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