Re: std::string and std::ostringstream performances

=?UTF-8?B?RXJpayBXaWtzdHLDtm0=?= <>
Wed, 31 Oct 2007 21:55:26 GMT
On 2007-10-31 21:58, Bala wrote:

On Oct 31, 3:21 pm, Erik Wikstr??m <> wrote:

On 2007-10-31 19:48, Bala wrote:

On Oct 31, 12:09 pm, "Jim Langston" <> wrote:

"Bala2508" <> wrote in message


I have a C++ application that extensively uses std::string and
std::ostringstream in somewhat similar manner as below

std::string msgHeader;

msgHeader = "<";
msgHeader += a;
msgHeader += "><";

msgHeader += b;
msgHeader += "><";

msgHeader += c;
msgHeader += ">";

Similarly it uses ostringstream as well and the function that uses
this gets called almost on every message that my application gets on
the socket. I am using this to precisely construct a XML Message to
be sent to another application.

What we observed when we ran a collect/analyzer on the application is
that it shows majority of the CPU spent in trying to deal with these 2
datatypes, their memory allocation using std::allocator and other
stuff. The CPU goes as high as 100% sometimes.

I would like to get an advice/suggestion on the following points
1. Is there a better way to use std::string / std::ostringstream than
the way I have been using it?
2. AM I using the wrong datatype for such kind of operations and
should move on to use something else? Any suggestions what the
datatype should be?

I eventually need these datatypes because the external library that I
am using to send this data out needs it in std::string /
std::ostringstream formats.

Would like to have some suggestions to bring down the CPU utilization.

One suggestion would be .reserve(). I E.
std::string msgHeader;
msgHeader.reserve( 100 );

That way the string msgHeader wouldn't need to try to allocate more memory
until it has used the initial 100 characters allocated. Some compilers are
better at preallocating a default number of bytes than others. Sometimes
they have to be given a hint. Figure out a good size to reserve (one big
enough where you won't need to be doing reallocatings, one small enough that
you're not running out of memory) and then try profiling it again and see if
it helps.- Hide quoted text -

I also clear the string using msgHeader.str("") method once i am done
with the sending of the message. Then again when this method gets
called, the same sequence of events happen. Wouldnt it clear the
allocated memory once i do a msgHeader.str("")? How do reserving
essentially help in this scenario?

To clear the string use clear() instead, that is what it is meant for.
clear() will not affect the capacity of the string so if you do
something like

  std::string str;

you will still be able to put 100 characters into the string before it
needs to reallocate.

Of course, if msgHeader is declared in the function that gets called it
will go out of scope when the function returns and will be reallocated
when it is called again, in which case a new string will be constructed
in which case the operations on the string will have not effect over two
different calls. If msgHeader on the other hand is external to the
function then you will probably benefit from using reserve. BTW, when
calling reserve() with an argument that is smaller than or equal to the
current capacity no action is taken.

Erik Wikstr??m- Hide quoted text -

- Show quoted text -

msgHeader is local to the function. And the maximum size would not be
more than a 100 bytes. So I plan to modify my code to use reserve and
clear as you suggested and will try a hand on the performance. I hope
it helps.

BTW, a general question.
If i dont use reserve and my function looks somewhat like this below

void somefunction ()
  std::string msgHeader
  msgHeader = "<";
  msgHeader += a;
  msgHeader += "><";

  msgHeader += b;
  msgHeader += "><";

  msgHeader += c;
  msgHeader += ">";

How is the actual memory allocation done? My understanding is that
the string library tries to reallocate memory on every statement.
That is, initially when it finds the statement "msgHeader = "<";", it
allocates say 1 byte to the msgHeader.
Then at the next statement it reallocates msgHeader as sizeof (a) +
current memory of msgHeader and so on.
Is this correct? If yes, then I am sure using reserve would improve
the performance dramatically.

I do not know, and I do not think the standard says anything about it.
But a good implementation will probably use a resizing scheme similar to
the one used for vectors, such as (at least) doubling the capacity every
time it resizes.

Erik Wikstr??m

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