内存不足什么原因

内存不足什么原因
mmap() failed: [12] Cannot allocate memory
PHP Fatal error: Out of memory (allocated 279924736) (tried to allocate 4664036 bytes) in phar:///usr/local/bin/composer/src/Composer/Util/RemoteFilesystem.php on line 451

Fatal error: Out of memory (allocated 279924736) (tried to allocate 4664036 byte s) in phar:///usr/local/bin/composer/src/Composer/Util/RemoteFilesystem.php on l ine 451