Re: [RFC][PATCH] Remove cgroup member from struct page

[Balbir Singh <balbir@linux.vnet.ibm.com>]

KAMEZAWA Hiroyuki wrote:
> On Sun, 31 Aug 2008 23:17:56 +0530
> Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
> 
>> This is a rewrite of a patch I had written long back to remove struct page
>> (I shared the patches with Kamezawa, but never posted them anywhere else).
>> I spent the weekend, cleaning them up for 2.6.27-rc5-mmotm (29 Aug 2008).
>>
> It's just because I think there is no strong requirements for 64bit count/mapcount.
> There is no ZERO_PAGE() for ANON (by Nick Piggin. I add him to CC.)
> (shmem still use it but impact is not big.)
> 

I understand the comment, but not it's context. Are you suggesting that the
sizeof _count and _mapcount can be reduced? Hence the impact of having a member
in struct page is not all that large? I think the patch is definitely very
important for 32 bit systems.

>> I've tested the patches on an x86_64 box, I've run a simple test running
>> under the memory control group and the same test running concurrently under
>> two different groups (and creating pressure within their groups). I've also
>> compiled the patch with CGROUP_MEM_RES_CTLR turned off.
>>
>> Advantages of the patch
>>
>> 1. It removes the extra pointer in struct page
>>
>> Disadvantages
>>
>> 1. It adds an additional lock structure to struct page_cgroup
>> 2. Radix tree lookup is not an O(1) operation, once the page is known
>>    getting to the page_cgroup (pc) is a little more expensive now.
>>
>> This is an initial RFC for comments
>>
>> TODOs
>>
>> 1. Test the page migration changes
>> 2. Test the performance impact of the patch/approach
>>
>> Comments/Reviews?
>>
> plz wait until lockless page cgroup....
> 

That depends, if we can get the lockless page cgroup done quickly, I don't mind
waiting, but if it is going to take longer, I would rather push these changes
in. There should not be too much overhead in porting lockless page cgroup patch
on top of this (remove pc->lock and use pc->flags). I'll help out, so as to
avoid wastage of your effort.

> And If you don't support radix-tree-delete(), pre-allocating all at boot is better.
> 

We do use radix-tree-delete() in the code, please see below. Pre-allocating has
the disadvantage that we will pre-allocate even for kernel pages, etc.

> BTW, why pc->lock is necessary ? It increases size of struct page_cgroup and reduce 
> the advantege of your patch's to half (8bytes -> 4bytes).
> 

Yes, I've mentioned that as a disadvantage. Are you suggesting that with
lockless page cgroup we won't need pc->lock?

> Thanks,
> -Kame

-- 
	Balbir

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