Re: NetBSD/vax current

[Anders Magnusson <ragge%ludd.ltu.se@localhost>]

[about NetBSD slowniness on vax]

For someone who is curious; some time ago (about 15 years) I noticed 
that NetBSD had become very slow on 11/750; unreasonable slow compared 
to other machines.

I started searching and found that quite much time were spent in 
hardclock.  This is an effective way to make a slow machine much more 
sluggish.

Here is a simple explanation for someone not a low-level hacker:

Hardclock runs each 10ms.  We can say that it takes 500 instructions to 
complete.  On a 1 MIPS machine we will spend 10000 instructions between 
each hardclock interrupt, of which 9500 will be for user programs 
(~95%).  On a 1GIPS machine we have 10000000 instructions, of which 
9999500 will be used for processes (~99.9%).

Suddenly something is added that makes hardclock take 5000 instructions 
instead.  On the 1MIPS machine we now only have 50% of the CPU left, but 
on the newer machine we still have 99.9% of the CPU left (== not 
noticeable).  Something like this can only be seen on a really slow 
machine, it's most likely not possible to find it even with fine-grain 
profiling on a modern PC.

Back then, it turned out to be some NTP and timekeeping using long long 
calculations that did eat up hardclock.   Just commenting out this made 
the machine just as slow as before :-)

-- Ragge