Re: Multithreaded /sbin.init? Is it possible?

[Jan-Benedict Glaw <jbglaw@lug-owl.de>]

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On Sun, 2003-09-14 12:40:55 -0500, Eric <eric@cisu.net>
wrote in message <200309141240.55800.eric@cisu.net>:
> Hello,
> 	Wouldn't the Linux Boot process be speeded up if the init program was 
> multithreaded? I know a little about threads.. and I think there might be 

Not really if they *run*, but if they sleep...

> 	 Each group would only take as long and the slowest script instead of the sum 
> of the execution time of the scripts. The init would start with the S1 
> scripts and maybe fork 3 times, wait for each to finish...., then fork maybe 
> twice for the S2 group if there are two scripts, wait for them to 
> finish...etc....

Well, if each group would only take to run as long as it's longest
running member, you'd need the CPU power to run them in parallel.
Threads (or multiple, concurrently running /etc/initd./ scripts) would
need several processors to really run in parallel. If you only have one
CPU, all processes/threads would get time slices to run in.

That is, in theory, their behaviour for *running*. However, init scripts
don't "run" all the time. Some of them actually sleep. Sleeping can even
be parallelized in a good manner with less CPUs than processes, so you
may gain a little speed, but from sleeping-in-parallel, not from
running-in-parallel. On the other hand, you hit the system with greater
memory consumption which, for low-RAM machines, may force you into swap,
which will really make you slow...

> 	Is this even possible?

Well, threads (on their own) are a great thing iff

	- you've got many (identical) tasks to _do_ in parallel while
	  having about the same number of CPUs.
	- you need to wait for rare events (ie. read from serial
	  interfaces and the like).

Threads in their own don't speed a well-written program up. They even
have the capacity to slow you down (by requiring unneccessary task
switches).

However, I've done a little test with my laptop (P2-266, 128MB RAM).
Groupwise parallelized, taking rc2 (not rcS) takes 5sec, vs. 20sec for
the non-parallelized traversal. But screen output now is even less
readable than before (Ever seen HP-UX starting?)...

MfG, JBG

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