From: | Rébeli-Szabó Tamás <pub(at)rblst(dot)info> |
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To: | pgsql-general(at)lists(dot)postgresql(dot)org |
Subject: | How to calculate shared memory size |
Date: | 2020-09-15 14:15:14 |
Message-ID: | 12b98406e50a52930bb96c8f76b73e1d@rblst.info |
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Lists: | pgsql-general |
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
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