Date: Sat, 22 Apr 2006 17:30:22 -0700 From: Don Dugger <dugger@hotlz.com> To: freebsd-chat@freebsd.org Cc: Benjamin Lutz <benlutz@datacomm.ch> Subject: Re: Why is not more FreeBSD software written in C++? Message-ID: <444ACA9E.5060602@hotlz.com> In-Reply-To: <200604230203.05209.benlutz@datacomm.ch> References: <200604221511.k3MFBxa2003218@mist.nodomain> <200604230203.05209.benlutz@datacomm.ch>
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Am I missing something here? What's the difference between test_iostream.cpp
and test_string.cpp?
Don 8)
Benjamin Lutz wrote:
>On Saturday 22 April 2006 17:11, Dan Strick wrote:
>
>
>>On Friday 21 Apr 2006 09:20, Don Dugger wrote:
>>
>>
>>>The example above is not exactly a realworld example. Even if you stick
>>>to plain C, a repeated putchar(' ') is 1-2 orders of magnitude slower
>>>than aggregating those characters and write()'ing them every few dozen
>>>chars.
>>>
>>>
>>This might seem obvious, but is it really true? I wrote a program that
>>tries it both ways with the output redirected to /dev/null and discovered
>>that filling a 24 character buffer and doing a write() takes about 11 times
>>as much cpu time as 24 putchar()s. Perhaps a buffer larger than a "few
>>dozen chars" would be useful. There must be a moral here somewhere. :-)
>>
>>
>
>Yes, it is really true. Since I don't know what your test is, I've created my
>own.
>
>test_putchar.c:
>----
>#include <stdlib.h>
>#include <stdio.h>
>int main(int argc, char** argv) {
> int i, j;
> for (i = 0; i < 50000; i++) {
> for (j = 0; j < 60; j++) {
> putchar('.');
> }
> putchar('\n');
> }
> return 0;
>}
>----
>
>test_write.c:
>----
>#include <stdlib.h>
>#include <unistd.h>
>int main(int argc, char** argv) {
> char buf[61];
> int i, j;
> for (i = 0; i < 50000; i++) {
> for (j = 0; j < 60; j++) {
> buf[j] = '.';
> }
> buf[60] = '\n';
> write(STDOUT_FILENO, buf, 61);
> }
> return 0;
>}
>----
>And for completeness' sake, two corresponding C++ iostream variants:
>test_iostream.cpp:
>----
>#include <iostream>
>using namespace std;
>int main(int argc, char** argv) {
> for (int i = 0; i < 50000; i++) {
> for (int j = 0; j < 60; j++) {
> cout << '.';
> }
> cout << '\n';
> }
> return 0;
>}
>----
>
>test_string.cpp
>----
>#include <iostream>
>using namespace std;
>int main(int argc, char** argv) {
> for (int i = 0; i < 50000; i++) {
> for (int j = 0; j < 60; j++) {
> cout << '.';
> }
> cout << '\n';
> }
> return 0;
>}
>----
>
>All files are compiled with gcc/g++ and the -O2 option. I'm running them on a
>Mac Mini under Linux.
>
>And the results are (I've run that command several times and picked a line
>that shows an average run time):
>
>$ time ./test_putchar > /dev/null
>./test_putchar > /dev/null 0.39s user 0.00s system 97% cpu 0.397 total
>$ time ./test_write > /dev/null
>./test_write > /dev/null 0.02s user 0.01s system 82% cpu 0.029 total
>$ time ./test_iostream > /dev/null
>./test_iostream > /dev/null 0.94s user 0.00s system 99% cpu 0.950 total
>$ time ./test_string > /dev/null
>./test_string > /dev/null 0.21s user 0.00s system 97% cpu 0.213 total
>
>
>That quite clearly shows that my prediction that using putchar is "1-2 orders
>of magnitude slower" is accurate. It also shows the points you and others
>have made that iostream is slower than C's I/O mechanisms.
>
>If we take the two most sensible variants, test_write and test_iostream, and
>have them write into a file, we get these numbers:
>
>$ time ./test_write > foo
>./test_write > foo 0.01s user 0.16s system 95% cpu 0.180 total
>$ time ./test_string > foo
>./test_string > foo 0.20s user 0.02s system 98% cpu 0.223 total
>
>Which is a small enough difference to be irrelevant. The large I/O performance
>advantage of the write(2) variant over the C++ string variant vanishes as
>soon as the code actually does something remotely useful.
>
>
>
>>I am not claiming that strings and iostreams are bad. I am observing
>>that some reasonable programs that use these facilities will run very
>>slowly and I am suggesting that it is not always obvious which programs
>>these are.
>>
>>
>
>You mentioned a pretty printer. I'm not convinced that the performance
>drawbacks you saw were inherent in iostream. Rather it seems that the comfort
>of C++'s strings and streams led the programmer to program in a less
>performance-conscious way.
>
>
>
>>I am also expressing disappointment because I want the
>>decision to use any feature of C++ or its standard library to be a
>>no-brainer.
>>
>>
>
>Personally, I don't hesitate to use features of C++. As for the STL... there
>are nice alternatives that are sometimes available. When writing Qt programs,
>I'll use Qt's data structures only, not the STL ones, since imo Qt's data
>structure classes are more comfortable to use.
>
>Cheers
>Benjamin
>
>
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