Linux shmmax and shmall - how to set correct unit?

Question

I have a server which has 16 GB memory.

Now I need to set my shmmax and shmall, because the server default is (checked with ipcs -l)

------ Messages Limits --------
max queues system wide = 32000
max size of message (bytes) = 8192
default max size of queue (bytes) = 16384
------ Shared Memory Limits --------
max number of segments = 4096
max seg size (kbytes) = 18014398509465599
max total shared memory (kbytes) = 18014398509465599
min seg size (bytes) = 1

------ Semaphore Limits --------
max number of arrays = 32000
max semaphores per array = 32000
max semaphores system wide = 1024000000
max ops per semop call = 500
semaphore max value = 32767

It seems terrible, shmall and shmmax is bigger than my 16 GB.

So I want to change the setting to

shmmax -> 16GB/4
shmall -> 16GB/2

But I can't be sure what unit I have set

shmmax --> 4420960256 
shmall --> 8620960256

But is the unit for my number? byte or KB? Because ipcs -l is showing KB....

echo "kernel.shmmax=4420960256" >> /etc/sysctl.conf
echo 4420960256> /proc/sys/kernel/shmmax
echo "kernel.shmall=8620960256" >> /etc/sysctl.conf
echo 8620960256> /proc/sys/kernel/shmall

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thanks for help, but the postgresql just crash and get killed by yesterday, it shows :

This error usually means that PostgreSQL's request for a shared memory segment exceeded available memory, swap space, or huge pages. To reduce the request size (currently 4420960256 bytes), reduce PostgreSQL's shared memory usage, perhaps by reducing shared_buffers or max_connections.

my setting =>

shared_buffers = 4GB
effective_cache_size = 12GB

Answer 1

Just use:

lsipc

On my Ubuntu 16.04 LTS I get:

RESOURCE DESCRIPTION                                    LIMIT USED  USE%
MSGMNI   Number of message queues                       32000    0 0.00%
MSGMAX   Max size of message (bytes)                     8192    -     -
MSGMNB   Default max size of queue (bytes)              16384    -     -
SHMMNI   Shared memory segments                          4096   20 0.49%
SHMALL   Shared memory pages                          2097152 4915 0.23%
SHMMAX   Max size of shared memory segment (bytes) 4294967296    -     -
SHMMIN   Min size of shared memory segment (bytes)          1    -     -
SEMMNI   Number of semaphore identifiers                  128    0 0.00%
SEMMNS   Total number of semaphores                     32000    0 0.00%
SEMMSL   Max semaphores per semaphore set.                250    -     -
SEMOPM   Max number of operations per semop(2)            100    -     -
SEMVMX   Semaphore max value                            32767    -     -

which clearly states the measure unit for the values I have specified in /etc/sysctl.conf. So for me SHMMAX is in bytes while SHMALL in pages (see getconf PAGE_SIZE).

Answer 2

Since PostgreSQL 9.3, PostgreSQL has been using anonymous shared memory rather than System V shared memory. So the rest of the answer only applies to older PostgreSQL releases.

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Just leave the setting the way it is – essentially, that means “unlimited” in your case. One less limit you could bang your head against!

The amount of shared memory allocated by PostgreSQL is fixed and mostly determined by shared_buffers. Just make sure you don't set that to exceed your RAM (4GB would be perfect), and there is no danger whatsoever.

For the record: experimentation on my system shows that the unit of kernel.shmmax is bytes, while the unit of kernel.shmall is memory pages (check getconf PAGESIZE).