Re: [RFC] 2.3.39 zone balancing

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

> 
> On Wed, 12 Jan 2000, Kanoj Sarcar wrote:
> 
> > --- mm/page_alloc.c	Tue Jan 11 11:00:31 2000
> > +++ mm/page_alloc.c	Tue Jan 11 23:59:35 2000
> > +		cumulative += size;
> > +		mask = (cumulative >> 7);
> > +		if (mask < 1) mask = 1;
> > +		zone->pages_low = mask*2;
> > +		zone->pages_high = mask*3;
> >  		zone->low_on_memory = 0;
> 
> I think that busier machines probably have a larger need
> for DMA memory than this code fragment will give us. I
> have the gut feeling that we'll want to keep about 512kB
> or more free in the lower 16MB of busy machines...

Note that as I point out in my documentation, and as Alan
also points out, 2.2 is doing fine. The 2.2 code does not
guarantee dma-zone balancing even if it is empty (if there
is enough regular free pages). Which means all dma requests
will fail. I have tried to fix that, since with HIGHMEM, 
the problem is actually more aggravated.

My aim is to fix a couple of problems, move to a zone based
balancing, and then maybe finetune it. For example, the 
>> 7 part can be replaced with >> N, where N is dependent 
on the zone type, or size of lower zones, etc. I mention this
in the doc too. The only problem is, if N < 7, you will probably
have degraded perfomance in certain cases due to more frequent
balancing.

> 
> (if only because such a large amount of free pages in
> such a small part of the address space will give us
> higher-order free pages)

I note that Andrea also commented about this. I am also
of the same opinion as him, we should not (as far as possible)
try to intermingle unrelated issues. In this case though,
I have no idea how having a large number of free dma pages
ensures more higher-order free pages. Can someone give me
the logic for this claim?

> 
> > --- mm/vmscan.c	Tue Jan 11 11:00:31 2000
> > +++ mm/vmscan.c	Tue Jan 11 23:29:41 2000
> > @@ -534,8 +534,11 @@
> >  	int retval = 1;
> >  
> >  	wake_up_process(kswapd_process);
> > -	if (gfp_mask & __GFP_WAIT)
> > +	if (gfp_mask & __GFP_WAIT) {
> > +		current->flags |= PF_MEMALLOC;
> >  		retval = do_try_to_free_pages(gfp_mask, zone);
> > +		current->flags &= ~PF_MEMALLOC;
> > +	}
> >  	return retval;
> >  }
> 
> Please note that kswapd still exits when the total number
> of free pages in the system is high enough. Balancing can
> probably better be done in the background by kswapd than
> by applications that happen to stumble across a nonbalanced
> zone...

Yes, we need to decide whether kswapd needs modification too. Its
just that I want to do incremental fixes, instead of change a 
huge bunch of code all at once. The question is, if I had a Linux
2.3 kernel, where I had completely deleted kswapd(), what problems 
would the kernel face? Ie, what is kswapd()'s purpose?

Kanoj

> 
> regards,
> 
> Rik
> --
> The Internet is not a network of computers. It is a network
> of people. That is its real strength.
> 

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