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https://github.com/eapache/wof_alloc

Fast C/C++ memory allocator with free-all operation
https://github.com/eapache/wof_alloc

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Fast C/C++ memory allocator with free-all operation

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README 

        
Wheel-of-Fortune Memory Allocator

=================================



A little while ago, the [Wireshark Project](https://www.wireshark.org/) was in

need of a new memory management framework to replace its aging 'emem' framework.

Nothing available seemed to suit, so I (mostly) wrote one from scratch, called

[wmem](https://code.wireshark.org/review/gitweb?p=wireshark.git;a=blob;f=doc/README.wmem).



The majority of wmem was bog-standard memory-pool code, but there was one

component that ended up turning into a *very* interesting exercise in

algorithms and data structures. Within wmem it is simply known as the "block"

allocator, but it was interesting enough that I extracted it from Wireshark,

and made it available stand-alone here. The wheel-of-fortune name will become

obvious once you understand how the recycler structure works.



It is one C source file that uses nothing but bare-bones standard C90, so it

should compile on basically any platform in existence.



Behaviour

---------



For a proper understanding of how the allocator works, read the rest of this

README. In brief though, `wof_alloc` is constant-time unless it needs to grab a

new block from the OS, the cost of which can be amortized. `wof_free` is always

constant-time. `wof_realloc` is constant-time if it doesn't have to move the

block, and is of course just a `wof_alloc`, `memcpy`, `wof_free` if it does has to move the

block.



`wof_free_all` is *very* fast - the design was optimized for this operation,

which in most allocators is either unavailable or quite slow. This operation

does not actually return any memory to the OS, permitting the pool to be

efficiently reused. `wof_gc` (also very fast) will return what memory it can to

the OS (so a `wof_free_all` followed by a `wof_gc` will actually return all

memory to the OS, with the exception of the pool struct itself).

`wof_allocator_destroy` does this automatically, so you don't have to worry

about leaking when destroying a pool.



Fragmentation depends heavily on the allocation patterns. It behaves quite well

(almost on par with more traditional allocators) under the common load of

fixed-size, short-lived allocs, and very few reallocs. However, it can perform

as much as 3x worse under many variable-sized reallocs. The best way to tell how

it will perform for you is, of course, to try it.



In Wireshark, a pool is created, and used for any allocations needed during the

dissection of a packet, most of which are not explicitly freed even when they

pass out of scope. When the packet has been dissected, `free_all` is called and

the same pool is reused for the next packet to arrive.



History

-------



Version 1 of this allocator was embedded in Wireshark's original emem framework.

It didn't have to handle realloc or free, so it was very simple: it just grabbed

a block from the OS and served allocations sequentially out of that until it

ran out, then allocated a new block. The old block was never revisited, so

it generally had a bit of wasted space at the end, but the waste was

small enough that it was simply ignored. This allocator provided very fast

constant-time allocation for any request that didn't require a new block from

the OS, and that cost could be amortized away.



Version 2 of this allocator was prompted by the need to support realloc and

free in wmem. The original version simply didn't save enough metadata to do

this, so I added a layer on top to make it possible. The primary principle

was the same (allocate sequentially out of big blocks) with a bit of extra

magic. Allocations were still fast constant-time, and frees were as well.

Some parts of that design are still present in this one. For more

details see older versions of the wmem block allocator from Wireshark's

subversion repository.



Version 3 of this allocator was written to address some issues that

eventually showed up with version 2 under real-world usage. Specifically,

version 2 dealt very poorly with memory fragmentation, almost never reusing

freed blocks and choosing to just keep allocating from the master block

instead. This led to particularly poor behaviour under certain allocation

patterns that showed up a lot in Wireshark.



Blocks and Chunks

-----------------



As in previous versions, allocations typically happen sequentially out of

large OS-level blocks. Each block has a short embedded header used to

maintain a doubly-linked list of all blocks (used or not) currently owned by

the allocator. Each block is divided into chunks, which represent allocations

and free sections (a block is initialized with one large, free, chunk). Each

chunk is prefixed with a `wof_chunk_hdr_t` structure, which is a short

metadata header (typically 8 bytes, depending on architecture and alignment

requirements) that contains the length of the chunk, the length of the previous

chunk, a flag marking the chunk as free or used, and a flag marking the last

chunk in a block. This serves to implement an inline sequential doubly-linked

list of all the chunks in each block. A block with three chunks might look

something like this:



```

         0                    _________________________

         ^      ___________  /        ______________   \       __________

||---||--|-----/-----------||--------/--------------||--\-----/----------||

||hdr|| prv | len |  body  || prv | len |   body    || prv | len | body  ||

||---||--------------------||--/--------------------||-------------------||

       \______________________/

```



When allocating, a free chunk is found (more on that later) and split into

two chunks: the first of the requested size and the second containing any

remaining memory. The first is marked used and returned to the caller.



When freeing, the chunk in question is marked as free. Its neighbouring

chunks are then checked; if either of them are free, the consecutive free

chunks are merged into a single larger free chunk. Induction can show that

applying this operation consistently prevents us ever having consecutive

free chunks.



Free chunks (because they are not being used for anything else) each store an

additional pair of pointers (see the `wof_free_hdr_t` structure) that form

the backbone of the data structures used to track free chunks.



Master and Recycler

-------------------



The extra pair of pointers in free chunks are used to build two doubly-linked

lists: the master and the recycler. The recycler is circular, the master is

a stack.



The master stack is only populated by chunks from new OS-level blocks,

so every chunk in this list is guaranteed to be able to serve any allocation

request (the allocator has special 'jumbo' logic to handle requests larger than

its block size). The chunk at the head of the master list shrinks as it serves

requests. When it is too small to serve the current request, it is popped and

inserted into the recycler. If the master list is empty, a new OS-level block

is allocated, and its chunk is pushed onto the master stack.



The recycler is populated by 'leftovers' from the master, as well as any

chunks that were returned to the allocator via a call to `wof_free()`. Although

the recycler is circular, we will refer to the element referenced from the

allocator as the 'head' of the list for convenience. The primary operation on

the recycler is called cycling it. In this operation, the head is compared

with its clockwise neighbour. If the neighbour is as large or larger, it

becomes the head (the list rotates counter-clockwise). If the neighbour is

smaller, then it is removed from its location and inserted as the counter-

clockwise neighbour of the head (the list still rotates counter-clockwise,

but the head element is held fixed while the rest of the list spins). This

operation has the following properties:

 - fast constant time

 - once the largest chunk is at the head, it remains at the head

 - more iterations increases the probability that the largest chunk will be

   the head (for a list with n items, n iterations guarantees that the

   largest chunk will be the head).



Allocating

----------



When an allocation request is received, the allocator first attempts to

satisfy it with the chunk at the head of the recycler. If that does not

succeed, the request is satisfied by the master list instead. Regardless of

which chunk satisfied the request, the recycler is always cycled.