Re: Fwd: Control page reclaim granularity

[Konstantin Khlebnikov <khlebnikov@openvz.org>]

Minchan Kim wrote:
> On Mon, Mar 12, 2012 at 06:18:21PM +0400, Konstantin Khlebnikov wrote:
>> Minchan Kim wrote:
>>> On Mon, Mar 12, 2012 at 04:14:14PM +0800, Zheng Liu wrote:
>>>> On 03/12/2012 02:20 PM, Konstantin Khlebnikov wrote:
>>>>> Minchan Kim wrote:
>>>>>> On Mon, Mar 12, 2012 at 10:06:09AM +0800, Zheng Liu wrote:
<CUT>
>>>>>>
>>>>>> Now problem is that
>>>>>>
>>>>>> 1. User want to keep pages which are used once in a while in memory.
>>>>>> 2. Kernel want to reclaim them because they are surely reclaim target
>>>>>>      pages in point of view by LRU.
>>>>>>
>>>>>> The most desriable approach is that user should use mlock to guarantee
>>>>>> them in memory. But mlock is too big overhead and user doesn't want to
>>>>>> keep
>>>>>> memory all pages all at once.(Ie, he want demand paging when he need
>>>>>> the page)
>>>>>> Right?
>>>>>>
>>>>>> madvise, it's a just hint for kernel and kernel doesn't need to make
>>>>>> sure madvise's behavior.
>>>>>> In point of view, such inconsistency might not be a big problem.
>>>>>>
>>>>>> Big problem I think now is that user should use madvise(WILLNEED)
>>>>>> periodically because such
>>>>>> activation happens once when user calls madvise. If user doesn't use
>>>>>> page frequently after
>>>>>> user calls it, it ends up moving into inactive list and even could be
>>>>>> reclaimed.
>>>>>> It's not good. :-(
>>>>>>
>>>>>> Okay. How about adding new VM_WORKINGSET?
>>>>>> And reclaimer would give one more round trip in active/inactive list
>>>>>> erwhen reclaim happens
>>>>>> if the page is referenced.
>>>>>>
>>>>>> Sigh. We have no room for new VM_FLAG in 32 bit.
>>>>> p
>>>>> It would be nice to mark struct address_space with this flag and export
>>>>> AS_UNEVICTABLE somehow.
>>>>> Maybe we can reuse file-locking engine for managing these bits =)
>>>>
>>>> Make sense to me.  We can mark this flag in struct address_space and check
>>>> it in page_refereneced_file().  If this flag is set, it will be cleard and
>>>
>>> Disadvantage is that we could set reclaim granularity as per-inode.
>>> I want to set it as per-vma, not per-inode.
>>
>> But with per-inode flag we can tune all files, not only memory-mapped.
>
> I don't oppose per-inode setting but I believe we need file range or mmapped vma,
> still. One file may have different characteristic part, something is working set
> something is streaming part.
>
>> See, attached patch. Currently I thinking about managing code,
>> file-locking engine really fits perfectly =)
>
> file-locking engine?
> You consider fcntl as interface for it?
> What do you mean?
>

If we set bits on inode we somehow account its users and clear AS_WORKINGSET and AS_UNEVICTABLE
at last file close. We can use file-locking engine for locking inodes in memory -- file lock automatically
release inode at last fput(). Maybe it's too tricky and we should add couple simple atomic counters to
generic strict inode (like i_writecount/i_readcount) but in this case we will add new code on fast-path.
So, looks like invention new kind of struct file_lock is best approach.
I don't want implement range-locking for now, but I can do it if somebody really wants this.

Yes, we can use fcntl(), but fadvise() is much better.

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