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SubjectRe: [PATCH -v2 -mm] add extra free kbytes tunable
On Thu, 1 Sep 2011, Rik van Riel wrote:

> Add a userspace visible knob to tell the VM to keep an extra amount
> of memory free, by increasing the gap between each zone's min and
> low watermarks.
> This is useful for realtime applications that call system
> calls and have a bound on the number of allocations that happen
> in any short time period. In this application, extra_free_kbytes
> would be left at an amount equal to or larger than than the
> maximum number of allocations that happen in any burst.
> It may also be useful to reduce the memory use of virtual
> machines (temporarily?), in a way that does not cause memory
> fragmentation like ballooning does.

I know this was merged into -mm, but I still have to disagree with it
because I think it adds yet another userspace knob that will never be
obsoleted, will be misinterepted, and is tied very closely to the
implementation of page reclaim, both synchronous and asynchronous. I also
think that it will cause regressions on other cpu intensive workloads
that don't require this extra freed memory because it works as a global
heuristic and is not tied to any specific application.

I think it would be far better to reclaim beyond above the high watermark
if the types of workloads that need this tunable can be somehow detected
(the worst case scenario is being a prctl() that does synchronous reclaim
above the watermark so admins can identify these workloads), or be able to
mark allocations within the kernel as potentially coming in large bursts
where allocation is problematic.

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