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    SubjectRe: [RFC][PATCH] Remove cgroup member from struct page
    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.)


    > >> 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.

    > 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.


    > > 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





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