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

From: Tang Chen <[email-masked]>
Date: Thu, 31 Jan 2013 17:44:06 +0800

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.

>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.

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. :)