1 /*! @page introduction Introduction to SimGrid
3 [SimGrid](http://simgrid.gforge.inria.fr/) is a toolkit
4 that provides core functionalities for the simulation of distributed
5 applications in heterogeneous distributed environments.
7 The specific goal of the project is to facilitate research in the area of
8 distributed and parallel application scheduling on distributed computing
9 platforms ranging from simple network of workstations to Computational
13 The goal of this practical session is to illustrate various usage of
14 the MSG interface. To this end we will use the following simple setting:
16 > Assume we have a (possibly large) bunch of (possibly large) data to
17 > process and which originally reside on a server (a.k.a. master). For
18 > sake of simplicity, we assume all input file require the same amount
19 > of computation. We assume the server can be helped by a (possibly
20 > large) set of worker machines. What is the best way to organize the
23 Although this looks like a very simple setting it raises several
24 interesting questions:
26 - Which algorithm should the master use to send workload?
28 The most obvious algorithm would be to send tasks to workers in a
29 round-robin fashion. This is the initial code we provide you.
31 A less obvious but probably more efficient approach would be to set up
32 a request mechanism where a client first ask for tasks, which allows
33 the server to decide which request to answer and possibly to send
34 the tasks to the fastest machines. Maybe you can think of a
37 - How many tasks should the client ask for?
39 Indeed, if we set up a request mechanism so that workers only
40 send request whenever they have no more task to process, they are
41 likely to be poorly exploited since they will have to wait for the
42 master to consider their request and for the input data to be
43 transferred. A client should thus probably request a pool of tasks
44 but if it requests too many tasks, it is likely to lead to a poor
47 - How is the quality of such algorithm dependent on the platform
48 characteristics and on the task characteristics?
50 Whenever the input communication time is very small compared to
51 processing time and workers are homogeneous, it is likely that the
52 round-robin algorithm performs very well. Would it still hold true
53 when transfer time is not negligible and the platform is, say,
54 a volunteer computing system ?
56 - The network topology interconnecting the master and the workers
57 may be quite complicated. How does such a topology impact the
60 When data transfers are the bottleneck, it is likely that a good
61 modeling of the platform becomes essential. In this case, you may
62 want to be able to account for complex platform topologies.
64 - Do the algorithms depend on a perfect knowledge of this
67 Should we still use a flat master worker deployment or should we
70 - How is such an algorithm sensitive to external workload variation?
72 What if bandwidth, latency and power can vary with no warning?
73 Shouldn't you study whether your algorithm is sensitive to such
76 - Although an algorithm may be more efficient than another, how
77 does it interfere with other applications?
79 %As you can see, this very simple setting may need to evolve way
80 beyond what you initially imagined.
82 <blockquote> Premature optimization is the root of all evil. -- D.E.Knuth</blockquote>
84 Furthermore, writing your own simulator is much harder than you
85 may imagine. This is why you should rely on an established and flexible
88 The following figure is a screenshot of [triva][fn:1] visualizing a [SimGrid
89 simulation][fn:2] of two master worker applications (one in light gray and
90 the other in dark gray) running in concurrence and showing resource
91 usage over a long period of time.
93 ![Test](./sc3-description.png)
99 A lot of information on how to install and use Simgrid are
100 provided by the [online documentation][fn:4] and by several tutorials:
102 - http://simgrid.gforge.inria.fr/tutorials/simgrid-use-101.pdf
103 - http://simgrid.gforge.inria.fr/tutorials/simgrid-tracing-101.pdf
104 - http://simgrid.gforge.inria.fr/tutorials/simgrid-platf-101.pdf
106 ## Installing SimGrid
108 Please see our Section on \ref install "installing SimGrid".
111 Please note that your distribution may ship with an old version of
112 SimGrid; you may want to use [a newer release](https://gforge.inria.fr/frs/?group_id=12)
113 or even [clone our git repository](https://gforge.inria.fr/frs/?group_id=12)
114 (a [GitHub mirror](https://github.com/mquinson/simgrid) is also available).
