[Spread-users] Spread and reliable message communication

Tuvi, Selim stuvi at SLAC.Stanford.EDU
Fri Aug 8 16:00:13 EDT 2003

 Actually the SP_receive call doesn't return any errors since we don't get any exceptions from the Python wrapper. It just waits as if no more mesages have been received.


Selim Tuvi, Research Engineering Group, SLAC
GLAST, I&T Online, MS 98
Tel:650-926-3376 Fax:650-926-4335

-----Original Message-----
From: Ryan Caudy [mailto:caudy at jhu.edu] 
Sent: Friday, August 08, 2003 12:10 PM
To: Tuvi, Selim
Cc: spread-users at lists.spread.org
Subject: Re: [Spread-users] Spread and reliable message communication

Spread does not block the sender for sending too fast, as long as there 
is room in Spread's internal buffers to hold the incoming (at which 
point it will block the application by not receiving from the socket at 
its end).  Spread's response to a "slow" receiver is to cut the receiver 
off.  This forces flow control to be done by the client.  So, likely 
what you're seeing is the client being cut off for being too slow... you 
can confirm this by examining the return code given to the SP_receive 
call that returns an error.


Tuvi, Selim wrote:

> Hi, we just started experimenting with the Python extension module for 
> Spread and we are having problems in reliably sending and receiving 
> messages. We tested this only under a Windows XP environment and only 
> under Python so bear with us. I e-mailed the same question to Guido 
> van Rossum, maintainer of the Python extension and he seems to think 
> the problem has nothing to do with the Python wrappers.
> The attached files demonstrate the problem. We read a file that 
> contains chunks of binary data about 800 bytes in size. In a loop we 
> send the contents (the file contains 5 chunks) 1000 times to the 
> receiver. If the CPU is loaded 100% then after a while the receiving 
> side stops receiving the data. It doesn't seem to drop packets since I 
> am also sending the message number as a value in the msg_type member 
> and the last msg_type value matches the received number of messages.
> We observed that The sending process finishes to completion and 
> reports that 4072000 bytes were sent. The receiving process on the other hand stops after about 600-800 packets depending on the load. I deliberatly slowed down the receiving side by dumping the contents of the received messages.
> As far as I could read from the documentation, if the sender is 
> sending faster than the receiving side can process, then the sender 
> should block, but this is not the case. We don't observe any blocking 
> on the sender at all. We tried using FIFO_MESS or RELIABLE_MESS as the 
> service type but that doesn't seem to make any difference.
> If you can shed some light on this problem or if we didn't understand 
> the Spread concepts fully we appreciate it if you can steer us in the 
> right direction.
> Testing was done under a Windows XP machine with ActiveState Python 
> 2.2.2 build 224. Spread extension version is 1.3 and Spread version is 
> version 3.17.01 built on 20/June/2003.
> Thanks
> -Selim
> Selim Tuvi, Research Engineering Group, SLAC
> GLAST, I&T Online, MS 98
> Tel:650-926-3376 Fax:650-926-4335

Ryan W. Caudy
Center for Networking and Distributed Systems
Department of Computer Science
Johns Hopkins University

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