Re: dynamic allocation of swap disk space

[Silviu Marin-Caea]

John Bradford wrote:
> Quote from Ray Bryant <raybry@xxxxxxx>:
>
> > Silviu Marin-Caea wrote:
> >
> > > My desktop has been thrashing the disk for a couple of hours because
> > > the swap space was exhausted.  And I have the ambition to leave it alone
> > > to see if it ever comes out of the thrashing.  Of course, it's not usable
> > > at all during this time, I'm writing this on the laptop.
> >
> > You've got a couple problems mixed together here.
> >
> > (1)  Swap space fills up because you have overcommitted memory.  In principle, 
> > filling up swap space has nothing to do with "thrashing".
> > (2)  "thrashing" is a characteristic of a poorly performing program in a
> > demand paging virtual memory system typically caused by trying to run a 
> > program with a resident size that is smaller than required.  Systems can 
> > thrash without filling up swap space.  It is true that systems can thrash AND 
> > fill up swap space, but it is not always so.
> >
> > You either need (1)  more main memory, (2) reduce the number of programs you 
> > are running simultaneously, or (3) better behaving programs, or all three. 
> > Increasing the amount of swap space will just use more disk.  It won't cause 
> > your system to stop thrashing since that is driven by what is going on in 
> > memory, not what is going on on the disk.
>
>
> Not necessarily.  Increasing swap can allow more physical RAM to be used for
> caching data from disk.
>
> Imagine a system with limited physical RAM, and limited swap space, running a
> process which causes a lot of filesystem activity on the same physical disk
> as is being used for swap.  If the total RAM, both physical and swap is almost
> completely full, increasing the swap space may allow some data from physical
> RAM to be swapped out, in favour of caching filesystem data from the disk.
>
> Without knowing more details of the original poster's machine, it's difficult
> to give specific advice about how to solve the problem.

1 GB RAM desktop machine, with 32 MB swap, kernel 2.6.3.  In normal 
operation, the swap is 80% free.  It started the thrashing when I loaded 
a HUGE page (generated from a database) in konqueror.

The issue is not to solve _my_ problem, I really don't care about it 
that much, the issue would be to solve this sort of problem for 
everyone.  I've had disk thrashing with filled up swap on a server 
(kernel 2.4) and that was much worse.  Anyway, I'll start using swapd 
and see what happens.  Some time in the future, I'll post here the 
conclusion.
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