Re: [RFC] vmevent: Implement pressure attribute

[Mel Gorman <mgorman@suse.de>]

On Mon, Oct 08, 2012 at 06:42:16PM -0700, John Stultz wrote:
> On 10/08/2012 02:46 AM, Mel Gorman wrote:
> >On Sun, Oct 07, 2012 at 01:14:17AM -0700, Anton Vorontsov wrote:
> >>And here we just try to let userland to assist, userland can tell "oh,
> >>don't bother with swapping or draining caches, I can just free some
> >>memory".
> >>
> >>Quite interesting, this also very much resembles volatile mmap ranges
> >>(i.e. the work that John Stultz is leading in parallel).
> >>
> >Agreed. I haven't been paying close attention to those patches but it
> >seems to me that one possiblity is that a listener for a vmevent would
> >set volatile ranges in response.
> 
> I don't have too much to comment on the rest of this mail, but just
> wanted to pipe in here, as the volatile ranges have caused some
> confusion.
> 
> While your suggestion would be possible, with volatile ranges, I've
> been promoting a more hands off-approach from the application
> perspective, where the application always would mark data that could
> be regenerated as volatile, unmarking it when accessing it.
> 
> This way the application doesn't need to be responsive to memory
> pressure, the kernel just takes what it needs from what the
> application made available.
> 
> Only when the application needs the data again, would it mark it
> non-volatile (or alternatively with the new SIGBUS semantics, access
> the purged volatile data and catch a SIGBUS), find the data was
> purged and regenerate it.
> 

Ok understood.

> That said, hybrid approaches like you suggested would be possible,
> but at a certain point, if we're waiting for a notification to take
> action, it might be better just to directly free that memory, rather
> then just setting it as volatile, and leaving it to the kernel then
> reclaim it for you.
> 

That's fine. I did not mean to suggest that volatile and vmevents on
memory pressure should be related or depending on each other in any way.


-- 
Mel Gorman
SUSE Labs

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