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# Memory leak somewhere?

My code is using massive amounts of memory (eventually), which I don't think should be the case. The memory use slowly increases until my computer runs out of memory. I tried enabling garbage collection, but that didn't help (or didn't help enough). I don't see any reason why this should use more and more memory. It takes somewhere between 5 and 10 hours for this program to use up my 16 GB of memory, but that memory user is increasing is clear quickly.

import numpy

trials = 5800
length = 5000
mean = 0
results = [0]*trials
data = [0]*length

for i in range(trials):
data[0] = numpy.random.normal(0,1)
for j in range(1, length):
data[j] = data[j-1] + numpy.random.normal(0,1)
for k in range(2, length):
if data[k] > 2*sqrt(k*log(log(k+1))):
results[i] += 1
mean += results[i]

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## 2 Answers

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We should inspect further, but there is indeed an issue. Thanks for reporting ! It has to do with the symbolic computations. Indeed, when you write 2*sqrt(k*log(log(k+1))), since k+1 is an integer, the computation is done symbolically, and not numerically, which takes more time. The quick way to avoid this is to add a dot . after the 1 to specify that it is a floating-point number and not an integer (anyway, you will compare with a float at the end):

So, just replace :

        if data[k] > 2*sqrt(k*log(log(k+1))):


with

        if data[k] > 2*sqrt(k*log(log(k+1.))):


What i do not understant though, is that this solution, while much faster than the initial code (and not leaking anymore), is still much slower than running the code from ipython (and adding the line from math import sqrt, log at the beginning), while the only remaining difference is about coercion.

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## Comments

1

sqrt and log in sage are wrappers that need to investigate their arguments to decide what to do. So with numerical arguments they are slower that the math sqrt and log, just for that reason. Furthermore, for numerical input they probably end up using mpfr rather than the math library, so that bit is probably slower too (but perhaps numerically a little more robust, and at least capable of dealing with multiprecision)

( 2016-08-07 23:17:38 +0200 )edit

As a complement to @tmonteil's answer, some hints about speed.

To gain speed, you can

• avoid Sage's preparser transforming Python ints into Sage Integers
• avoid conversions and coercions that occur when you add, multiply, compare numbers of different types
• avoid creating lists you don't need; use iterators instead
• use methods instead of functions, eg for log and sqrt
• square instead of taking square root
• set apart some cases where you can save yourself some computation and comparison.

Here are four variants of a function inspired by your code, with 'trials' and 'length' as arguments.

import numpy

def compute_a(trials, length):
mean = 0
results = [0]*trials
data = [0]*length
for i in range(trials):
data[0] = numpy.random.normal(0,1)
for j in range(1, length):
data[j] = data[j-1] + numpy.random.normal(0,1)
for k in range(2, length):
if data[k] > 2*sqrt(k*log(log(k+1))):
results[i] += 1
mean += results[i]
return mean

def compute_b(trials, length):
mean = 0
results = [0]*trials
data = [0]*length
for i in range(trials):
data[0] = numpy.random.normal(0,1)
for j in range(1, length):
data[j] = data[j-1] + numpy.random.normal(0,1)
for k in range(2, length):
if data[k] > 2*sqrt(k*log(log(k+1.))):
results[i] += 1
mean += results[i]
return mean

def compute_c(trials, length):
two = RDF(2)
mean = 0
results = [0]*trials
data = [0]*length
for i in xrange(trials):
data[0] = numpy.random.normal(0,1)
for j in range(1, length):
data[j] = data[j-1] + numpy.random.normal(0,1)
for k in range(2, length):
if RDF(data[k]) > two*(RDF(k)*RDF(k+1).log().log()).sqrt():
results[i] += 1
mean += results[i]
return mean

def compute_d(trials, length):
zero = RDF.zero()
four = RDF(4r)
mean = 0r
results = [0r]*trials
data = [0r]*length
for i in xrange(trials):
data[0r] = numpy.random.normal(0r,1r)
for j in range(1r, length):
data[j] = data[j-1r] + numpy.random.normal(0r,1r)
for k in range(2r, length):
a = RDF(data[k])
if a > zero and a*a > four*RDF(k)*RDF(k+1r).log().log():
results[i] += 1r
mean += results[i]
return mean

• compute_a is just making your code into a function.
• compute_buses @tmonteil's trick of k+1. so that comparisons are done in RR rather than SR.
• compute_c makes sure that all multiplications, logs, sqrts, comparisons are in RDF; we also use xrange instead of range to avoid creating a list we don't need.
• compute_d tries to avoid Sage's preparser, and to avoid square roots; we also avoid a lot of computation when data[k] < 0, because we don't even have to compute the right-hand side in that case.

For (trials, length) = (2000, 2000), I get timings of roughly: (a) 4 s, (b) 40 ms, (c) 5 ms, (d) 2 ms.

sage: timeit('compute_a(2000,2000)')
5 loops, best of 3: 3.99 s per loop ...
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Asked: 2016-08-07 20:04:22 +0200

Seen: 896 times

Last updated: Aug 09 '16