120 This [software][fn:1] will be useful to make fancy graph or treemap
121 visualizations and get a better understanding of simulations. You
122 will first need to install pajeng:
125 sudo apt-get install git cmake build-essential libqt4-dev libboost-dev freeglut3-dev ;
126 git clone https://github.com/schnorr/pajeng.git
127 cd pajeng && mkdir -p build && cd build && cmake ../ -DCMAKE_INSTALL_PREFIX=$HOME && make -j install
131 Then you can install viva.
134 sudo apt-get install libboost-dev libconfig++-dev libconfig8-dev libgtk2.0-dev freeglut3-dev
135 git clone https://github.com/schnorr/viva.git
136 cd viva && mkdir -p build_graph && cd build_graph && cmake ../ -DTUPI_LIBRARY=ON -DVIVA=ON -DCMAKE_INSTALL_PREFIX=$HOME && make -j install
142 This [software][fn:5] provides a Gantt-chart visualization.
145 sudo apt-get install paje.app
150 This software provides a [Gantt-chart visualization][fn:6].
153 sudo apt-get install vite
157 ## Setting up and Compiling
159 The corresponding archive with all source files and platform files
160 can be obtained [here](http://simgrid.gforge.inria.fr/tutorials/msg-tuto/msg-tuto.tgz).
168 %As you can see, there is already a nice Makefile that compiles
169 everything for you. Now the tiny example has been compiled and it
170 can be easily run as follows:
173 ./masterworker0 platforms/platform.xml deployment0.xml 2>&1
176 If you create a single self-content C-file named foo.c, the
177 corresponding program will be simply compiled and linked with
184 For a more "fancy" output, you can try:
187 ./masterworker0 platforms/platform.xml deployment0.xml 2>&1 | simgrid-colorizer
190 For a really fancy output, you should use [viva/triva][fn:1]:
193 ./masterworker0 platforms/platform.xml deployment0.xml --cfg=tracing:yes \
194 --cfg=tracing/uncategorized:yes --cfg=viva/uncategorized:uncat.plist
195 LANG=C ; viva simgrid.trace uncat.plist
198 For a more classical Gantt-Chart visualization, you can produce a
202 ./masterworker0 platforms/platform.xml deployment0.xml --cfg=tracing:yes \
203 --cfg=tracing/msg/process:yes
204 LANG=C ; Paje simgrid.trace
207 Alternatively, you can use [vite][fn:6].
210 ./masterworker0 platforms/platform.xml deployment0.xml --cfg=tracing:yes \
211 --cfg=tracing/msg/process:yes --cfg=tracing/basic:yes
215 ## Getting Rid of Workers in the Deployment File
217 In the previous example, the deployment file `deployment0.xml`
218 is tightly connected to the platform file `platform.xml` and a
219 worker process is launched on each host:
222 <?xml version='1.0'?>
223 <!DOCTYPE platform SYSTEM "http://simgrid.gforge.inria.fr/simgrid.dtd">
224 <platform version="3">
225 <!-- The master process (with some arguments) -->
226 <process host="Tremblay" function="master">
227 <argument value="20"/> <!-- Number of tasks -->
228 <argument value="50000000"/> <!-- Computation size of tasks -->
229 <argument value="1000000"/> <!-- Communication size of tasks -->
230 <argument value="Jupiter"/> <!-- First worker -->
231 <argument value="Fafard"/> <!-- Second worker -->
232 <argument value="Ginette"/> <!-- Third worker -->
233 <argument value="Bourassa"/> <!-- Last worker -->
234 <argument value="Tremblay"/> <!-- Me! I can work too! -->
236 <!-- The worker process (with no argument) -->
237 <process host="Tremblay" function="worker" on_failure="RESTART"/>
238 <process host="Jupiter" function="worker" on_failure="RESTART"/>
239 <process host="Fafard" function="worker" on_failure="RESTART"/>
240 <process host="Ginette" function="worker" on_failure="RESTART"/>
241 <process host="Bourassa" function="worker" on_failure="RESTART"/>
