Re: Counter-intuitive io vs no-io time readings

From:

Ian Collins <ian-news@hotmail.com>

Newsgroups:

comp.lang.c++

Date:

Thu, 10 Apr 2014 08:26:17 +1200

Message-ID:

<bqlon9F4creU1@mid.individual.net>

Ney Andr?? de Mello Zunino wrote:

Hello.

A discussion at work about how we might need to reconsider our
understanding about basic data structures and their impact on the
performance of algorithms has led me to start writing a simple
benchmarking program.

The full code is pasted below (still a work in progress) and consists
basically of an abstract base class /timed_task/ which takes care of
calling a virtual /do_run/ method and measuring the (wallclock) elapsed
time. The time readings are done with facilities from sys/time.h (I'm
running Ubuntu 13.10 64-bit).

The only test implemented so far consists of sequentially accessing
elements in an int vector and checking if their are odd or even (see
class /vec_seq_access/).

The test data is dynamically generated and consists of a hundred million
random numbers. The test data generation time is of no importance.

/*** BEGIN CODE ***/

<snip>

class vec_seq_access : public timed_task {
public:
    vec_seq_access(const std::vector<unsigned int>& data) : data(data) {
    }
protected:
    virtual void do_run() const {
        int odd_count = 0;
        int even_count = 0;
        for (const auto& i : data) {
            if (i % 2 != 0) ++odd_count; else ++even_count;
        }
        std::cout << odd_count << " odd numbers and " << even_count << " even
numbers.\n";

If this is the line you refer to later, I would expect an optimiser to
remove the body of this function if you remove this line.

<snip>

auto generate_random_data(int count) {

This auto declaration is missing the return type.

<snip>

As you can see, the average time is roughly 1.97 seconds per run. Now,
the counter-intuitive aspect mentioned in the subject came up when I
decided to remove the IO call from the test algorithm, expecting time
readings to decrease a bit. Here is the output with the IO stream
operation removed:

The one I highlighted?

Generating random data...
Done.
[TEST] Vector sequential access (5 runs)
Run 1: 2141563114 nanoseconds ~= 2.1416 seconds
Run 2: 2142123171 nanoseconds ~= 2.1421 seconds
Run 3: 2141130097 nanoseconds ~= 2.1411 seconds
Run 4: 2140915057 nanoseconds ~= 2.1409 seconds
Run 5: 2141052016 nanoseconds ~= 2.1411 seconds

Surprisingly, the average run time has actually increased to about 2.14
seconds. Surely, I must be missing something or maybe there's some weird
combination of factors leading to this outcome. Can anybody see what's
going on?

Probably the latter! Did you look at the generated assembly code?

Compiler: g++ 4.8.1
Compilation command: g++ --std=c++1y -o ds-perf ds-perf.cpp

--std=c++1y ?

--
Ian Collins