Troubleshoot - VMC and ICM dowm / IPC basics

It happened that after restart of SAP and underlying OS, I have VMC down. Not just Java part like usually (in that case just restart VMC in prod client - I could write some some very short article about this - on two-three lines), but whole VMC.
As newbie I was confused till I found out that I have down ICM on same application server and in log of it, I found out some more information. I missed this from start, because I forgot that ICM has to be running and checked after each restart on all SAP instances, not just central one  (yeah, I'm still junior).
And reason? Another of my wrong assumption. Even if you restart OS, IPC shared memory is not cleared. It actually saved to some meta file. So in my case, thanks to not cleared IPC, ICM does not have enough memory, fell down on start and VMC was unable to start.

Lessons learned?

- first part:
SMC are important on all application servers - not only central one (my bad memory from basic education).
VMC and ICM processes are using shared memory separately from rest of SAP AS, so they can fall or did not start because of full shared memory on OS.

- now second, more interesting part:
IPC (Inter-Program Communication) is way, how shared memory of OS is managed (I will not get to details now, this sentence is not completely correct, right). Important info is, that memory allocation is stored in /dev/mem and it's not cleared after restart of OS so during restart of SAP you need to free it always manually by submitting usual command:
cleanipc XX remove
(where XX is number of instance/s you need to clear)

if you want to be sure, all is cleared, you could check it by command:
ipcs -mS
or
ipcs -mS|grep -i <SID>adm

Not so precise information in blog - intro of #notsure hastag

Sometimes you will overlook some easy issue, because you will forgot or you are not so skilled. Like me. For that and for my reminder there will be this renewed blog and part of it. It will not be so accurate, because sometimes I can be one, who thinks "I get it", but opposite can be true. In that case i will correct that entry once I found, I'm wrong.

You can find such blog entries with hastag #notsure .
In such blog posts, some theories can be wrong, based on my experience, but not true. I will be more then glad if you will correct any "lie" in them in comments, so I can adjust that post.

But yes, you could argue even about other blog post and their missing info.