Re: why swap at all?

[Helge Hafting]

Buddy Lumpkin wrote:

> > Hi Buddy,
> > Even for systems that don't *need* the extra memory space, swap can
> > actually provide performance improvements by allowing unused memory
> > to be replaced with often-used memory.
> >    
>
>  
>
> > For example, I have 57MB swapped right now. It allows me to instantly
> > grep the kernel tree. If I turned swap off, each grep would probably
> > take 30 seconds.
> >    
>
> Your analogy is flawed. There are many reasons why this doesn't work in the
> real world.
>
> I don't think any modern and popular OS contains mechanisms that silently
> stage old pages to disk.
Linux is modern and popular . . .

> The constant twitching of the hard drive this
> causes for no apparent reason drives people insane 
Stupid people then. If they really expect the disk to work
only when they hit save or start up something.  Sheesh.

> and drains precious
> battery life on laptops. (see description for the pages_min, pages_low and
> pages_high watermarks for clarity)
>  
This is a valid concern. Laptop users may want to sacrifice performance
for battery life. Linux can be tweaked quite a bit for this, more
development is probably a good idea. We who use AC power don't
want a performance loss on our machines though, so any such tweaks
must be optional.


[...]

> One thing that can be done to minimize the problem where heavy filesystem
> I/O flushes important pages from memory like pages from shared libraries and
> executables only for them to fault back in as soon as they become runnable,
> is to implement something similar to what Sun implemented in Solaris 8
> called the cyclical page cache. The idea is that the pagecache pages against
> itself and is actually considered free memory from an anonymous memory
> perspective. The pagecache is free to grow all it wants, but since it is
> counted as free memory, anonymous memory allocation will cause the pagecache
> to shrink because it is considered free memory.
>  
Linux counts cache as free memory too, of course. 
Allocate memory, and cache will go away.
It has been like this for many years.

Helge Hafting
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