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

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

KAMEZAWA Hiroyuki wrote:
> On Mon, 01 Sep 2008 08:58:32 +0530
> Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
> 
>> 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.
> Maybe they cannot be reduced. For 32bit systems, if the machine doesn't equip
> crazy amounts of memory (as 32GB) I don't think this 32bit is not very large.
> 
> Let's calculate. 1GB/4096 x 4 bytes = 1 MB per 1GB. 
> But you adds spinlock_t, then what this patch reduce is not so big. Maybe only
> hundreds of kilobytes. (All pages in HIGHMEM will be used with structpage_cgroup.)
> 

There are other things like sizeof(struct page) crossing cacheline boundaries
and if we pass cgroup_disabled=memory, we save on the radix tree slots and
memory used there.

> 
>>>> 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. 
> The development of lockless-page_cgroup is not stalled. I'm just waiting for
> my 8cpu box comes back from maintainance...
> If you want to see, I'll post v3 with brief result on small (2cpu) box.
> 

I understand and I am not pushing you to completing it, but at the same time I
don't want to queue up behind it for long. I suspect the cost of porting
lockless page cache on top of my patches should not be high, but I'll never know
till I try :)

>> 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.
>>
> Sorry. I missed pc==NULL case.
> 

No Problem

> 
>>> 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?
>>
> Not so clear at this stage. 
> 
> Thanks,
> -Kame
> 
> 
> 


-- 
	Balbir

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