Demo: What malloc() Asks the Kernel For

Ten malloc() calls of a given size, under strace.

Run

make
strace -e trace=brk,mmap,munmap ./malloc_kb 100
strace -e trace=brk,mmap,munmap ./malloc_kb 1000

Ask

• Ten malloc()s, six brk()s. What is the allocator doing with the extra?
• The last brk() goes down. What did the program do to cause that?
• At 1000 KB every malloc() is an mmap(). Why is a big block not worth keeping in the heap?
• Where have you seen "libc decides when to cross" before?