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# Carrier Migration

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## Introduction

The ERTS memory allocators manage memory blocks in two types of raw memory chunks. We call these chunks of raw memory carriers. Single-block carriers which only contain one large block, and multi-block carriers which contain multiple blocks. A carrier is typically created using `mmap()` on unix systems. However, how a carrier is created is of minor importance. An allocator instance typically manages a mixture of single- and multi-block carriers.

## Problem

When a carrier is empty, i.e. contains only one large free block, it is deallocated. Since multi-block carriers can contain both allocated blocks and free blocks at the same time, an allocator instance might be stuck with a large amount of poorly utilized carriers if the memory load decreases. After a peak in memory usage it is expected that not all memory can be returned since the blocks still allocated are likely to be dispersed over multiple carriers. Such poorly utilized carriers can usually be reused if the memory load increases again. However, since each scheduler thread manages its own set of allocator instances, and memory load is not necessarily correlated to CPU load, we might get into a situation where there are lots of poorly utilized multi-block carriers on some allocator instances while we need to allocate new multi-block carriers on other allocator instances. In scenarios like this, the demand for multi-block carriers in the system might increase at the same time as the actual memory demand in the system has decreased which is both unwanted and quite unexpected for the end user.

## Solution

In order to prevent scenarios like this we've implemented support for migration of multi-block carriers between allocator instances.

### Management of Free Blocks

In order to be able to remove a carrier from one allocator instance and add it to another we need to be able to move references to the free blocks of the carrier between the allocator instances. The allocator instance specific data structure referring to the free blocks it manages often refers to the same carrier from multiple places. For example, when the address order best-fit strategy is used this data structure is a binary search tree spanning all carriers that the allocator instance manages. Free blocks in one specific carrier can be referred to from potentially every other carrier that is managed, and the amount of such references can be huge. That is, the work of removing the free blocks of such a carrier from the search tree will be huge. One way of solving this could be not to migrate carriers that contain lots of free blocks, but this would prevent us from migrating carriers that potentially need to be migrated in order to solve the problem we set out to solve.

By using one data structure of free blocks in each carrier and an allocator instance-wide data structure of carriers managed by the allocator instance, the work needed in order to remove and add carriers can be kept to a minimum. When migration of carriers is enabled on a specific allocator type, we require that an allocation strategy with such an implementation is used. Currently we've implemented this for three different allocation strategies. All of these strategies use a search tree of carriers sorted so that we can find the carrier with the lowest address that can satisfy the request. Internally in carriers we use yet another search tree that either implement address order first fit, address order best fit, or best fit. The abbreviations used for these different allocation strategies are `aoff`, and `aoffcaobf`, `aoffcbf`.

### Carrier Pool

In order to migrate carriers between allocator instances we move them through a pool of carriers. In order for a carrier migration to complete, one scheduler needs to move the carrier into the pool, and another scheduler needs to take the carrier out of the pool.

The pool is implemented as a lock-free, circular, double linked, list. The list contains a sentinel which is used as the starting point when inserting to, or fetching from, the pool. Carriers in the pool are elements in this list.

The list can be modified by all scheduler threads simultaneously. During modifications the double linked list is allowed to get a bit "out of shape". For example, following the `next` pointer to the next element and then following the `prev` pointer does not always take you back to were you started. The following is however always true:

- Repeatedly following `next` pointers will eventually take you to the sentinel.
- Repeatedly following `prev` pointers will eventually take you to the sentinel.
- Following a `next` or a `prev` pointer will take you to either an element in the pool, or an element that used to be in the pool.

When inserting a new element we search for a place to insert the element by only following `next` pointers, and we always begin by skipping the first element encountered. When trying to fetch an element we do the same thing, but instead only follow `prev` pointers.

By going different directions when inserting and fetching, we avoid contention between threads inserting and threads fetching as much as possible. By skipping one element when we begin searching, we preserve the sentinel unmodified as much as possible. This is beneficial since all search operations need to read the content of the sentinel. If we were to modify the sentinel, the cache line containing the sentinel would unnecessarily be bounced between processors.

The `prev` and `next` fields in the elements of the list contain the value of the pointer, a modification marker, and a deleted marker. Memory operations on these fields are done using atomic memory operations. When a thread has set the modification marker in a field, no-one except the thread that set the marker is allowed to modify the field. If multiple modification markers need to be set, we always begin with `next` fields followed by `prev` fields in the order following the actual pointers. This guarantees that no deadlocks will occur.

When a carrier is being removed from a pool, we mark it with a thread progress value that needs to be reached before we are allowed to modify the `next` and `prev` fields. That is, until we reach this thread progress we are not allowed to insert the carrier into the pool again, and we are not allowed to deallocate the carrier. This ensures that threads inspecting the pool always will be able to traverse the pool and reach valid elements. Once we have reached the thread progress value that the carrier was tagged with, we know that no threads may have references to it via the pool.

### Migration

Each allocator instance keeps track of the current utilization of its multi-block carriers. When the total utilization falls below the "abandon carrier utilization limit" it starts to inspect the utilization of the current carrier when deallocations are made. If also the utilization of the carrier falls below the "abandon carrier utilization limit" it unlinks the carrier from its data structure of available free blocks and inserts the carrier into the pool.

Since the carrier has been unlinked from the data structure of available free blocks, no more allocations will be made in the carrier.

The allocator instance that created a carrier is called its owner. Ownership never changes.

