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SubjectRe: 'disposable' dirty pages [was: Out Of Memory in v. 2.1]
On Fri, Oct 09, 1998 at 12:50:03PM -0400, Kenneth Albanowski wrote:
> Yes, uniformly yucky. It feels like disappearing 4K pages could be of use
> in some contexts (places where either the cached data fits easily in a 4K
> page, or the data is continuous, and 4K chunks can be recalculated as
> needed.) but not especially easy to use -- and also not portable. Just
> move over to an Alpha, and you're working with 8K pages all of a sudden...

See my suggestion for "unmap-on-discard" pages, that would solve this in
a simpler way and has other useful properties (especially the ability to
reclaim unused portions of cached objects). And no need for a VMA.

> Yes, the only overhead is the extra (internal) trap to turn off the
> discardable flag, and then the syscall to turn the bit back on (if you are
> using the memory as a cache). That should be a miniscule overhead, though.

You don't need the internal trap -- check if a page is discardable when
it's aged enough to remove it. The normal page ageing will detect "next
touch", you just have to clear the touched/dirty bits when marking a
page discardable.

Of course, architectures which don't have a touched bit per pte will
need a trap, but they need it for page ageing anyway, it's presumably
already done and implemented.

-- Jamie

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