Re: Control page reclaim granularity

[Minchan Kim <minchan@kernel.org>]

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.

> 
> Regards,
> Zheng

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