Thursday, August 6, 2015

Digest for comp.lang.c++@googlegroups.com - 11 updates in 3 topics

legalize+jeeves@mail.xmission.com (Richard): Aug 06 05:56PM

[Please do not mail me a copy of your followup]
 
c.milanesi.bg@gmail.com spake the secret code
 
>I recently released the open source library "cpp-mmf":
>https://github.com/carlomilanesi/cpp-mmf
 
There's already a boost library that covers this.
 
<http://www.boost.org/doc/libs/1_58_0/libs/iostreams/doc/classes/mapped_file.html>
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Victor Bazarov <v.bazarov@comcast.invalid>: Aug 06 02:31PM -0400

On 8/6/2015 1:56 PM, Richard wrote:
>> https://github.com/carlomilanesi/cpp-mmf
 
> There's already a boost library that covers this.
 
> <http://www.boost.org/doc/libs/1_58_0/libs/iostreams/doc/classes/mapped_file.html>
 
Did you read past the second line of the announcement?
 
The OP wrote:
>> It has the same purpose and the same run-time speed of the
>> "Memory-Mapped Files" Boost library, but it offers the following
>> advantages: [...]
 
V
--
I do not respond to top-posted replies, please don't ask
woodbrian77@gmail.com: Aug 05 10:24PM -0700

On Wednesday, August 5, 2015 at 3:12:27 AM UTC-5, Juha Nieminen wrote:
 
> > http://stackoverflow.com/questions/21946447/how-much-performance-difference-when-using-string-vs-char-array
 
> They are not doing the same thing. Formatted printing into a static buffer
> is not the same thing as building a dynamic string by appending stuff.
 
How about comparing these two programs:
 
#include <string.h>
 
int main ()
{
for (int i = 0; i < 4000000; ++i){
char s[200];
::strncpy(s, "We few; we happy few; we band of brothers.", sizeof(s));
}
}
 
and
 
#include <string>
 
int main ()
{
for (int i = 0; i < 4000000; ++i){
::std::string s = "We few; we happy few; we band of brothers.";
}
}
 
uname -a
Linux localhost.localdomain 4.0.4-301.fc22.x86_64 #1 SMP Thu May 21 13:10:33 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux
 
g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-redhat-linux/5.1.1/lto-wrapper
Target: x86_64-redhat-linux
Configured with: ../configure --enable-bootstrap --enable-languages=c,c++,objc,obj-c++,fortran,ada,go,lto --prefix=/usr --mandir=/usr/share/man --infodir=/usr/share/info --with-bugurl=http://bugzilla.redhat.com/bugzilla --enable-shared --enable-threads=posix --enable-checking=release --enable-multilib --with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions --enable-gnu-unique-object --enable-linker-build-id --with-linker-hash-style=gnu --enable-plugin --enable-initfini-array --disable-libgcj --with-default-libstdcxx-abi=c++98 --with-isl --enable-libmpx --enable-gnu-indirect-function --with-tune=generic --with-arch_32=i686 --build=x86_64-redhat-linux
Thread model: posix
gcc version 5.1.1 20150422 (Red Hat 5.1.1-1) (GCC)
 
 
size oldschool stdstr
text data bss dec hex filename
1504 596 4 2104 838 oldschool
1875 604 40 2519 9d7 stdstr
 
 
The std::string version is over 4 times slower than the
old school version on this machine and even slower on a
FreeBSD machine.
 
time ./oldschool
 
real 0m0.085s
user 0m0.084s
sys 0m0.002s
 
time ./stdstr
 
real 0m0.365s
user 0m0.363s
sys 0m0.003s
 
 
 
Brian
Ebenezer Enterprises - In G-d we trust.
http://webEbenezer.net
Ian Collins <ian-news@hotmail.com>: Aug 06 05:42PM +1200

