[COFF] Re: Terminology query - 'system process'?

[Bakul Shah <bakul@iitbombay.org>]

I remember calling them kernel processes as they had no code running in user mode. Not sure now of the year but sometime in ‘80s. Now I’d probably call them kernel threads as they don’t have a separate address space.

> On Dec 14, 2023, at 1:48 PM, jnc@mercury.lcs.mit.edu wrote:
> 
> So Lars Brinkhoff and I were chatting about daemons:
> 
>  https://gunkies.org/wiki/Talk:Daemon
> 
> and I pointed out that in addition to 'standard' daemons (e.g. the printer
> spooler daemon, email daemon, etc, etc) there are some other things that are
> daemon-like, but are fundamentally different in major ways (explained later
> below). I dubbed them 'system processes', but I'm wondering if ayone knows if
> there is a standard term for them? (Or, failing that, if they have a
> suggestion for a better name?)
> 
> 
> Early UNIX is one of the first systems to have one (process 0, the "scheduling (swapping)
> process"), but the CACM "The UNIX Time-Sharing System" paper:
> 
>  https://people.eecs.berkeley.edu/~brewer/cs262/unix.pdf
> 
> doesn't even mention it, so no guidance there. Berkeley UNIX also has one,
> mentioned in "Design and Implementation of the Berkeley Virtual Memory
> Extensions to the UNIX Operating System":
> 
>  http://roguelife.org/~fujita/COOKIES/HISTORY/3BSD/design.pdf
> 
> where it is called the "pageout daemon".("During system initialization, just
> before the init process is created, the bootstrapping code creates process 2
> which is known as the pageout daemon. It is this process that .. writ[es]
> back modified pages. The process leaves its normal dormant state upon being
> waken up due to the memory free list size dropping below an upper
> threshold.") However, I think there are good reasons to dis-favour the term
> 'daemon' for them.
> 
> 
> For one thing, typical daemons look (to the kernel) just like 'normal'
> processes: their object code is kept in a file, and is loaded into the
> daemon's process when it starts, using the same mechanism that 'normal'
> processes use for loading their code; daemons are often started long after
> the kernel itself is started, and there is usually not a special mechanism in
> the kernel to start daemons (on early UNIXes, /etc/rc is run by the 'init'
> process, not the kernel); daemons interact with the kernel through system
> calls, just like 'ordinary' processes; the daemon's process runs in 'user'
> CPU mode (using the same standard memory mapping mechanisms, just like
> blah-blah).
> 
> 'System processes' do none of these things: their object code is linked into
> the monolithic kernel, and is thus loaded by the bootstrap; the kernel
> contains special provision for starting the system process, which start as
> the kernel is starting; they don't do system calls, just call kernel routines
> directly; they run in kernel mode, using the same memory mapping as the
> kernel itself; etc, etc.
> 
> Another important point is that system processes are highly intertwined with
> the operation of the kernel; without the system process(es) operating
> correctly, the operation of the system will quickly grind to a halt. The loss
> of ordinary' daemons is usually not fatal; if the email daemon dies, the
> system will keep running indefinitely. Not so, for the swapping process, or
> the pageout daemon
> 
> 
> Anyway, is there a standard term for these things? If not, a better name than
> 'system process'?
> 
>    Noel