The allocator instance that has the responsibility to perform deallocations in a carrier is called its employer. The employer may also perform allocations if the carrier is not in the pool. Employment may change when a carrier is fetched from or inserted into the pool.

Deallocations in a carrier, while it remains in the pool, is always performed the owner. That is, all pooled carriers are employed by their owners.

Each carrier has an atomic word containing a pointer to the employing allocator instance and three bit flags; IN_POOL, BUSY and HOMECOMING.

When fetching a carrier from the pool, employment may change and further deallocations in the carrier will be redirected to the new employer using the delayed dealloc functionality.

When a foreign allocator instance abandons a carrier back into the pool, it will also pass it back to its owner using the delayed dealloc queue. When doing this it will set the HOMECOMING bit flag to mark it as "enqueued". The owner will later clear the HOMECOMING bit when the carrier is dequeued. This mechanism prevents a carrier from being enqueued again before it has been dequeued.

When a carrier becomes empty, it will be deallocated. Carrier deallocation is always done by the owner that allocated the carrier. By doing this, the underlying functionality of allocating and deallocating carriers can remain simple and doesn't have to bother about multiple threads. In a NUMA system we will also not mix carriers originating from multiple NUMA nodes.

If a carrier in the pool becomes empty, it will be withdrawn from the pool and be deallocated by the owner which already employs it.

If a carrier employed by a foreign allocator becomes empty, it will be passed back to the owner for deallocation using the delayed dealloc functionality.

In short:

- The allocator instance that created a carrier owns it.
- An empty carrier is always deallocated by its owner.
- Ownership never changes.
- The allocator instance that uses a carrier employs it.
- An employer can abandon a carrier into the pool.
- Pooled carriers are not allocated from.
- Pooled carriers are always employed by their owner.
- Employment can only change from owner to a foreign allocator when a carrier is fetched from the pool.

### Searching the pool

When an allocator instance needs more carrier space, it inspects the pool. If no carrier could be fetched from the pool, it will allocate a new carrier. Regardless of where the allocator instance gets the carrier from, it just links in the carrier into its data structure of free blocks.

To harbor real time characteristics, searching the pool is limited. We only inspect a limited number of carriers. If none of those carriers had a free block large enough to satisfy the allocation request, the search will fail. A carrier in the pool can also be BUSY if another thread is currently doing block deallocation work on the carrier. A BUSY carrier will also be skipped by the search as it cannot satisfy the request. The pool is lock-free and we do not want to block, waiting for the other thread to finish.

### The bad cluster problem

Before OTP-17.4 the search algorithm had a problem as the search always started at the same position in the pool, the sentinel. This could lead to contention from concurrent searching processes. But even worse, it could lead to a "bad" state when searches fail with a high rate leading to new carriers instead being allocated. These new carriers may later be inserted into the pool due to bad utilization. If the frequency of insertions into the pool is higher than successful fetching from the pool, memory will eventually get exhausted.

This "bad" state consists of a cluster of small and/or highly fragmented carriers located at the sentinel in the pool. The largest free block in such a "bad" carrier is rather small, making it unable to satisfy most allocation requests. As the search always started at the sentinel, any such "bad" carriers that had been left in the pool would eventually cluster together at the sentinel. All searches first have to skip past this cluster of "bad" carriers to reach a "good" carrier. When the cluster gets to the same size as the search limit, all searches will essentially fail.

To counter the "bad cluster" problem and also ease the contention, the search will now always start by first looking at the allocators own carriers. That is, carriers that were initially created by the allocator itself and later had been abandoned to the pool. If none of our own abandoned carrier would do, then the search continues into the pool, as before, to look for carriers created by other allocators. However, if we have at least one abandoned carrier of our own that could not satisfy the request, we can use that as entry point into the pool.

The result is that we prefer carriers created by the thread itself, which is good for NUMA performance. And we get more entry points when searching the pool, which will ease contention and clustering.

### Our own pooled tree

To do the first search among own carriers, every allocator instance has a `pooled_tree` of carriers. This tree is only accessed by the allocator itself and can only contain its own carriers. When a carrier is abandoned and put in the pool, it is also inserted into `pooled_tree`. This is either done direct, if the carrier was already employed by its owner, or by first passing it back to the owner via the delayed dealloc queue.

When we search our `pooled_tree` and find a carrier that is no longer in the pool, we remove that carrier from `pooled_tree` and mark it as TRAITOR, as it is now employed by a foreign allocator. We will not find any carriers in `pooled_tree` that are marked as BUSY by other threads.

If no carrier in `pooled_tree` had a large enough free block, we search it again to find any carrier that may act as an entry point into the shared list of all pooled carriers. This in order to, if possible, avoid starting at the sentinel and thereby ease the "bad clustering" problem.

Furthermore, the search for own carriers that are scheduled for deallocation is done as the last search option. The idea is that it is better to reuse a poorly utilized carrier than to resurrect an empty carrier that was just about to be released back to the OS.

### Result

The use of this strategy of abandoning carriers with poor utilization and reusing them in allocator instances with an increased carrier demand is extremely effective and completely eliminates the problems that otherwise sometimes occurred when CPU load dropped while memory load did not.

When using the `aoffcaobf` or `aoff` strategies compared to `gf` or `bf`, we loose some performance since we get more modifications in the data structure of free blocks. This performance penalty is however reduced using the `aoffcbf` strategy. A trade off between memory consumption and performance is however inevitable, and it is up to the user to decide what is most important.

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