245 This is ok as the platform is rather small but will be painful when
246 using larger platforms. Instead, modify the simulator
247 `masterworker0.c` into `masterworker1.c` so that the master
248 launches a worker process on all the other machines at startup. The
249 new deployment file `deployment1.xml` should thus now simply be:
252 <?xml version='1.0'?>
253 <!DOCTYPE platform SYSTEM "http://simgrid.gforge.inria.fr/simgrid.dtd">
254 <platform version="3">
255 <!-- The master process (with some arguments) -->
256 <process host="Tremblay" function="master">
257 <argument value="20"/> <!-- Number of tasks -->
258 <argument value="50000000"/> <!-- Computation size of tasks -->
259 <argument value="1000000"/> <!-- Communication size of tasks -->
264 To this end you may need the following MSG functions (click on the links
265 to see their descriptions):
268 int MSG_get_host_number(void);
269 xbt_dynar_t MSG_hosts_as_dynar(void);
270 void * xbt_dynar_to_array (xbt_dynar_t dynar);
271 msg_process_t MSG_process_create(const char *name, xbt_main_func_t code,
272 void *data, msg_host_t host);
276 It may avoid bugs later to avoid launching a worker on
277 the master host so you probably want to remove it from the host
280 The `data` field of the @ref MSG_process_create can be used to pass
281 a channel name that will be private between master
282 and workers (e.g., `master_name:worker_name`). Adding the
283 `master_name` in the channel name will allow to easily have several
284 masters and a worker per master on each machine. To this end, you
285 may need to use the following functions:
288 msg_host_t MSG_host_self(void);
289 const char * MSG_host_get_name(msg_host_t host);
290 msg_process_t MSG_process_self(void);
291 void * MSG_process_get_data(msg_process_t process);
294 If you are not too familiar with string
295 manipulation in C, you may want to use the following functions
296 (see the C reference for details):
299 char *strcpy(char *dest, const char *src);
300 char *strcat(char *dest, const char *src);
303 ## Setting up a Time Limit Mechanism
305 In the current version, the number of tasks is defined through the
306 worker arguments. Hence, tasks are created at the very beginning of
307 the simulation. Instead, create tasks as needed and provide a time
308 limit indicating when it stops sending tasks. To this end, you will
309 obviously need to know what time it is:
312 double MSG_get_clock(void);
315 Otherwise, a quite effective way of terminating the simulation
316 would be to use some of the following functions:
319 void MSG_process_kill(msg_process_t process);
320 int MSG_process_killall(int reset_PIDs);
323 Anyway, the new deployment `deployment2.xml` file should thus look
327 <?xml version='1.0'?>
328 <!DOCTYPE platform SYSTEM "http://simgrid.gforge.inria.fr/simgrid.dtd">
329 <platform version="3">
330 <process host="Tremblay" function="master">
331 <argument value="3600"/> <!-- Simulation timeout -->
332 <argument value="50000000"/> <!-- Computation size of tasks -->
333 <argument value="1000000"/> <!-- Communication size of tasks -->
338 It may also be a good idea to transform most of the `XBT_INFO` into
339 `XBT_DEBUG` (e.g., keep the information on the total number of
340 tasks processed). These debug messages can be activated as follows:
343 ./masterworker2 platforms/platform.xml deployment2.xml --log=msg_test.thres:debug
346 ## Using the Tracing Mechanism
348 SimGrid can trace all resource consumption and the outcome can be
349 displayed with viva as illustrated in the section "Setting up and Compiling". However, when several
350 masters are deployed, it is hard to understand what happens.
