On Wed, 21 Nov 2012 15:01:50 +0000
Mel Gorman [off-list ref] wrote:

On Tue, Nov 20, 2012 at 10:12:28AM -0800, David Rientjes wrote:

quoted

On Mon, 19 Nov 2012, Anton Vorontsov wrote:

quoted

We try to make userland freeing resources when the system becomes low on
memory. Once we're short on memory, sometimes it's better to discard
(free) data, rather than let the kernel to drain file caches or even start
swapping.

To add another usecase: its possible to modify our version of malloc (or 
any malloc) so that memory that is free()'d can be released back to the 
kernel only when necessary, i.e. when keeping the extra memory around 
starts to have a detremental effect on the system, memcg, or cpuset.  When 
there is an abundance of memory available such that allocations need not 
defragment or reclaim memory to be allocated, it can improve performance 
to keep a memory arena from which to allocate from immediately without 
calling the kernel.

A potential third use case is a variation of the first for batch systems. If
it's running low priority tasks and a high priority task starts that
results in memory pressure then the job scheduler may decide to move the
low priority jobs elsewhere (or cancel them entirely).

A similar use case is monitoring systems running high priority workloads
that should never swap. It can be easily detected if the system starts
swapping but a pressure notification might act as an early warning system
that something is happening on the system that might cause the primary
workload to start swapping.

I hope Anton's writing all of this down ;)


The proposed API bugs me a bit.  It seems simplistic.  I need to have a
quality think about this.  Maybe the result of that think will be to
suggest an interface which can be extended in a back-compatible fashion
later on, if/when the simplistic nature becomes a problem.

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