how can tmpfs larger than SHMALL parameter?

Question

The amount of Shared memory being utilized:

#> df -h /dev/shm Filesystem Size Used Avail Use% Mounted on tmpfs 125G 96G 30G 77% /dev/shm

The max amount of shared memory system-wide, set by the SHMALL kernel parameter:

ipcs -lm | grep shared max total shared memory (kbytes) = 67108864

Question: How is this possible? How can the max total shared memory be set to 64GB in the kernel, but df is reporting that 96G shared memory is being used?

Answer 1

The shmall parameter refers to traditional UNIX IPC shared memory. You can see the memory allocations of that using the command ipcs. The tmpfs temporary filesystem is a totally different subsystem used as a ram based, swap backed filesystem.

tmpfs is used as the basis to perform posix shared memory in fact, which is a different type of shared memory system to unix IPC.