353 <?xml version='1.0'?>
354 <!DOCTYPE platform SYSTEM "http://simgrid.gforge.inria.fr/simgrid.dtd">
355 <platform version="3">
356 <process host="Tremblay" function="master">
357 <argument value="3600"/> <!-- Simulation timeout -->
358 <argument value="50000000"/> <!-- Computation size of tasks -->
359 <argument value="10"/> <!-- Communication size of tasks -->
361 <process host="Fafard" function="master">
362 <argument value="3600"/> <!-- Simulation timeout -->
363 <argument value="50000000"/> <!-- Computation size of tasks -->
364 <argument value="10"/> <!-- Communication size of tasks -->
366 <process host="Jupiter" function="master">
367 <argument value="3600"/> <!-- Simulation timeout -->
368 <argument value="50000000"/> <!-- Computation size of tasks -->
369 <argument value="10"/> <!-- Communication size of tasks -->
374 So let's use categories to track more precisely who does what and when:
377 void TRACE_category(const char *category);
378 void MSG_task_set_category (msg_task_t task, const char *category);
381 The outcome can then be visualized as follows:
384 ./masterworker3 platforms/platform.xml deployment3.xml --cfg=tracing:yes\
385 --cfg=tracing/categorized:yes --cfg=viva/categorized:viva_cat.plist
386 LANG=C; viva simgrid.trace viva_cat.plist
389 Right now, you should realize that nothing is behaving like you
390 expect. Most workers are idle even though input data are ridiculous
391 and there are several masters deployed on the platform. Using a
392 Gantt-chart visualization may help:
395 ./masterworker3 platforms/platform.xml deployment3.xml --cfg=tracing:yes \
396 --cfg=tracing/msg/process:yes
397 LANG=C; Paje simgrid.trace
400 OK, so it should now be obvious that round robin is actually
403 ## Improving the Scheduling
405 Instead of a round-robin scheduling, let's implement a first-come
406 first-served mechanism. To this end, workers need to send a tiny
407 request first. A possible way to implement such a request with MSG
408 is to send on a specific channel (e.g., the name of the master
409 name) a task with payload 0 and whose attached data is the worker
410 name. This way, the master can keep track of which workers are idle
413 To know whether it has pending requests, the master can use the
414 following [function][fn:7]:
417 int MSG_task_listen(const char *alias);
420 If so, it should get the request and push the corresponding host
421 into a dynar so that they can later be retrieved when sending a
425 xbt_dynar_t xbt_dynar_new(const unsigned long elm_size,
426 void_f_pvoid_t const free_f);
427 void xbt_dynar_push(xbt_dynar_t const dynar, const void *src);
428 void xbt_dynar_shift(xbt_dynar_t const dynar, void *const dst);
429 unsigned long xbt_dynar_length(const xbt_dynar_t dynar);
432 %As you will soon realize, with such simple mechanisms, simple
433 deadlocks will soon appear. They can easily be removed with a
434 simple polling mechanism, hence the need for the following
438 msg_error_t MSG_process_sleep(double nb_sec);
441 %As you should quickly realize, on the simple previous example, it
442 will double the throughput of the platform but will be quite
443 ineffective when input size of the tasks is not negligible anymore.
445 From this, many things can easily be added. For example, you could:
446 - add a performance measurement mechanism;
447 - enable the master to make smart scheduling choices using
448 measurement information;
449 - allow workers to have several pending requests so as to overlap
450 communication and computations as much as possible;
453 ## Using More Elaborate Platforms
455 SimGrid offers a rather powerful platform modeling mechanism. The
456 `src/platform/` repository comprises a variety of platform ranging
457 from simple ones to quite elaborated ones. Associated to a good
458 visualization tool to ensure your simulation is meaningful, they
459 can allow you to study to which extent your algorithm scales...
461 What is the largest number of tasks requiring 50e6 flops and 1e5
462 bytes that you manage to distribute and process in one hour on
463 `g5k.xml` (you should use `deployment_general.xml`)?
465 # Points to improve for the next time
467 - Propose equivalent exercises and skeleton in java.
468 - Propose a virtualbox image with everything (simgrid, paje, viva,
470 - Ease the installation on mac OS X (binary installer) and
472 - Explain that programming in C or java and having a working
473 development environment is a prerequisite.
475 [fn:1]: http://triva.gforge.inria.fr/index.html
476 [fn:2]: http://hal.inria.fr/inria-00529569
477 [fn:3]: http://hal.inria.fr/hal-00738321
478 [fn:4]: http://simgrid.gforge.inria.fr/documentation.html
479 [fn:5]: http://paje.sourceforge.net/
480 [fn:6]: http://vite.gforge.inria.fr/