How to i calculate the size of shared memory available to the system

Very early UNIX systems did not have MMUs, and so effectively, all memory in the system was shared between all processes in memory. UNIX V7 was the first one that had memory management, AFAIK. The PDP-11 did not even have a MMU when it was released; see this PDF book, page 35.

As time moved forward and MMUs became a commonplace thing, UNIX began to require it. And then memory could be separated between processes. In the 1980s we saw more IPC mechanisms, including shared memory managed by the OS (which was new in SVR1, circa 1983). SVR1 also introduced messages and semaphors, and the System V APIs are still available on modern systems for all three of these things.

I just went ahead and see what happens, when you write too many files to /dev/shm.

OSError: [Errno 28] No space left on device

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/mnt/fastssd/bot_subreddit_recom/write_user_docs.py", line 85, in <module>
    f.write(subreddits)
OSError: [Errno 28] No space left on device
inFile: RC_2018-01-02
outDir: tmp
outputToScreenOnly: 0
OSError: [Errno 28] No space left on device

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/mnt/fastssd/bot_subreddit_recom/write_user_docs.py", line 85, in <module>
    f.write(subreddits)
OSError: [Errno 28] No space left on device

It “ran out of space” after 33 days of input files, as shown above.

>>> 36.*33/12
99.0

So an estimated 99GB -- which is very close to 100GB -- of files, is where the “no space left” error happened. This is exactly the size of the /dev/shm device which I had set to be 100G. This means swap was not really able to be used when it was needed. This has demonstrated that unfortunately, /dev/shm simply runs out, and does not let you start to use swap as virtual memory to keep going, when there’s a shortage. So now we know thanks to this experiment. You just need real physical RAM for /dev/shm to work, and virtual RAM aka swap is not helpful for /dev/shm at all.

I have posted this answer to share with others who may benefit from the information gained.