How to calculate shared memory size

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How to calculate shared memory size

Rébeli-Szabó Tamás
Hello all,

I want to do a proper calculation of the number of huge pages needed for
PostgreSQL to start.

To do that, I need a good approximation of how much shared memory will
be requested at server startup.

Looking at the shared memory calculation in the source (ipci.c), I can
see that two terms stand out:

   size = add_size(size, BufferShmemSize()); ---> shared_buffers
   size = add_size(size, XLOGShmemSize()); --> 1/32 * shared_buffers, but
max WAL segment size

If I use only those two terms ( shared buffers + WAL segment size) for
my calculation, the result will be off by a lot, especially with bigger
shared buffers (e.g. 32 GB).

What are any other major addends (in the order of 10 megabytes or more)
that I need to take into account to get a better result?

Regards,

tamas