Re: [PATCH/RFC] A method for clearing out page cache

[Ingo Molnar <mingo@elte.hu>]

* Andrew Morton <akpm@osdl.org> wrote:

> >  . enable users to
> >  specify an 'allocation priority' of some sort, which kicks out the
> >  pagecache on the local node - or something like that.
> 
> Yes, that would be preferable - I don't know what the difficulty is
> with that.  sys_set_mempolicy() should provide a sufficiently good
> hint.

yes. I'm not against some flushing mechanism for debugging or test
purposes (it can be useful to start from a new, clean state - and as
such the sysctl for root only and depending on KERNEL_DEBUG is probably
better than an explicit syscall), but the idea to give a flushing API to
applications is bad i believe.

It is the 'easy and incorrect path' to a number of NUMA (and non-NUMA)
VM problems and i fear that it will destroy the evolution of VM
priority/placement/affinity APIs (NUMAlib, etc.).

At least making it sufficiently painful to use (via the originally
proposed root-only sysctl) could still preserve some of the incentive to
provide a clean solution for applications. 'Time to market' constraints
should not be considered when adding core mechanisms.

	Ingo