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# Super Carrier

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A super carrier is large memory area, allocated at VM start, which can be used during runtime to allocate normal carriers from.

The super carrier feature was introduced in OTP R16B03. It is enabled with command line option `+MMscs <size in Mb>` and can be configured with other options.

## Problem

The initial motivation for this feature was customers asking for a way to pre-allocate physical memory at VM start for it to use.

Other problems were different experienced limitations of the OS implementation of mmap:

- Increasingly bad performance of mmap/munmap as the number of mmap'ed areas grow.
- Fragmentation problem between mmap'ed areas.

A third problem was management of low memory in the halfword emulator. The implementation used a naive linear search structure to hold free segments which would lead to poor performance when fragmentation increased.

## Solution

Allocate one large continuous area of address space at VM start and then use that area to satisfy our dynamic memory need during runtime. In other words: implement our own mmap.

### Use cases

If command line option +MMscrpm (Reserve Physical Memory) is set to false, only virtual space is allocated for the super carrier from start. The super carrier then acts as an "alternative mmap" implementation without changing the consumption of physical memory pages. Physical pages will be reserved on demand when an allocation is done from the super carrier and be unreserved when the memory is released back to the super carrier.

If +MMscrpm is set to true, which is default, the initial allocation will reserve physical memory for the entire super carrier. This can be used by users that want to ensure a certain minimum amount of physical memory for the VM.

However, what reservation of physical memory actually means highly depends on the operating system, and how it is configured. For example, different memory overcommit settings on Linux drastically change the behaviour.

A third feature is to have the super carrier limit the maximum amount of memory used by the VM. If +MMsco (Super Carrier Only) is set to true, which is default, allocations will only be done from the super carrier. When the super carrier gets full, the VM will fail due to out of memory. If +MMsco is false, allocations will use mmap directly if the super carrier is full.

### Implementation

The entire super carrier implementation is kept in erl_mmap.c. The name suggest that it can be viewed as our own mmap implementation.

A super carrier needs to satisfy two slightly different kinds of allocation requests; multi block carriers (MBC) and single block carriers (SBC). They are both rather large blocks of continuous memory, but MBCs and SBCs have different demands on alignment and size.

SBCs can have arbitrary size and do only need minimum 8-byte alignment.

MBCs are more restricted. They can only have a number of fixed sizes that are powers of 2. The start address need to have a very large alignment (currently 256 kb, called "super alignment"). This is a design choice that allows very low overhead per allocated block in the MBC.

To reduce fragmentation within the super carrier, it is good to keep SBCs and MBCs apart. MBCs with their uniform alignment and sizes can be packed very efficiently together. SBCs without demand for alignment can also be allocated quite efficiently together. But mixing them can lead to a lot of memory wasted when we need to create large holes of padding to the next alignment limit.

The super carrier thus contains two areas. One area for MBCs growing from the bottom and up. And one area for SBCs growing from the top and down. Like a process with a heap and a stack growing towards each other.

### Data structures

The MBC area is called sa as in super aligned and the SBC area is called sua as in super un-aligned.

Note that the "super" in super alignment and the "super" in super carrier has nothing to do with each other. We could have chosen another naming to avoid confusion, such as "meta" carrier or "giant" alignment.

    +-------+ <---- sua.top
    |  sua  |
    |       |
    |-------| <---- sua.bot
    |       |
    |       |
    |       |
    |-------| <---- sa.top
    |       |
    |  sa   |
    |       |
    +-------+ <---- sa.bot

When a carrier is deallocated a free memory segment will be created inside the corresponding area, unless the carrier was at the very top (in `sa`) or bottom (in `sua`) in which case the area will just shrink down or up.

We need to keep track of all the free segments in order to reuse them for new carrier allocations. One initial idea was to use the same mechanism that is used to keep track of free blocks within MBCs (alloc_util and the different strategies). However, that would not be as straight forward as one can think and can also waste quite a lot of memory as it uses prepended block headers. The granularity of the super carrier is one memory page (usually 4kb). We want to allocate and free entire pages and we don't want to waste an entire page just to hold the block header of the following pages.

Instead we store the meta information about all the free segments in a dedicated area apart from the `sa` and `sua` areas. Every free segment is represented by a descriptor struct (`ErtsFreeSegDesc`).

    typedef struct {
        RBTNode snode;      /* node in 'stree' */
        RBTNode anode;      /* node in 'atree' */
        char* start;
        char* end;
    }ErtsFreeSegDesc;

To find the smallest free segment that will satisfy a carrier allocation (best fit), the free segments are organized in a tree sorted by size (`stree`). We search in this tree at allocation. If no free segment of sufficient size was found, the area (`sa` or `sua`) is instead expanded. If two or more free segments with equal size exist, the one at lowest address is chosen for `sa` and highest address for `sua`.

At carrier deallocation, we want to coalesce with any adjacent free segments, to form one large free segment. To do that, all free segments are also organized in a tree sorted in address order (`atree`).

So, in total we keep four trees of free descriptors for the super carrier; two for `sa` and two for `sua`. They all use the same red-black-tree implementation that support the different sorting orders used.

When allocating a new MBC we first search after a free segment in `sa`, then try to raise `sa.top`, and then as a fallback try to search after a free segment in `sua`. When an MBC is allocated in `sua`, a larger segment is allocated which is then trimmed to obtain the right alignment. Allocation search for an SBC is done in reverse order. When an SBC is allocated in `sa`, the size is aligned up to super aligned size.

### The free descriptor area

As mentioned above, the descriptors for the free segments are allocated in a separate area. This area has a constant configurable size (+MMscrfsd) that defaults to 65536 descriptors. This should be more than enough in most cases. If the descriptors area should fill up, new descriptor areas will be allocated first directly from the OS, and then from `sua` and `sa` in the super carrier, and lastly from the memory segment itself which is being deallocated. Allocating free descriptor areas from the super carrier is only a last resort, and should be avoided, as it creates fragmentation.

### Halfword emulator

The halfword emulator uses the super carrier implementation to manage its low memory mappings thar are needed for all term storage. The super carrier can here not be configured by command line options. One could imagine a second configurable instance of the super carrier used by high memory allocation, but that has not been implemented.

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