Re: new OOM heuristic failure (was: Re: VM: qsbench)

[Stephan von Krawczynski <skraw@ithnet.com>]

>                                                                     
> On Fri, 2 Nov 2001, Stephan von Krawczynski wrote:                  
> >                                                                   
> > To clarify this one a bit:                                        
> > shrink_cache is thought to do what it says, it is given a number  
of                                                                    
> > pages it should somehow manage to free by shrinking the cache.    
What my                                                               
> > patch does is go after the _whole_ list to fulfill that.          
>                                                                     
> I would suggest a slight modification: make "max_mapped" grow as the
> priority goes up.                                                   
>                                                                     
> Right now max_mapped is fixed at "nr_pages*10".                     
>                                                                     
> You could have something like                                       
>                                                                     
> 	max_mapped = nr_pages * 60 / priority;                             
>                                                                     
> instead, which might also alleviate the problem with not even       
bothering to                                                          
> scan much of the inactive list simply because 99% of all pages are  
mapped.                                                               
>                                                                     
> That way you don't waste time on looking at the rest of the inactive
list                                                                  
> until you _need_ to.                                                
                                                                      
Ok. I re-checked the code and found out this approach cannot stand.   
the list scan _is_ already exited early when priority is low:         
                                                                      
        int max_scan = nr_inactive_pages / priority;                  
                                                                      
        while (--max_scan >= 0 && (entry = inactive_list.prev) !=     
&inactive_list) {                                                     
                                                                      
It will not make big sense to do it again in max_mapped.              
                                                                      
On the other hand I am also very sure, that refining:                 
                                                                      
        if (max_mapped==0)                                            
                swap_out(priority, gfp_mask, classzone);              
                                                                      
        return nr_pages;                                              
                                                                      
in the end to:                                                        
                                                                      
        if (max_mapped==0 && nr_pages>0)                              
                swap_out(priority, gfp_mask, classzone);              
                                                                      
        return nr_pages;                                              
                                                                      
is a good thing. We don't need swap_out if we gained all the pages    
requested, no matter if we _could_ do it or not.                      
                                                                      
Is there some performance difference in this approach, Lorenzo? I     
guess it should.                                                      
                                                                      
Regards,                                                              
Stephan