Lock-free node allocator for fifo-queues

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Dmitriy V'jukov

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May 31, 2007, 3:11:41 PM5/31/07
to lock-free
Another crosspost from comp.programming.threads just to create some
initial content :)

I propose the special lock-free allocator for allocating nodes for
fifo-queues.
The feature of this allocator is that allocation and deallocation
issues no atomic operations.
Here goes code:

template<typename type>
class node_allocator
{
private:
struct header
{
header* next_;
bool free_;
// actually this struct can locate in one word
// we can use lsb of next pointer as free flag
};

// head of fifo queue of allocated items
// here we place newly allocated item
header* head_;

// tail of fifo queue of allocated items
// here is situated the oldest item
header* tail_;

// count of allocated items
unsigned count_;

public:
node_allocator()
: head_()
, tail_()
, count_()
{}

type* alloc()
{
// this method must be called by only one thread
// that owns this allocator - the "producer" -
// that allocates the nodes and enqueue them into
// the fifo-queue

header* h;
if (tail_ && tail_->free_)
{
h = tail_;
tail_ = tail_->next_;
}
else
{
h = reinterpret_cast<header*>(new
char[sizeof(header) +
sizeof(type)]);
++count_;
}
h->free_ = false;
h->next_ = 0;
if (head_)
head_->next_ = h;
if (!tail_)
tail_ = h;
head_ = h;
return reinterpret_cast<type*>(h + 1);
}

static void free(type* node)
{
// this method is called by any thread
// that free the node that owned by this allocator -
// the "consumer"

header* h = reinterpret_cast<header*>(node) - 1;
// release_membar
h->free_ = true;
}

};

This version doesn't free any nodes to system allocator and only
accumulates nodes. But it seems that freeing of nodes can be added to
algorithm - especially since most of the data is touched by only one
thread - so addition of freeing should not add any "CAS-overhead".
Nevertheless if we want really high performance we must accumulate
_many_ nodes anyway.

Usage example:

template<typename type>
class mpsc_queue
{
public:
struct node
{
node* next_;
type* data_;
};

typedef node_allocator<node> allocator_t;

private:
node* head_;
node* tail_;

public:
mpsc_queue()
: head_()
, tail_()
{
}

void enqueue(allocator_t& a, type* v)
{
// here we allocate from thread-local allocator
node* n = a.alloc();
n->data_ = v;
do {n->next_ = head_;}
while (n->next_ != (node*)atomic_cas((long
volatile*)&head_,
(long)n, (long)n->next_));
}

type* dequeue()
{
if (!tail_)
{
node* head = (node*)atomic_exchange((long
volatile*)&head_, 0);
if (!head)
return 0;
node* prev = 0;
node* curr = head;
do
{
node* next = curr->next_;
curr->next_ = prev;
prev = curr;
curr = next;
}
while (curr);
tail_ = prev;
}
type* v = tail_->data_;
node* n = tail_;
tail_ = n->next_;
// here we free the node
allocator_t::free(n);
return v;
}

};

int main()
{
typedef mpsc_queue<int> queue_t;
typedef queue_t::allocator_t allocator_t;

queue_t q;
allocator_t a;

int i1 = 11;
int i2 = 12;
int i3 = 13;

q.enqueue(a, &i1);
q.enqueue(a, &i2);
q.enqueue(a, &i3);

int* ii1 = q.dequeue();
int* ii2 = q.dequeue();

q.enqueue(a, &i1);
q.enqueue(a, &i2);
q.enqueue(a, &i3);

int* ii3 = (int*)q.dequeue();
int* ii4 = (int*)q.dequeue();

}

I think that usage of such allocator can be very beneficial in some
scenarios, particular with various heavy load fifo queues.

Chris Thomasson distributed lock-free slab-allocator:
http://groups.google.com/group/comp.programming.threads/msg/e3efa5628...
doesn't handle such situations very good - requires one CAS on
deallocation and zero CAS'es on allocation amortized.
It seems that with my node_allocator mpsc_queue::dequeue() operation
doesn't issue any atomic operations at all (amortized :) ).

Dmitriy V'jukov

Dmitriy V'jukov

unread,
May 31, 2007, 3:12:04 PM5/31/07
to lock-free
The main feature:
Allocator tracks not free nodes (like freelists in other allocators),
but it tracks currently allocated in-use nodes.
Allocator tracks currently allocated nodes in some particular order
(fifo or lifo).
Allocator checks the tail node (wrt fifo or lifo order) whether it is
freed by some other thread by checking the boolean field "free" of
node.
The free() procedure of allocator is no more than setting the boolean
field "free" of node to "true".

I think it is new. And it is fast.
No one memory location is written by two threads. So no CASes at all.

Dmitriy V'jukov

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