1. Programming the cloud
with Skywriting"
Derek Murray"

2. Outline"
• Existing parallel programming models"
• Introduction to Skywriting"
• Ongoing work"

3. SMP programming"
• Symmetric multiprocessing"
– All cores share the same address space"
– Usually assumes cache-coherency"
• Multi-threaded programs"
– Threads created dynamically"
– Shared, writable data structures"
– Synchronisation"
• Atomic compare-and-swap"
• Mutexes, Semaphores"
• {Software, Hardware} Transactional Memory"

4. Distributed programming"
• Shared nothing*"
– Processors communicate by message passing"
– Standard assumption for large supercomputers"
– ...and data centres"
– ...and recent manycore machines (e.g. Intel SCC)"
• Explicit messages"
– MPI, Pregel, Actor-based"
• Implicit messages: task parallelism"
– MapReduce, Dryad"

5. Task parallelism"
• Master-worker architecture"
– Master maintains task queue"
– Workers execute independent tasks in parallel"
• Fault tolerance"
– Re-execute tasks on failed workers"
– Speculatively re-execute “slow” tasks"
• Load balancing"
– Workers consume tasks at their own rate"
– Task granularity must be optimised"

6. Task graph"
A runs before B"
A" B"
B depends on A"

7. MapReduce"
• Two kinds of task: map and reduce"
• User-specified map and reduce functions"
– map() a record to a list of key-value pairs"
– reduce() a key and the list of related values"
• Tasks apply functions to data partitions"
M" R"
M" R"
M" R"

8. Dryad"
• Task graph is first class"
– Vertices run arbitrary code"
– Multiple inputs and inputs"
– Channels specify data transport"
• Graph must be acyclic and finite"
– Permits topological sorting"
– Prevents unbounded iteration"

9. Architecture"
Code"
Driver Results"
program"

10. while
(!converged)
do
work
in
parallel;

11. Existing systems"
Code"
Driver Results"
program"
Code"
Driver
program" Results"
while
(…)
Code"
submitJob();
Results"
Code"
Results"

12. Existing systems"
Code"
Driver Results"
program"
Code"
Driver
program"
while
(…)
submitJob();

13. Skywriting"
while
(…)
doStuff();
Code"
Results"
• Turing-complete language
for job specification"
• Whole job executed on the
cluster"

14. Spawning a Skywriting task"
function
f(arg1,
arg2)
{
…
}
result
=
spawn(f,
[arg1,
arg2]);
//
result
is
a
“future”
value_of_result
=
*result;

15. Building a task graph"
function
f(x,
y)
{
…
}
function
g(x,
y)
{
…
}
f
function
h(x,
y)
{
…
}
a
a
a
=
spawn(f,
[7,
8]);
g
g
b
=
spawn(g,
[a,
0]);
c
=
spawn(g,
[a,
1]);
d
=
spawn(h,
[b,
c]);
b
c
return
d;
h
d

16. Iterative algorithm"
current
=
…;
do
{
prev
=
current;
a
=
spawn(f,
[prev,
0]);
b
=
spawn(f,
[prev,
1]);
c
=
spawn(f,
[prev,
2]);
current
=
spawn(g,
[a,
b,
c]);
done
=
spawn(h,
[current]);
while
(!*done);

17. Iterative algorithm"
f
f
f
g
h
f
f
f

18. Aside: recursive algorithm"
function
f(x)
{
if
(/*
x
is
small
enough
*/)
{
return
/*
do
something
with
x
*/;
}
else
{
x_lo
=
/*
bottom
half
of
x
*/;
x_hi
=
/*
top
half
of
x
*/;
return
[spawn(f,
[x_lo]),
spawn(f,
[x_hi])];
}
}

19. Performance case studies"
• All experiments used Amazon EC2"
– m1.small instances, running Ubuntu 8.10"
• Microbenchmark"
• Smith-Waterman"

20. Job creation overhead"
60"
Overhead (seconds)"
50"
40"
30"
Hadoop"
20" Skywriting"
10"
0"
0" 20" 40" 60" 80" 100"
Number of workers"

21. Smith-Waterman data flow"

22. Parallel Smith-Waterman"

23. Parallel Smith-Waterman"
350"
300"
Time (seconds)"
250"
200"
150"
100"
50"
0"
1" 10" 100" 1000" 10000"
Number of tasks"

24. Future work: manycore"
• Trade-offs are different"
– Centralised master may become a bottleneck"
– Switch to local work-queues and work-stealing"
– Distributed scoreboards for futures"
– Optimised interpreter/compilation?"
• Multi-scale hybrid approaches"
– Multiple cores"
– Multiple machines"
– Multiple clouds..."

25. Future work: message-passing"
• Language designed for batch processing"
– However, batches may be infinitely long!"
• Can we apply it to streaming data?"
– Log files"
– Financial reports"
– Sensor data"
• Can we include data dependencies?"
– High- and low-frequency streams"

26. Conclusions"
• Turing-complete programming language
for cloud computing"
• Runs real jobs with low overhead"
• Lots more still to do!"

27. Questions?"
• Email"
– Derek.Murray@cl.cam.ac.uk"
• Project website"
– http://www.cl.cam.ac.uk/netos/skywriting/