Implement malloc which is backed by a disk file (dmalloc)

Question

General malloc and mmap description

malloc (or any allocation function) is supposed to allocate memory for applications. Standard glibc malloc implementation uses sbrk() system call to allocate the memory. The memory allocated to an application is not backup by disk. Only when the application is swept out, the contents of memory are moved to disk (pre-configured swap disk).

The other method to allocate the memory is through the use of mmap. mmap system call creates mapping in virtual address space for calling process. Following is mmap function declaration as per POSIX standard.

void *mmap(void *addr, size_t length, int prot, int flags, int fd, off_t offset);

/* Describe the meaning for few important parameters to mmap */

mmap system call can also be used to allocate memory. Typically this is used to load, application binaries or static libraries. For example following mmap call will allocate memory, without a backing file.

address = __mmap (0, length, PROT_READ|PROT_WRITE,MAP_ANONYMOUS|MAP_PRIVATE, -1, 0);

Flags MAP_ANONYMOUS: The mapping is not backed by any file; its contents are initialized to zero.

MAP_PRIVATE: Create a private copy-on-write mapping. Updates to the mapping are not visible to other processes mapping the same file, and are not carried through to the underlying file.

dmalloc dmalloc is a new API which allocates memory using a disk backed file i.e. without MAP_ANONYMOUS and MAP_PRIVATE to mmap. The dmalloc would be particularly useful with SSDs, which has very low read/write latency as compared to HDD. Since the file is mapped into the RAM, the dlmalloc will also benefit from high speed RAM.

Alternatives

A SSD can also be configured as a highest priority swap device, however this approach suffers from HDD optimized swapping algorithm inside Linux kernel. The swapping algorithm tries to cluster application pages on swap. When a data from swap is needed it reads the complete cluster (read-ahead). If an application is doing random IOs, the read-ahead data would cause unnecessary IOs to disk.

Question:-

1. what is ment by "allocates memory using a disk backed file i.e. without MAP_ANONYMOUS and MAP_PRIVATE to mmap." which flag i should use apart from those two.
2. how i creat on-write backup of memory allocated to an application.

Answer 1

I never heard from dmalloc but like you mention it it looks like a mix between malloc (pure memory allocation) and mmap (direct mapping of memory to disk). dmalloc seems to allocate memory backed by disk, but more performant than mmap on slow disks (e.g. SSD). I could imagine that it groups write operations before actually flushing the writes to disk, whereas mmap is more or less a "virtual memory window" on a disk file.

As for your questions.

1) MAP_ANONYMOUS and MAP_PRIVATE are flags for use with mmap. The fact that these flags are mentioned as not being used, makes me think dmalloc is a fresh implementation and has no relationship to mmap.

2) dmalloc seems to be suited for what you say: it "backs up" memory to disk, similar to mmap. You may need to read the details of the documentation to know when exactly you have guarantee that the data is effectively on disk (caching,...)