This document discusses combining GNU Prolog with PM2 to enable distributed multi-threading capabilities. It presents an implementation where PM2 and GNU Prolog are connected without modifications. This allows GNU Prolog programs to execute across multiple computers through message passing. An execution model is described where a master node distributes work to worker nodes, which perform tasks and return results. Preliminary results show this approach achieves speedups for some real-world problems.

1. Distributed Multi-Threading
in GNU Prolog
Nuno Morgadinho, Salvador Abreu
{nm, spa}@di.uevora.pt
Departamento de Informática
Évora 2007

2. Introduction
• Distributed - one or more computers
communicating over a network
• Multi-Threading - more than one thread of
execution
• Thread - a flow of execution
Slide 2 ICLP ’07 CICLOPS

3. Introduction
• Obtain results faster for problems where
performance is critical
• The problem can be divided into smaller tasks
which can be carried out simultaneously
• Parallelism
Slide 3 ICLP ’07 CICLOPS

4. What We Present
• PM2 - a distributed multi-threading
programming environment
• GNU Prolog - efficient native Prolog compiler
• Combination of both
Slide 4 ICLP ’07 CICLOPS

5. GNU Prolog and PM2: Why?
• GNU Prolog produces stand-alone executables
• The size of the executables is relatively small
• Doesn’t work with machine independent saved-
states
Slide 5 ICLP ’07 CICLOPS

6. Implementation
• Established a model for connecting PM2 and
GNU Prolog
• The approach doesn’t involve modifications to
GNU Prolog neither to PM2
• Compatibility with GNU Prolog libraries is
retained
Slide 6 ICLP ’07 CICLOPS

7. Approach
• Tabard - new program that manages distributed
instances of GNU Prolog engines
• pm2prolog - new library that allows the
development of distributed multithreaded
Prolog applications
Slide 7 ICLP ’07 CICLOPS

8. Functionality
• Allow to execute computations remotely
• Manage the engines responsible for the
computations
• Manage the communication involved between
the several machines
• Based on distributed memory and explicit
message-passing
Slide 8 ICLP ’07 CICLOPS

9. Example
:- initialization(init).
:- include('pm2prolog').
% thread with rank 0
init:-
pm2_is_master, !,
pm2_max_rank(MaxRank),
start_prolog_workers(MaxRank),
test_prolog_workers(MaxRank),
read_test(MaxRank),
stop_prolog_workers.
% thread != 0
init:-
worker_code.
Slide 9 ICLP ’07 CICLOPS

10. Before Running
• Configuration that specifies the list of machines
• Each machine is mapped to one or more
processing nodes or virtual processors (VPs)
Slide 10 ICLP ’07 CICLOPS

11. Execution Model
• Binary is copied to all machines
• In VP0 (master) a gprolog engine is created
• In the other VPs (workers) a pthread in C is
created and stands awaiting messages
Slide 11 ICLP ’07 CICLOPS

12. Execution Model
• In the master, now in the Prolog thread, a
predicate is called to send a message to every
worker
• The workers receive the message, initiate a
gprolog engine and the thread stands awaiting
more messages to come
Slide 12 ICLP ’07 CICLOPS

13. Execution Model
• In the master, work is distributed throughout
the workers through message-passing
• The workers receive tasks which they execute
locally. As soon as they finish, they send their
results back to the master
Slide 13 ICLP ’07 CICLOPS

14. Execution Model
• The master assembles the work results by
reading as many messages as the number of
previously sent messages
• The master redistributes work again or orders
the workers to finish their execution
• The workers terminate
• The master reiniciates the workers or
terminates itself
Slide 14 ICLP ’07 CICLOPS

15. Inside a Virtual Processor
UNLOCK Message Queue
LOCK
WRITE READ
WRITE
...
C Thread Listener
Prolog Thread
write_message_queue() thread_send_message/2
Start_Prolog() thread_get_message/2
Stop_Prolog() pm2_self/1
... ...
RECEIVE SEND
SOCKET
Slide 15 ICLP ’07 CICLOPS

16. • Several VPs per physical machine is possible
Node 1 Node 2
VP 2
VP 0 Prolog
Proc
Prolog
Proc
Node 3
VP 1
VP 2
Prolog Prolog
Proc Proc
...
...
Slide 16 ICLP ’07 CICLOPS

17. ISO Support
• ISO/IEC Draft Technical Report 13211-5:2007,
Prolog Multi-Threading Support
• Extensions to take into consideration remote
threads
Slide 17 ICLP ’07 CICLOPS

18. PM2-Prolog Remote Threads
• thread_create/2
• vid(Rank, ThreadID)
• Rank - VP identifier
• ThreadID - identifier of the thread inside
Slide 18 ICLP ’07 CICLOPS

19. Experimental Evaluation
100
75
50
25
0
2004 2005 2006 2007
Results obtained with 7x Intel(R) Pentium(R) 4 2.80 Mhz each, Hyperthreading
enabled, 512 Mbytes each, Linux 2.4.19 kernel, 100 Mbits TCP/IP Ethernet network
Slide 19 ICLP ’07 CICLOPS

20. Conclusions
• We presented a distributed multi-threading
GNU Prolog system on top of PM2
• First results show that it can obtain substantial
speedups, even for real-world
• Proved the approach to be technically possible
and can be of use to other implementers
Slide 20 ICLP ’07 CICLOPS

21. Further Work
• Improving our proposal
• Extend the API with introspection and
monitoring predicates
• Experiment with more programs and bigger
configurations
• Build our own applications using this
technology
Slide 21 ICLP ’07 CICLOPS