Andrew Morton wrote:

Nick Piggin [off-list ref] wrote:

quoted

No, it is not that code I am worried about, you're actually doing
this too (disregarding the admin's wishes):

         dirty_ratio = vm_dirty_ratio;
         if (dirty_ratio > unmapped_ratio / 2)
                 dirty_ratio = unmapped_ratio / 2;

         if (dirty_ratio < 5)
                 dirty_ratio = 5;

hm, OK, that's some "try to avoid writeback off the LRU" stuff.

Yep

But you said "This ensures we should always attempt to start background
writeout before synchronous writeout.".  Does not the current code do that?

Basically what the above code, is scale the dirty_ratio with the
amount of unmapped pages, however it doesn't also scale the
dirty_background_ratio (it does after my patch).

So it isn't difficult to imagine this causing dirty_ratio to become
very close to dirty_background_ratio.

The crude check prevents the values from becoming exactly equal.

	if (background_ratio >= dirty_ratio)
		background_ratio = dirty_ratio / 2;

quoted

So if the admin wants a dirty_ratio of 40 and dirty_background_ratio of 10
then that's good, but I'm sure if they knew you're moving dirty_ratio to 10
here, they'd want something like 2 for the dirty_background_ratio.

I contend that the ratio between these two values is more important than
their absolue values -- especially considering one gets twiddled here.

Maybe true, maybe false.  These things are demonstrable via testing, no?

Might be, I don't know how. Seemed straightforward (to me).
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