Re: Fwd: Control page reclaim granularity

[Zheng Liu <gnehzuil.liu@gmail.com>]

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:
>>> On Mon, Mar 12, 2012 at 09:29:34AM +0900, Minchan Kim wrote:
>>>> I forgot to Ccing you.
>>>> Sorry.
>>>>
>>>> ---------- Forwarded message ----------
>>>> From: Minchan Kim<minchan@kernel.org>
>>>> Date: Mon, Mar 12, 2012 at 9:28 AM
>>>> Subject: Re: Control page reclaim granularity
>>>> To: Minchan Kim<minchan@kernel.org>, linux-mm<linux-mm@kvack.org>,
>>>> linux-kernel<linux-kernel@vger.kernel.org>, Konstantin Khlebnikov<
>>>> khlebnikov@openvz.org>, riel@redhat.com, kosaki.motohiro@jp.fujitsu.com
>>>>
>>>>
>>>> On Fri, Mar 09, 2012 at 12:54:03AM +0800, Zheng Liu wrote:
>>>>> Hi Minchan,
>>>>>
>>>>> Sorry, I forgot to say that I don't subscribe linux-mm and
>>>>> linux-kernel
>>>>> mailing list.  So please Cc me.
>>>>>
>>>>> IMHO, maybe we should re-think about how does user use mmap(2).  I
>>>>> describe the cases I known in our product system.  They can be
>>>>> categorized into two cases.  One is mmaped all data files into memory
>>>>> and sometime it uses write(2) to append some data, and another uses
>>>>> mmap(2)/munmap(2) and read(2)/write(2) to manipulate the files.  In
>>>>> the
>>>>> second case,  the application wants to keep mmaped page into memory
>>>>> and
>>>>> let file pages to be reclaimed firstly.  So, IMO, when application
>>>>> uses
>>>>> mmap(2) to manipulate files, it is possible to imply that it wants
>>>>> keep
>>>>> these mmaped pages into memory and do not be reclaimed.  At least
>>>>> these
>>>>> pages do not be reclaimed early than file pages.  I think that
>>>>> maybe we
>>>>> can recover that routine and provide a sysctl parameter to let the
>>>>> user
>>>>> to set this ratio between mmaped pages and file pages.
>>>>
>>>> I am not convinced why we should handle mapped page specially.
>>>> Sometimem, someone may use mmap by reducing buffer copy compared to
>>>> read
>>>> system call.
>>>> So I think we can't make sure mmaped pages are always win.
>>>>
>>>> My suggestion is that it would be better to declare by user explicitly.
>>>> I think we can implement it by madvise and fadvise's WILLNEED option.
>>>> Current implementation is just readahead if there isn't a page in
>>>> memory
>>>> but I think
>>>> we can promote from inactive to active if there is already a page in
>>>> memory.
>>>>
>>>> It's more clear and it couldn't be affected by kernel page reclaim
>>>> algorithm change
>>>> like this.
>>>
>>> Thank you for your advice.  But I still have question about this
>>> solution.  If we improve the madvise(2) and fadvise(2)'s WILLNEED
>>> option,  it will cause an inconsistently status for pages that be
>>> manipulated by madvise(2) and/or fadvise(2).  For example, when I call
>>> madvise with WILLNEED flag, some pages will be moved into active list if
>>> they already have been in memory, and other pages will be read into
>>> memory and be saved in inactive list if they don't be in memory.  Then
>>> pages that are in inactive list are possible to be reclaim.  So from the
>>> view of users, it is inconsistent because some pages are in memory and
>>> some pages are reclaimed.  But actually the user hopes that all of pages
>>> can be kept in memory.  IMHO, this inconsistency is weird and makes
>>> users
>>> puzzled.
>>
>> 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
>> when reclaim happens
>> if the page is referenced.
>>
>> Sigh. We have no room for new VM_FLAG in 32 bit.
> 
> 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
the function returns referenced > 1.  Then this page can be promoted into
activate list.  But I prefer to set/clear this flag in madvise.

PS, I have subscribed linux-mm mailing list. :-)

Regards,
Zheng

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