Memory use

I am attempting to determine the amount of memory which is being used by my 2 PostgreSQL instances (8.4.1 and 9.2.10).

OS Version: Red Hat Enterprise Linux Server release 5.10 (Tikanga) (Linux xxxxxxx 2.6.18-371.11.1.el5 #1 SMP Mon Jun 30 04:51:39 EDT 2014 x86_64 x86_64 x86_64 GNU/Linux)

When I use top, I have recently observed memory usage of up to 190 GB but I am being told by our Server support team that it never uses more than about 12 GB of computational memory (including memory used by a small Oracle instance). I have read on-line that because PostgreSQL uses shared memory that top (and pmap) tends to overstate the actual memory in use.

top - 09:47:30 up 330 days, 23:12, 1 user, load average: 2.79, 3.37, 3.14
Tasks: 1525 total, 3 running, 1520 sleeping, 0 stopped, 2 zombie
Cpu(s): 3.0%us, 0.3%sy, 0.0%ni, 96.4%id, 0.4%wa, 0.0%hi, 0.0%si, 0.0%st
Mem: 264114472k total, 193001540k used, 71112932k free, 497484k buffers
Swap: 16779852k total, 697304k used, 16082548k free, 179534676k cached

Using the information found in Table 17-2. PostgreSQL Shared Memory Usage (http://www.postgresql.org/docs/9.2/interactive/kernel-resources.html) I have calculated the following for the instances:

8.4.1 instance:
Usage Approximate shared memory bytes required (as of 8.3) megs Observed Observed (megs)
Connections (1800 + 270 * max_locks_per_transaction) * max_connections 332,316,000 316.92
Autovacuum workers (1800 + 270 * max_locks_per_transaction) * autovacuum_max_workers 3,323,160 3.17
Prepared transactions (770 + 270 * max_locks_per_transaction) * max_prepared_transactions - -
Shared disk buffers (block_size + 208) * shared_buffers 1,033,476,505,600 985,600.00 102,400.00 0.0977
WAL buffers (wal_block_size + 8) * wal_buffers 16,777,216.00 16.00 2,048.00 0.0020
Fixed space requirements 770 *1024 788,480 0.75
1,033,494,071,296 985,616.75

Variables
max_locks_per_transaction 4096
Max_connections 300
autovacuum_max_workers 3
max_prepared_transactions 0 0 means not used
block_size 1024
shared_buffers 838860800
wal_block_size 8192
wal_buffers 16777216
work_mem 16777216

9.2.10 instance:
Usage Approximate shared memory bytes required (as of 8.3) megs Observed Observed (megs)
Connections (1800 + 270 * max_locks_per_transaction) * max_connections 332,316,000 316.92
Autovacuum workers (1800 + 270 * max_locks_per_transaction) * autovacuum_max_workers 3,323,160 3.17
Prepared transactions (770 + 270 * max_locks_per_transaction) * max_prepared_transactions - -
Shared disk buffers (block_size + 208) * shared_buffers 2,645,699,854,336 2,523,136.00 262,144.00 0.2500
WAL buffers (wal_block_size + 8) * wal_buffers 16,777,216.00 16.00 2,048.00 0.0020
Fixed space requirements 770 *1024 788,480 0.75
2,645,717,420,032 2,523,152.75

Variables
max_locks_per_transaction 4096
Max_connections 300
autovacuum_max_workers 3
max_prepared_transactions 0 0 means not used
block_size 1024
shared_buffers 2147483648
wal_block_size 8192 4.1015625
wal_buffers 16777216
work_mem 67108864

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The observed values above for Shared disk buffers and WAL Buffers were captured using the "Current Value" in the Backend Configuration Editor of the pgAdmin III (v1.18) tool. They are a point in time value.

I question the calculation for Shared disk buffers as it is way too high. I saw another post stating the calculation could be off by the block size. If that is the case, the displayed values above would be divided by 1024. In the postgresql.conf files, the shared_buffer values were entered as 800MB and 2048MB.

The small Oracle mentioned above has the following SGA:
Fixed Size 2,254,952
Variable Size 1,392,510,872
Database Buffers 704,643,072
Redo Buffers 38,477,824

Bottom line, does it seem reasonable based upon the above that the server could only be using around 12 GB of memory? Is there another way to determine the actual memory being used by the instances?
Thank you,

Tim Hearne

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