Re: [2.6 patch] i386: always use 4k stacks

[Adrian Bunk <bunk@stusta.de>]

On Sat, Dec 17, 2005 at 06:44:07PM +0100, Andi Kleen wrote:
> Kyle Moffett <mrmacman_g4@mac.com> writes:
> 
> > On Dec 16, 2005, at 10:35, Diego Calleja wrote:
> > > I know, but there's too much resistance to the "pure" 4kb patch.
> > 
> > I have yet to see any resistance to the 4Kb patch this time around
> > that was not "*whine* don't break my ndiswrapper plz".  
> 
> My comment from last time about the missing safety net still applies 100%.
> 
> Kernel code is getting more complex all the time and running with
> very tight stack is just risky.

My patch reduces it from roughly 6kB to 4kB.

I'm with you that we need a safety net, but I don't see a problem with 
this being between 3kB and 4kB. The goal should be to _never_ use more 
than 3kB stack having a 1kB safety net.

And in my experience, many stack problems don't come from code getting 
more complex but from people allocating 1kB structs or arrays of
> 2k chars on the stack. In these cases, the code has to be fixed and 
"make checkstack" makes it easy to find such cases.

And as a data point, my count of bug reports for problems with in-kernel 
code with 4k stacks after Neil's patch went into -mm is still at 0.

> > The point is to force it in -mm so most people can't just disable it
> > because it fixes their problem.  We want 8k stacks to go away, and
> 
> Who is we? And why? 
> 
> About the only half way credible arguments I've seen for it were:  
> 
> - "it might reduce stalls in the VM with order 1". Didn't quite
> convince me because there were no numbers presented and at least on
> x86-64 I've never noticed or got reported significant stalls because
> of this.
> 
> - "it allows more threads for 32bit which might run out of lowmem" - i
> think everybody agrees that the 10k threads case is not really
> something to encourage. And even when you want to add it then only a factor
> two increase (which this patch brings) is not really too helpful.
>...

Unfortunately, "is not really something to encourage" doesn'a make 
"happens in real-life applications" impossible...

Reducing the stack by one third brings a factor two reduction in the 
memory usage of threads - I wouldn't say this sounds too bad.

> -Andi

cu
Adrian

-- 

       "Is there not promise of rain?" Ling Tan asked suddenly out
        of the darkness. There had been need of rain for many days.
       "Only a promise," Lao Er said.
                                       Pearl S. Buck - Dragon Seed

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