Re: m68k 2.6.26-1 vs 2.4.30 comparison

[Lance Tagliapietra <lancetag@Luminet.net>]

On Wed, May 06, 2009 at 08:44:41AM +0200, Kolbj??rn Barmen wrote:
> On Sun, 3 May 2009, Lance Tagliapietra wrote:
> 
> > Observations: a). I 2.4.30 kernel compile was about 6 hours on this hardware (GCC 2.95.4).  The 2.6.29 took 4 days 
> > (GCC 4.1.2, Debian).  That was without the modules, too.  Now, it did select the config option for smallest code 
> > size, and perhaps that is not well supported for m68k and also added to the compile time. Make was done as nice -n 
> > 17 but the system is mostly idle, otherwise, but that is how I compile the 2.4.30.
> 
> Most of the times is spent "entering directory bla; : nothing to do here; leaving directory bla" - 2.6 is much bigger 
> in terms of number of directories/files to parse through, and on slow IO that certainly matters :)

Also, what seemed to be different is that this compile called a shell script for each file being compiled.  I'm not sure 
if that was generated by the Makefile at build time yet.  My other thought is that GCC 4.1.x has a larger footprint than 
GCC 2.95 as it seemed to swap more than under the 2.4.30 / GCC 2.95.x.

> 
> > b). My custom 2.4.30 kernel size is about 750K uncompressed. With setting the options to remove support for hardware 
> > that I don't have and features that I don't need, I still came up with a kernel of 2.7M.  The goal is to have the 
> > smallest footprint kernel possible.
> 
> My amiga kernel, which is not optimized for size, has ipv6 and lots of stuff,, is 2170192 bytes, stripped. I suspect 
> you have not stripped yours?

Point of clarification: I was changing the .config to remove support for hardware (don't even make as a module) and 
features that I don't need in the kernel.  Was there another method being referred to with the term 'stripped' above?

> 
> > c). The 2.6.26 kernel seems to want to keep more memory free and hit the swap much more than the 2.4.30 kernel 
> > according to vmstat.  Under 2.4.30 I see the free memory go as low as about 200K, and it will remain at that level 
> > as long as is necessary.  Under 2.6.26, the free memory stays at about 800K, and if it drops below that, it will 
> > come back to that level relatively quickly.
> 
> What does "sysctl vm.min_free_kbytes" say? Here it says "vm.min_free_kbytes = 1763"

500
> 
> > d). The real time clock came up on the worng month, going from 2.4.30 to 2.6.26 (or 28), March vs April, in this 
> > case.
> 
> Hm, this sounds familiar, allthough I cant pinpoint it.

Geert responded to this in a previous mail to the group, I have to get some information back to him.

> 
> > e). Is there an option which tells the kernel the minimum amount of free RAM to maintain as I describe in (c) above?  
> > RAM is relatively precious in my m68k environment, and having 500k being held in reserve seems a bit much?
> 
> I'd try with "sysctl -w vm.min_free_kbytes=500" and see if that helps. If it does, make it permanent by adding it to 
> /etc/sysctl.conf

Thanks so much for this suggestion!!!  I set the value to 200, and I so far have not seen the value (via vmstat) go 
below 200, lowest observed so far has been 244.  This might be a good suggestion for m68k to make permanent?

--Lance