Re: [RFC][PATCH] mm: merge as soon as possible when pcp alloc/free

[Xishi Qiu <qiuxishi@huawei.com>]

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.
> 
> https://www.kernel.org/doc/gorman/html/understand/understand009.html
> "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?

Thanks,
Xishi Qiu

> 
> .
> 



--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>