RE: [RFC][PATCH 0/2] Tunable watermark

[Satoru Moriya]

On 01/07/2011 05:39 PM, David Rientjes wrote:

> The semantics of any watermark is to trigger events to happen at a 
> specific level, so they should be static with respect to a frame of 
> reference (which in the VM case is the min watermark with respect to the 
> size of the zone).  If you're going to adjust the min watermark, it's then 
> _mandatory_ to adjust the others to that frame of reference, you shouldn't 
> need to tune them independently.

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.

> The problem that Satoru is reporting probably has nothing to do with the 
> watermarks themselves but probably requires more aggressive action by 
> kswapd and/or memory compaction.

More aggressive action may reduce the possibility of the problem reported.
But we can't avoid the problem completely because applications may
allocate/access faster than reclaiming/compaction.

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