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

"Alexander Grigoriev" <>
Fri, 29 Aug 2008 07:26:48 -0700
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, m_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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