fix: set M_MMAP_THRESHOLD to prevent memory fragmentation

[mmstick]

I read some articles about how a default behavior of glibc malloc causes some applications to consume large amounts of RAM until reaching OOM. There's a tunable parameter called M_MMAP_THRESHOLD (or the MALLOC_MMAP_THRESHOLD_ environment variable) which defines when an allocation will use mmap instead of sbrk.

Small sbrk buffers are stored by malloc in arenas for reuse by the application. mmap buffers are released by the OS instantly on drop. The problem is that the default behavior of malloc is to dynamically increase the threshold if this tunable is not set, which leads to some applications storing lots of randomly-sized gigantic buffers in its arenas, which may never be freed.

This lowered memory usage of cosmic-bg on my system from 96 MB to 25 MB.

[kskarthik]

This lowered memory usage of cosmic-bg on my system from 96 MB to 25 MB.

wow! that's a drastic amount