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SubjectRe: [RFC][PATCH] mm: merge as soon as possible when pcp alloc/free
On 2016/11/5 20:29, Anshuman Khandual wrote:

> On 11/05/2016 01:27 PM, Xishi Qiu wrote:
>> Usually the memory of android phones is very small, so after a long
>> running, the fragment is very large. Kernel stack which called by
>> alloc_thread_stack_node() usually alloc 16K memory, and it failed
>> frequently.
>> However we have CONFIG_VMAP_STACK now, but it do not support arm64,
>> and maybe it has some regression because of vmalloc, it need to
>> find an area and create page table dynamically, this will take a short
>> time.
>> I think we can merge as soon as possible when pcp alloc/free to reduce
>> fragment. The pcp page is hot page, so free it will cause cache miss,
>> I use perf to test it, but it seems the regression is not so much, maybe
>> it need to test more. Any reply is welcome.
> The idea of PCP is to have a fast allocation mechanism which does not depend
> on an interrupt safe spin lock for every allocation. I am not very familiar
> with this part of code but the following documentation from Mel Gorman kind
> of explains that the this type of fragmentation problem which you might be
> observing as one of the limitations of PCP mechanism.
> "Per CPU page list" sub header.

"The last potential problem is that buddies of newly freed pages could exist
in other pagesets leading to possible fragmentation problems."
So we should not change it, and this is a known issue, right?

Xishi Qiu

> .

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