Can't allocate memory

Here is the error code:

RuntimeError: [enforce fail at CPUAllocator.cpp:64] . DefaultCPUAllocator: can't allocate memory: you tried to allocate 452984832 bytes. Error code 12 (Cannot allocate memory)

I have seen the post

And I have checked my RAM by free -h as shown below:

              total        used        free      shared  buff/cache   available

Mem:           6.8G        1.2G        5.0G         71M        533M        5.3G
Swap:          2.0G        1.9G        119M

What’s wrong?452984832 bytes seems not to be very large space?