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SubjectRE: [RFC][PATCH 0/2] Tunable watermark
On Thu, 13 Jan 2011, Satoru Moriya wrote:

> Currently watermark[low,high] are set by following calculation (lowmem case).
>
> watermark[low] = watermark[min] * 1.25
> watermark[high] = watermark[min] * 1.5
>
> So the difference between watermarks are following:
>
> min <-- min/4 --> low <-- min/4 --> high
>
> I think the differences, "min/4", are too small in my case.
> Of course I can make them bigger if I set min_free_kbytes to bigger value.
> But it means kernel keeps more free memory for PF_MEMALLOC case unnecessarily.
>
> So I suggest changing coefficients(1.25, 1.5). Also it's better
> to make them accessible from user space to tune in response to application
> requirements.
>

Userspace can't possibly be held responsible for tuning internal VM
parameters in response to certain workloads like this; if you have
evidence that different coefficients work better in different
circumstances, then present the criteria for which you intend to change
them from the command line via your new tunables and let's work to make
the VM more extendable to serve those workloads well. This should be done
by showing how background reclaim is ineffective, we enter direct
compaction or reclaim too aggressively, we don't wait for writeout long
enough, we prematurely kill applications when unnecessary, etc, which
would undoubtedly have if you're going to make any sane adjustments via
these new tunables.


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