> ::std::string s = "We few; we happy few; we band of brothers.";
> }
> }
 
A fairer comparison would be
 
int main ()
{
std::string s;
for (int i = 0; i < 4000000; ++i){
s = "We few; we happy few; we band of brothers.";
}
}
 
--
Ian Collins
woodbrian77@gmail.com: Aug 05 10:53PM -0700

On Thursday, August 6, 2015 at 12:42:31 AM UTC-5, Ian Collins wrote:
> s = "We few; we happy few; we band of brothers.";
> }
> }
 
Here's the time for that version.
 
time ./stdstr
 
real 0m0.120s
user 0m0.118s
sys 0m0.001s
 
In most cases I can't lift a std::string out of the loop like that
though.
woodbrian77@gmail.com: Aug 05 11:32PM -0700

On Wednesday, August 5, 2015 at 10:03:59 AM UTC-5, Bo Persson wrote:
> > a number of std::string objects to arrays of char in your application.
 
> And you don't think that actually opening the file will hide the
> difference (being fractions of a microsecond)?
 
I'm doing what I can to make things run fast. This is trading
some space for some speed. That seems like a good trade off to me.
I can't do much to make opening files faster.
 
If you don't have a lot of money for hardware, software optimizations
can help.
 
Brian
Ebenezer Enterprises - Bless G-d, America.
http://webEbenezer.net
"Öö Tiib" <ootiib@hot.ee>: Aug 06 12:20AM -0700

> sys 0m0.001s
 
> In most cases I can't lift a std::string out of the loop like that
> though.
 
We run profiler on full application. We do not write code that has no
input nor output in tight loops. Therefore such tests are deceiving.
On all cases of composing multi-megabyte XML, JSON or the like (why else
you need to copy 4 millions of texts in tight loop?) that 'strncat' and
the like lose to 'string::append' and the like.
 
We can win both by hand optimizing usage of bytes (in vector of bytes)
for concrete case but that does give minor returns because the real
bottle-neck is in transfer of data. So if we instead use the effort of
that raw byte level tinkering to find a way to reduce the data transferred
by half then the program is twice faster.
Christopher Pisz <nospam@notanaddress.com>: Aug 06 10:10AM -0500

> sys 0m0.001s
 
> In most cases I can't lift a std::string out of the loop like that
> though.
 
A fairer comparison would be
 
int main ()
{
std::string s;
s.reserve(200);
for (int i = 0; i < 4000000; ++i)
{
s = "We few; we happy few; we band of brothers.";
}
}
 
 
 
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---
Bo Persson <bop@gmb.dk>: Aug 06 05:29PM +0200

On 2015-08-06 07:42, Ian Collins wrote:
> s = "We few; we happy few; we band of brothers.";
> }
> }
 
Or try doing this with C strings:
 
int main ()
{
std::string s;
for (int i = 0; i < 4000000; ++i){
s += "We few; we happy few; we band of brothers.";
}
}
 
 
 
Bo Persson
woodbrian77@gmail.com: Aug 06 09:38AM -0700

On Thursday, August 6, 2015 at 10:29:38 AM UTC-5, Bo Persson wrote:
> s += "We few; we happy few; we band of brothers.";
> }
> }
 
No thanks. I don't want to totally stop using std::string, but
I have some cases where I'm not appending anything to the initial
value.
 
Previously I was more comfortable using std::unique_ptr than
I am now. I still use unique_ptr, but not as much as before.
This is similar with std::string.
 
 
Brian
Ebenezer Enterprises
http://webEbenezer.net
"Osmium" <r124c4u102@comcast.net>: Aug 06 10:23AM -0500

"red floyd" wrote:
 
>> another so I thought I'd give it a shot here.
 
> I wanted to buy some meat, but the butcher was closed, so I tried to buy
> some at the greengrocer's instead.
 
He followed the most obvious alternative he could think of. And it worked
for him, didn't it? I wasn't paying attention, nevertheless I saw at least
three useful answers.
 
I'm happy for him and wish him well.
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