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Re: Swappiness and Virtual Memory Management



On Wed, 2005-04-27 at 22:40 -0400, Peter Arremann wrote:
> On Wednesday 27 April 2005 22:31, Ow Mun Heng wrote:
> > I've got a question regarding VM management in Linux.
> >
> > I've got 512MB of RAM on my laptop and at times it can still be slow esp
> > when the RAM is used up and starts accessing the swap.
> >
> > Setting swappiness to 10 doesn't really make a difference at all.
> >
> > However, (what I've heard) users of FreeBSD has attested that hitting
> > swap on those BSD system does not make any noticeable effect on their
> > system. Even when there's up to like 700MB of swap in use.
> >
> > On Linux, it seems that the opposite is true, when we hit swap, and then
> > the disk starts to thrash violently and I get slow load-up of programs
> > and even switching between virtual Desktops can be a real pain.
> >

> I do some work where I can't ever get enough ram - My current system is about 
> 14GB in swap and I don't feel any real issue with that... 

Wow..

> 
> It mostly depends on your workload. If you run a lot of server tasks you 
> notice paging less then if you run interactive stuff. Worst is, if you try to 
> work with a huge picture in gimp - a few meg in swap can already drag you 
> down consierably... 
> 
> Also, you're running a laptop - laptop drives are horribly slow. We did the 
> same work on desktops and laptops - both with 2GB ram, 3.2GHz desktop P4 and 
> otherwise also almost the same - difference was the disk... our ram usage was 
> about 2.5GB and the resulting performance panelty was about 40%... 

Yes.. this is on a laptop with a 5400rpm drive. Though I know that it is
an issue, the friend running on FreeBSD on a thinkpad X40 doesn't see
this issue when getting > 700MB in swap. If I hit like 200MB in swap,
I'm already thrashing. Moving from Evo to Firefox is a wait as it swaps
in/out.

> Other than that, yes FreeBSD I can agree acts a little nicer when it comes to 
> swapping - but in all honesty, the difference is so little that you really 
> have to test the system and create the swapping on purpose and then time the 
> apps to notice any difference. 

How much that is true I can't tell but what I do know is the difference
is noticeable in the sense of the word. It just feels slower on Linux
rather than on BSD.

I've seen one other poster that mentioned it's cause linux pushes around
4k for every read of the swap and thus makes things slower.

How much that is true, again, I don't really know

> 
> Peter.
> 

-- 
Ow Mun Heng
Gentoo/Linux on DELL D600 1.4Ghz 
98% Microsoft(tm) Free!! 
Neuromancer 11:03:06 up 2 days, 1:43, 6 users, load average: 0.34, 0.52,
0.52 



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