Re: [RFC 1/3] mm: Add VM pressure notifications

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Thu, Nov 08, 2012 at 05:01:24PM +0000, Mel Gorman wrote:
> (Sorry about being very late reviewing this)
> 
> On Wed, Nov 07, 2012 at 03:01:28AM -0800, Anton Vorontsov wrote:
> > This patch introduces vmpressure_fd() system call. The system call creates
> > a new file descriptor that can be used to monitor Linux' virtual memory
> > management pressure. There are three discrete levels of the pressure:
> > 
> 
> Why was eventfd unsuitable? It's a bit trickier to use but there are
> examples in the kernel where an application is required to do something like
> 
> 1. open eventfd
> 2. open a control file, say /proc/sys/vm/vmpressure or if cgroups
>    /sys/fs/cgroup/something/vmpressure
> 3. write fd_event fd_control [low|medium|oom]. Can be a binary structure
>    you write
> 
> and then poll the eventfd. The trickiness is awkward but a library
> implementation of vmpressure_fd() that mapped onto eventfd properly should
> be trivial.
> 
> I confess I'm not super familiar with eventfd and if this can actually
> work in practice

You've described how it works for memory thresholds and oom notifications
in memcg. So it works. I also prefer this kind of interface.

See Documentation/cgroups/cgroups.txt section 2.4 and
Documentation/cgroups/memory.txt sections 9 and 10.

-- 
 Kirill A. Shutemov

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