On 11/22/2012 03:35 AM, Andrew Morton wrote:
> On Wed, 21 Nov 2012 22:52:29 +0800
> Jiang Liu <liu...@gmail.com> wrote:
>
>> On 11/21/2012 03:19 AM, Andrew Morton wrote:
>>> On Tue, 20 Nov 2012 23:18:34 +0800
>>> Jiang Liu <liu...@gmail.com> wrote:
>>>
>>>>>> +static unsigned long calc_memmap_size(unsigned long spanned_pages,
>>>>>> + unsigned long present_pages)
>>>>>> +{
>>>>>> + unsigned long pages = spanned_pages;
>>>>>> +
>>>>>> + /*
>>>>>> + * Provide a more accurate estimation if there are big holes
>>>>>> within
>>>>>> + * the zone and SPARSEMEM is in use.
>>>>>> + */
>>>>>> + if (spanned_pages > present_pages + (present_pages >> 4) &&
>>>>>> + IS_ENABLED(CONFIG_SPARSEMEM))
>>>>>> + pages = present_pages;
>>>>>> +
>>>>>> + return PAGE_ALIGN(pages * sizeof(struct page)) >> PAGE_SHIFT;
>>>>>> +}
>>>>>
>>>>> Please explain the ">> 4" heuristc more completely - preferably in both
>>>>> the changelog and code comments. Why can't we calculate this
>>>>> requirement exactly? That might require a second pass, but that's OK for
>>>>> code like this?
>>>> Hi Andrew,
>>>> A normal x86 platform always have some holes within the DMA ZONE,
>>>> so the ">> 4" heuristic is to avoid applying this adjustment to the DMA
>>>> ZONE on x86 platforms.
>>>> Because the memmap_size is just an estimation, I feel it's OK to
>>>> remove the ">> 4" heuristic, that shouldn't affect much.
>>>
>>> Again: why can't we calculate this requirement exactly? That might
>>> require a second pass, but that's OK for code like this?
>>
>> Hi Andrew,
>> If there are holes within a zone, it may cost us one or two extra pages
>> for each populated region within the zone due to alignment because memmap
>> for
>> each populated regions may not naturally aligned on page boundary.
>
> Right. So with an additional pass across the zone and a bit of
> arithmetic, we can calculate the exact space requirement for memmap?
> No need for kludgy heuristics?
Hi Andrew:
Happy Thanksgiving!
The way to calculate the exact space requirement for memmap seems a little
complex, it depends on:
CONFIG_SPARSEMEM_VMEMMAP
CONFIG_SPARSEMEM_ALLOC_MEM_MAP_TOGETHER
arch implemenation of alloc_remap()
Actually the original motivation is to reduce the deviation on a platform
such as:
node 0: 0-2G,4G-255G (a 2G hole between 2-4G)
node 1: 256G - 511G
node 2: 512G - 767G
node 3: 768G - 1023G
node 0: 1024G - 1026G (memory recovered from the hole)
So I just tried to reduce the deviation instead of accurate calculation of
memmap.
Regards!
Gerry
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