Re: Problem with STL vector peformance, benchmarks included

From:
peter koch <peter.koch.larsen@gmail.com>
Newsgroups:
comp.lang.c++
Date:
30 Apr 2007 13:12:44 -0700
Message-ID:
<1177963964.443915.88700@h2g2000hsg.googlegroups.com>
On 30 Apr., 21:53, StephQ <askmeo...@mailinator.com> wrote:

On Apr 30, 7:58 pm, peter koch <peter.koch.lar...@gmail.com> wrote:

On 30 Apr., 17:35, StephQ <askmeo...@mailinator.com> wrote:

On Apr 30, 4:32 pm, StephQ <askmeo...@mailinator.com> wrote:

Have you turned off checked iterators? (see:http://www.codeproject.com/vcpp/stl/checkediterators.asp)


Thank you for very usefull suggestion. I didn't know that checked
iterators were turned on even in release mode in vc8 by default.

The new results (with checked iterators turned off) are:

Microsoft:
vector: 94
array: 94
stupid: 94
ptr: 141
ptr: 96

Intel:
vector: 141
array: 141 //62 if I eanble SSE2
stupid: 141 //62 if I enable SSE2 and disable exception handling
ptr: 141
ptr: 140

The situation is now much better.
Howere is seems that the Microsofr compiler is still doing 35% better
in all the situations except the "vector iterator" one.

Do you have any other suggestion to try?
I know nothing of lowe level instructions, but if I post the
"assembler - like" code here would it be of any help for you?

Thank you

Cheers
StephQ


I reply to myself just to tell you that I don't mind investigating any
more these issues.
I ran the test using doubles instead of int and the results are very
similar, with the microsoft compiler having something like 3% more
performance.

However the Stepanov Abstraction test favours the intel compiler by a
large margin.
Abstraction penalty with Intel:
0.85
0.68 with sse2

With Microsoft:
1.11

A curiosity..... how is it possible to get an abstraction penalty
below 1 ?


Perhaps because you had a bad test? Rerun the benchmarks more than one
time and remember that caching has a huge effect on results (I believe
a factor of ten is quite normal). So you should know how to e.g. clear
(or fill) the cache as appropriate.
Writing a good benchmark is not easy.

/Peter


I'm quite a newbie....
Do you suggest that the initial run is the "right" one, while
subsequent runs get distrorted by caching or the opposite thing?
By caching you mean that the objects of interests are loaded in the L1/
L2 cache right?

Yes.

But I obtained these results in a stable way with different runs....

I remember that caching influences the results of subsequent runs of
benchmarks, but I don't understand why. Isn't cache/memory freed after
the software exit?

No. Caching takes place at the hardware level, so no freeing takes
place just as freeing memory does not remove physical memory.

Anyway I increased the number of calculations in the test becouse it
was taking too few time to run.


Right. But try to follow Roland Pibingers advice and see if that
explains anything.

/Peter

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