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

"Alexander Grigoriev" <>
Sat, 30 Aug 2008 13:06:56 -0700
Unfortunately, it doesn't look like the algorithm shown in codeproject
guarantees that ::WSASend would be called in order. The buffers will be
fetched in order, yes, but if those WSASend loops shown at the example can
ever run in more than one thread for a socket, then the send order for data
is not guaranteed.

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

It seems we are talking about same solution. I use 2 states
(InOrder-NotInOrder) while you use 5 (0, 1, >1, 80000001, >80000001). In
my handler I check order, if out of order data received then I just store
it to process later as you explain below.

"Alexander Grigoriev" <> wrote in message

For me, it looks like an overengineered solution to self-inflicted

I suggest to use "single-entered workitem" concept instead.

IOCP threads would simply queue the recv packets in arbitrary order into
the socket context. Then they would either use QUeueUserWorkitem, or
PostQueuedCompletionStatus to execute a handler for that data.

To make sure that at any moment there is only one handler queued or
running for this socket context, use a per-socket LONG flag with the
following states:

0 - no WI is queued or executed;
1 - the WI is queued, not picked for execution;

1 - WI is queued, not picked for execution, more data is queued;

80000001 - WI is currently executing

80000001 - WI is currently executing and there is more data queued to it.

I'll leave it up to you to devise a code to manipulate the flag. There,
when you get IOCP notification for data, you queue the data and check the
flag. If it's >=1, or >80000001, you don't have to do anything; your new
data will get handled. InterlockedIncrement the flag; if it's >80000001
or >1, you don't have to do anything; your new data will get handled;
otherwise it's 1, queue your workitem. In the workitem, do {} while loop.
In the beginning of the loop, set the flag to 80000001, then handle all
data you can handle. Loop end condition is when your
InterlockedCompareExchange(&flag, 0, 0x80000001) succeeds.

The code inside said do{}while loop will deal with all data ordering
issues, by simply resorting the buffers according to their seq#. If it
doesn't have a buffer to close a hole, it will just continue the loop,
where it either exit, or gives the data one more shot.

And yes, such code works. The only synchronization primitives required is
the said flag, and a workitem/DPC/APC facility.

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

The concept is explained in this article by Len Holgate :
He also impelemented the concept in his free IOCP Socket Server. I only
implement a sequencer as a non-blocking or lock-free algorithm.

"Alexander Grigoriev" <> wrote in message

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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