free_pages_low=0 and "fork: Cannot allocate memory"

[furio ercolessi <furio spin it>]

I occasionally experience "fork: Cannot allocate memory" errors in 
situations where networking load is fairly high (I have rather
busy DNS, SMTP and www caching services on this box, which is
a LX 533 MHz with 512MB RAM), but there is still plenty of memory
available (~150MB, as reported by "free" after subtraction of
buffers/cache), and swap is untouched.

This is basically the same problem I had back in june
( http://www.lib.uaa.alaska.edu/axp-list/archive/1999-06/ ,
  thread "fork: Cannot allocate memory" ). With 2.2.7 I had it
every few days or hours; with 2.0.37 every few months, but is
now increasing as load grows and becoming annoying again.

Now I noticed with horror that
# cat /proc/sys/vm/freepages
536     0       804
that is (see /usr/src/linux/Documentation/memory-tuning.txt)
the free_pages_low parameter is zero ; I observe the same in
other alphalinux systems running 2.0.36.  
So this 0 is likely to be the reason of my errors:
	free_pages_low should probably be about double of min_free_pages,
say the docs.

Now I was wondering where this 0 is coming from, as the same file
also says:
	The values selected as boot defaults are the following:  For a
	machine with n>=8 Megabytes of memory, set min_free_pages = n*2,
	free_pages_low = n*3 and free_pages_high = n*4.  Machines with
	8 Megabytes or less behave as if they had 8 Megabytes.
so at boot time I should have had
1024	1536	2048
and so all three these parameters are falling down.  Why?
Anybody else seeing vm parameters falling down?  

furio ercolessi
Spin - Trieste