping(I mean direct mapping part),
>>> why can't put kernel direct mapping memory into one memory device, and
>>> other memory into the other devices?
>>
>> We cannot do that because in that way, we will lose NUMA performance.
>>
>> If you know NUMA, you will understand the following example:
>>
>> node0:                    node1:
>>     cpu0~cpu15                cpu16~cpu31
>>     memory0~memory511         memory512~memory1023
>>
>> cpu16~cpu31 access memory16~memory1023 much faster than memory0~memory511.
>> If we set direct mapping area in node0, and movable area in node1, then
>> the kernel code running on cpu16~cpu31 will have to access 
>> memory0~memory511.
>> This is a terrible performance down.
> 
> So if config NUMA, kernel memory will not be linear mapping anymore? For
> example, 
> 
> Node 0  Node 1 
> 
> 0 ~ 10G 11G~14G
> 
> kernel memory only at Node 0? Can part of kernel memory also at Node 1?
> 
> How big is kernel direct mapping memory in x86_64? Is there max limit?


Max kernel direct mapping memory in x86_64 is 64TB.

> It seems that only around 896MB on x86_32. 
> 
>>
>>> As you know x86_64 don't need
>>> highmem, IIUC, all kernel memory will linear mapping in this case. Is my
>>> idea available? If is correct, x86_32 can't implement in the same way
>>> since highmem(kmap/kmap_atomic/vmalloc) can map any address, so it's
>>> hard to focus kernel memory on single memory device.
>>
>> Sorry, I'm not quite familiar with x86_32 box.
>>
>>> 3. In current implementation, if memory hotplug just need memory
>>> subsystem and ACPI codes support? Or also needs firmware take part in?
>>> Hope you can explain in details, thanks in advance. :)
>>
>> We need firmware take part in, such as SRAT in ACPI BIOS, or the firmware
>> based memory migration mentioned by Liu Jiang.
> 
> Is there any material about firmware based memory migration?
> 
>>
>> So far, I only know this. :)
>>
>>> 4. What's the status of memory hotplug? Apart from can't remove kernel
>>> memory, other things are fully implementation?
>>
>> I think the main job is done for now. And there are still bugs to fix.
>> And this functionality is not stable.
>>
>> Thanks. :)
> 
> 
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