Re: swapping and the value of /proc/sys/vm/swappiness

[Con Kolivas <kernel@kolivas.org>]

Marcelo Tosatti wrote:
> On Wed, Sep 15, 2004 at 08:53:21AM +1000, Con Kolivas wrote:
> 
>>Marcelo Tosatti wrote:
>>
>>>On Tue, Sep 14, 2004 at 11:31:53AM -0700, Florin Andrei wrote:
>>>
>>>
>>>>On Mon, 2004-09-06 at 16:27, Andrew Morton wrote:
>>>>
>>>>
>>>>>Con Kolivas <kernel@kolivas.org> wrote:
>>>>
>>>>>>The change was not deliberate but there have been some other people 
>>>>>>report significant changes in the swappiness behaviour as well (see 
>>>>>>archives). It has usually been of the increased swapping variety 
>>>>>>lately. It has been annoying enough to the bleeding edge desktop users 
>>>>>>for a swag of out-of-tree hacks to start appearing (like mine).
>>>>>
>>>>>All of which is largely wasted effort.
>>>>
>>>>>From a highly-theoretical, ivory-tower perspective, maybe; i am not the
>>>>one to pass judgement.
>>>>>From a realistic, "fix it 'cause it's performing worse than MSDOS
>>>>without a disk cache" perspective, definitely not true.
>>>>
>>>>I've found a situation where the vanilla kernel has a behaviour that
>>>>makes no sense:
>>>>
>>>>http://marc.theaimsgroup.com/?l=linux-kernel&m=109237941331221&w=2
>>>>http://marc.theaimsgroup.com/?l=linux-kernel&m=109237959719868&w=2
>>>>http://marc.theaimsgroup.com/?l=linux-kernel&m=109238126314192&w=2
>>>>
>>>>A patch by Con Kolivas fixed it:
>>>>
>>>>http://marc.theaimsgroup.com/?l=linux-kernel&m=109410526607990&w=2
>>>>
>>>>I cannot offer more details, i have no time for experiments, i just need
>>>>a system that works. The vanilla kernel does not.
>>>
>>>
>>>Have you tried to decrease the value of /proc/sys/vm/swappiness 
>>>to say 30 and see what you get?
>>>
>>>Andrew's point is that we should identify the problem - Con's patch
>>>rewrites swapping policy.  
>>
>>I already answered this. That hard swappiness patch does not really 
>>rewrite swapping policy. It identifies exactly what has changed because 
>>it does not count "distress in the swap tendency". Therefore if the 
>>swappiness value is the same, the mapped ratio is the same (in the 
>>workload) yet the vm is swappinig more, it is getting into more 
>>"distress". The mapped ratio is the same but the "distress" is for some 
>>reason much higher in later kernels, meaning the priority of our 
>>scanning is getting more and more intense. This should help direct your 
>>searches.
> 
> 
>>These are the relevant lines of code _from mainline_:
>>
>>distress = 100 >> zone->prev_priority
>>mapped_ratio = (sc->nr_mapped * 100) / total_memory;
>>swap_tendency = mapped_ratio / 2 + distress + vm_swappiness
>>if (swap_tendency >= 100)
>>-		reclaim_mapped = 1;
>>
>>
>>That hard swappiness patch effectively made "distress == 0" always.
> 
> So isnt it true that decreasing vm_swappiness should compensate 
> distress and have the same effect of your patch? 

Nope. We swap large amounts with the wrong workload at swappiness==0 
where we wouldn't before at swappiness==60. ie there is no workaround 
possible without changing the code in some way.

> To be fair I'm just arguing, haven't really looked at the code.

Thats cool ;)

Cheers,
Con