Question

I read the document Understanding Virtual Memory and it said one method for changing tunable parameters in the Linux VM was the command:

sysctl -w vm.max_map_count=65535

I want to know what the number 65535 means and how much memory could vm use by the setting.

Answer 1

From the Linux kernel documentation:

max_map_count:

This file contains the maximum number of memory map areas a process
may have. Memory map areas are used as a side-effect of calling
malloc, directly by mmap and mprotect, and also when loading shared
libraries.

While most applications need less than a thousand maps, certain
programs, particularly malloc debuggers, may consume lots of them,
e.g., up to one or two maps per allocation.

The default value is 65536.

Bottom line: this setting limits the number of discrete mapped memory areas – on its own it imposes no limit on the size of those areas or on the memory that is usable by a process.

And yes, this:

sysctl -w vm.max_map_count=65535

is just a nicer way of writing this:

echo 65535 > /proc/sys/vm/max_map_count