Re: My -short- lock-free sequencer class, I want to see your comments

"Alexander Grigoriev" <>
Fri, 29 Aug 2008 20:00:57 -0700
There is no such concept as "order" when you're talking about two or more
different threads. The completion packets are posted to IOCP queue in order,
they are fetched by threads that call GetQueuedCompletionStatus
sequentially. As soon as the kernel code that's under GQCS function releases
the queue lock, there is no "order" concept anymore. Your code may happen to
call IsInOrder function and get TRUE, and at the next instance the thread
that was "ahead" may now run "behind". If you want "ordered" processing, you
need to handle that in one thread.

"K?r?at" <> wrote in message

As a configurable property of my server, it is possible to post more than
one overlapped WSARecv for a given socket at the same time. Those calls
always completes in order but it doesn't mean that they will be processed
in order.

Not my model, the IO Completion Port model needs multiple threads. It
doesn't need multiple threads indeed but using IOCP framework with single
thread is meaningless and completely disables it's advantages.

Anyway, what do you think about the class's correctness without
considering IO related issues?

"Alexander Grigoriev" <> wrote in message

Considering that single socket RX happens always in order, why do you
think you need interlocked operations? Just make sure you get all data
from your socket in the same thread. Describe, why your model need
multiple threads?

"K?r?at" <> wrote in message


Recently I needed a sequencer for my IOCP based socket server and
developed one. I try to implement it in lock-free manner. Your comments
will bee appreciated.

The class is very small and all it do is to maintain two numbers in
thread safe manner. Before every WSARecv call I get next available
sequence number from sequencer and put that number into my PerIoContext
object. When a recv completion occured then I check the sequence number
to decide if the completion occured in order. So one of the two numbers
(m_lCurrentSequence) represents call sequence and the other
(m_lRunningOrder) represents completion sequence. Here is the class :

class Sequencer
    Sequencer (LONG lMaxSequence) : m_lCurrentSequence (0),
m_lRunningOrder (0), m_lMaxSequence (lMaxSequence)

    LONG getNextSequence ()
         LONG lCurrentSequence, lNextSequence;
         while (true)
              InterlockedExchange (&lCurrentSequence,
              lNextSequence = (lCurrentSequence == m_lMaxSequence ? 0 :
lCurrentSequence + 1);
              if (lCurrentSequence == InterlockedCompareExchange
(&m_lCurrentSequence, lNextSequence, lCurrentSequence))
       return lNextSequence;

bool isInOrder (const LONG lSequence)
     return (lSequence == m_lRunningOrder);

bool updateRunningOrder (const LONG lSequence)
     if (isInOrder (lSequence))
          // Safe region...
          LONG lNewRunningOrder = (lSequence + 1) > m_lMaxSequence ? 0 :
(lSequence + 1);
          InterlockedExchange (&m_lRunningOrder, lNewRunningOrder);
          return true;
     return false;

LONG m_lMaxSequence;
LONG m_lCurrentSequence;
LONG m_lRunningOrder;

Thanks in advance for your comments.

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