[Mpi-22] [Mpi-forum] MPI 2.2 proposal: resolving MPI_Request_free issues

[Erez Haba]

-----Original Message-----
From: mpi-22-bounces_at_[hidden] [mailto:mpi-22-bounces_at_[hidden]] On Behalf Of Underwood, Keith D
Sent: Wednesday, July 16, 2008 4:32 PM
To: MPI 2.2
Subject: Re: [Mpi-22] [Mpi-forum] MPI 2.2 proposal:resolving MPI_Request_free issues

>> I don't quite understand examples 1 and 2 (how would they cause segv's
>> in the TCP stack).  It is permissible to (pseudocode):
>>
>>   while (bytes_to_send > 0) {
>>      rc = write(fd, buffer, bytes_to_send);
>>      if (rc > 0) {
>>         buffer += rc;
>>         bytes_to_send -= rc;
>>      } else {
>>         ...error...
>>      }
>>   }
>>   free(buffer);
>>
>> regardless of what the receiver does.  I'm not a kernel guy; does
>> updating TCP sequence numbers also interact with the payload buffer?
>
>[erezh] it will never happen with your code above; but you are not using
>async zcopy.
>The pattern in windows is to use overlapped send (write) which is still
>active when the function returns, and is the most efficient way to send
>your buffer. I know it's possible with Linux but I don't' have the exact
>pattern.

> [Keith]
> So, how does this work for other users of zero copy?  How do they know
> when the send buffer is truly free?

[erezh] you get a notification from your local resource manager; on windows with the overlapped mechanism you have few ways to get notified. 1) The overlapped event get set, the thread can wait on it. 2) a completion port notification that the thread can dequeue. 3) an APC (similar to unix signal) get queued to the thread (thread need to be in an alertable state)