Computational Social Science, Lecture 07: Counting Fast, Part I

Counting Fast
(Part I)

Sergei Vassilvitskii
Columbia University
Computational Social Science
March 8, 2013

Computers are fast!

Servers:
– 3.5+ Ghz

Laptops:
– 2.0 - 3 Ghz

Phones:
– 1.0-1.5 GHz

Overall: Executes billions of operations per second!

2 Sergei Vassilvitskii

But Data is Big!

Datasets are huge:
– Social Graphs (Billions of nodes, each with hundreds of edges)
• Terabytes (million million bytes)
– Pictures, Videos, associated metadata:
• Petabytes (million billion bytes!)


Computers are getting faster
Moore’s law (1965!):
– Number of transistors on a chip doubles every two years.


Computers are getting faster

Moore’s law (1965!):
– Number of transistors on a chip doubles every two years.

For a few decades:
– The speed of chips doubled every 24 months.

Now:
– The number of cores doubling
– Speed staying roughly the same


But Data is Getting Even Bigger

Unknown author, 1981 (?):
– “640K ought to be enough for anyone”

Eric Schmidt, March 2013:
– “There were 5 exabytes of information created between the dawn of
civilization through 2003, but that much information is now created
every 2 days, and the pace is increasing.”


Data Sizes
What is Big Data?
– MB in 1980s Hard Drive Capacity
– GB in 1990s
– TB in 2000s
– PB in 2010s


Working with Big Data

Two datasets of numbers:
– Want to ﬁnd the intersection (common values)
– Why?
• Data cleaning (these are missing values)
• Data mining (these are unique in some way)


Working with Big Data

Two datasets of numbers:
– Want to ﬁnd the intersection (common values)
– Why?
• Data cleaning (these are missing values)
• Data mining (these are unique in some way)

– How long should it take?
• Each dataset has 10 numbers?
• Each dataset has 10k numbers?
• Each dataset has 10M numbers?
• Each dataset has 10B numbers?
• Each dataset has 10T numbers?


How to Find Intersections?


Idea 1: Scan

Look at every number in list 1:
– Scan through dataset 2, see if you ﬁnd a match

common_elements = 0
for number in dataset1:
for number2 in dataset2:
if number1 == number2:
common_elements +=1


Idea 1: Scanning

For each element in dataset 1, scan through dataset 2, see if it’s present

common_elements = 0
for number in dataset1:
for number2 in dataset2:
if number1 == number2:
common_elements +=1

Analysis: Number of times if statement executed?
– |dataset2| for every iteration of outer loop
– |dataset1| * |dataset2| in total


Idea 1: Scanning


Running time:
– 100M * 100M = 1016 comparisons in total
– At 1B (109) comparisons / second


Idea 1: Scanning


Running time:
– 100M * 100M = 1016 comparisons in total
– At 1B (109) comparisons / second
– 107 seconds ~ 4 months!

– Even with 1000 computers: 104 seconds -- 2.5 hours!


Idea 2: Sorting

Suppose both sets are sorted
– Keep pointers to each
– Check for match, increase the smaller pointer

[Blackboard]


Idea 2: Sorting

sorted1 = sorted(dataset1)
sorted2 = sorted(dataset2)
pointer1, pointer2 = 0
common_elements = 0
while pointer1 < size(dataset1) and pointer2 < size(dataset2):
if sorted[pointer1] == sorted[pointer2]:
common_elements+=1
pointer1+=1; pointer2+=1
else if sorted[pointer1] < sorted[pointer2]:
pointer1+=1
else:
pointer2+=1

Analysis:
– Number of times if statement executed?
– Increment a counter each time: |dataset1|+|dataset2|


Idea 2: Sorting

Analysis:
– Number of times if statement executed?
– Increment a counter each time: |dataset1|+|dataset2|

Running time:
– At most 100M + 100M comparisons
– At 1B comparisons/second ~ 0.2 seconds
– Plus cost of sorting! ~1 second per list
– Total time = 2.2 seconds


Reasoning About Running Times (1)

Worry about the computation as a function of input size:
– “If I double my input size, how much longer will it take?”
• Linear time (comparisons after sorting): twice as long!
• Quadratic time (scan): four (22) times as long
• Cubic time (very slow): 8 (23) time as long
• Exponential time (untenable):
• Sublinear time (uses sampling, skips over input)


Reasoning About Running Times (2)

Worry about the computation as a function of input size.
Worry about order of magnitude, not exact running time:
– Difference between 2 seconds and 4 seconds much smaller than
between 2 seconds and 3 months!
• The scan algorithm does more work in the while loop (but only a constant more
work) -- 3 comparisons instead of 1.
• Therefore, still call it linear time


Reasoning about running time

Worry about order of magnitude, not exact running time.

Captured by the Order notation: O(.)
– For an input of size n, approximately how long will it take?
– Scan: O(n2)
– Comparisons after sorted: O(n)


Reasoning about running time

Worry about order of magnitude, not exact running time.

Captured by the Order notation: O(.)
– For an input of size n, approximately how long will it take?
– Scan: O(n2)
– Comparisons after sorted: O(n)
– Sorting = O(n log n)
• Slightly more than n,
• But much less than n2.


Avoiding Sort: Hashing

Idea 3.
– Store each number in list1 in a location unique to it
– For each element in list2, check if its unique location is empty

[Blackboard]


Idea 3: Hashing

table = {}
for i in range(total):
table.add(dataset1[i])
common_elements = 0
for i in range(total):
if (table.has(dataset2[i])):
common_elements+=1

Analysis:
– Number of additions to the table: |dataset1|
– Number of comparisons: |dataset2|
– If Additions to the table and comparisons are 1B/second
– Total running time is: 0.2s


Lots of Details

Hashing, Sorting, Scanning:
– All have their advantages
– Scanning: in place, just passing through the data
– Sorting: in place (no extra storage), much faster
– Hashing: not in place, even faster


Lots of Details

Hashing, Sorting, Scanning:
– All have their advantages
– Scanning: in place, just passing through the data
– Sorting: in place (no extra storage), much faster
– Hashing: not in place, even faster

Reasoning about algorithms:
– Non trivial (and hard!)
– A large part of computer science
– Luckily mostly abstracted


Break


Distributed Computation

Working with large datasets:
– Most datasets are skewed
– A few keys are responsible for most of the data
– Must take skew into account, since averages are misleading


Additional Cost

Communication cost
– Prefer to do more on a single machine (even if it’s doing more work) to
constantly communicating

– Why? If you have 1000 machines talking to 1000 machines --- that’s
1M channels of communication
– The overall communication cost grows quadratically, which we have
seen does not scale...


Analysis at Scale


Doing the study

Suppose you had the data available. What would you do?

If you have a hypothesis:
– “Taking both Drug A and Drug B causes a side effect C”?


Doing the study

Look at the ratio of observed
symptoms over expected
- Expected: fraction of people who
took drug A and saw effect C.
A B - Observed: fraction of people who
took drugs A and B and saw effect C.

C


Doing the study

Look at the ratio of observed
symptoms over expected
- Expected: fraction of people who
took drug A and saw effect C.
A B - Observed: fraction of people who
took drugs A and B and saw effect C.

This is just counting!
C


Doing the study

Suppose you had the data available. What would you do?

Discovering hypotheses to test:
– Many pairs of drugs, some co-occur very often
– Some side effects are already known


Computational Social Science, Lecture 07: Counting Fast, Part I

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