[Buildroot] gunzip slows booting

[Mike Sander]

Hi All,

I'm building a 2.6.25 kernel and initramfs for an atmel at91sam9260ek 
using buildroot.  I'm not sure if I'm experiencing a buildroot problem 
per se, but hopefully someone here might have some insight.

With the default uImage (uses compressed zImage) and default compressed 
initramfs I have a boot time from exiting u-boot to my user space 
application running (started from iniittab) on the order of 3.0 sec.

If I generate the uImage from Image (uncompressed kernel image),  my 
boot time drops to approx 2.0 sec.    As far as I can see, the only 
difference is that gunzip() is not called for the uncompressed kernel.  
It becomes a simple data copy.  I don't have the image sizes at hand, 
but gzip typically gives a 2:1 compression ratio.

Similarly, if I generate an uncompressed initramfs ( I simply removed 
the gzip from gen_initramfs_list.sh) my boot time further drops to 1.2 
sec.   The filesystem is ~600k compressed and 1.3 MB uncompressed.    
Again, the unzip is replaced by a data copy operation.

I guess the first question is whether there is a problem or not with 
gzip appearing to be slow?   I had expected the compressed kernel & 
initramfs to have quicker boot times than uncompressed.  I suspect the 
default kernel boot may be optimized for relatively slow disk copy and 
very fast processors [typical desktop].    In such a configuration a 
long unzip operation probably is faster than coping a bigger image from 
a slow disk.

Assuming there is a problem here, I have started to debug this.  
basically gunzip() calls inflate().   After a few more calls we get to 
inflate_codes() which ultimate ends in a call to memcpy().  What I have 
observed and find very strange is that memcpy() is being used to 
repeatedly copy a very small number of bytes (typically 1 to 4).  
Occasionally it is called to copy a larger # of bytes.  There are 
literally  tens (or hundreds) of thousands of memcpy to move a few MB 
data.   I suspect this is where the vast majority of the time is being 
spent.   

Is it possible that I am seeing an architecture specific problem?


Any comments/suggestions are welcome.

Mike