Re: [patch] first bit of vm balancing fixes for 2.3.52-1

[kanoj@google.engr.sgi.com (Kanoj Sarcar)]

> 
> 
> On Mon, 13 Mar 2000, Ben LaHaise wrote:
> >
> > This is the first little bit of a few vm balancing patches I've been
> > working on.  It does two main things: moves the lru_cache list into the
> > per-zone structure, and slightly reworks the kswapd wakeup logic so the
> > zone_wake_kswapd flag is cleared in free_pages_ok.  Moving the lru_cache
> > list into the zone structure means we can make much better progress when
> > trying to free a specific type of memory.  Moving the clearing of the
> > zone_wake_kswapd flag into the free_pages routine stops kswapd from
> > continuing to swap out ad nausium: my box will discard the entire page
> > cache when it hits low memory when doing a simple sequential read.  With
> > this patch in place it hovers around 3MB free as it should.
> 
> Looks sane to me.
> 
> 		Linus

I am not sure about the zone lru_cache, since any claims without extensive
performance testing is meaningless ... but it does look more cleaner
theoretically. 

About the zone_wake_kswapd clearing in free_pages, yes, it is the right
thing to do ... looking at 2.2, a similar thing was done (ie, nr_free_pages
was updated, which was a signal to the balancing code). Unfortunately, 
this leads to a classfree() call in both pagefree and pagealloc, but I guess
thats the cost of fixing the memory class bugs present in 2.2. 

I do have a problem though with the way the zone_wake_kswapd flag is
otherwise being manipulated in the patch. The rules by which low_on_memory,
zone_wake_kswapd and kswapd poking is done is in Documentation/vm/balance.
I think free_pages_ok also needs to clear the low_on_memory (its never
being cleared in your code). I think the code in the (free <= z->pages_high)
in __alloc_pages() should stay the way it is, unless you can come up for 
a logic for changing it.

Kanoj
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