Valgrind can't run: out of memory?

One thing that it important to note here is that running a program with valgrind will require about twice as much memory as normal usage. This is because it keeps around a shadow copy of allocated memory.

The section on valgrind in this blog post about debugging tools was really helpful for me in understanding what it is up to underneath the hood:

To do this, the first thing memcheck does is to maintain a registry of all allocated memory. Every time a new chunk memory is allocated memcheck keeps track of it by remembering the returned pointer, the amount of memory allocated as well as the backtrace from which it has been allocated. Additionally, it adds some redzones around the allocated memory that cannot be allocated in order to easily detect out-of-buffer accesses.

Needless to say it will also catch every single deallocation in order to keep its registry up to date. The deallocation does not immediately remove the entry from the registry, it marks it as deallocated and remembers the deallocation backtrace. By putting deallocated memory in quarantine, it ensures that use-after-free accesses can be caught as such since that memory cannot be reused for other purpose too rapidly.