473,383 Members | 1,892 Online
Bytes | Software Development & Data Engineering Community
Post Job

Home Posts Topics Members FAQ

Join Bytes to post your question to a community of 473,383 software developers and data experts.

Cannot optimize 64bit Linux code

I am porting a piece of C code to 64bit on Linux. I am using 64bit
integers. It is a floating point intensive code and when I compile
(gcc) on 64 bit machine, I don't see any runtime improvement when
optimizing -O3. If I construct a small program I can get significant
(>4x) speed improvement using -O3 versus -g. If I compile on a 32 bit
machine, it runs 5x faster on the 64 bit machine than does the 64bit
compiled code.

It seems like something is inhibiting the optimization. Someone on
comp.lang.fortran suggested it might be an alignment problem. I am
trying to go through and eliminate all 32 bit integers righ now (this
is a pretty large hunk of code). But thought I would survey this
group, in case it is something naive I am missing.

Any opinion is welcomed. I really need this to run up to speed, and I
need the big address space. Thanks in advance.

Dick
Feb 28 '08 #1
4 3275
Thanks for all the hints and thoughts.

My small program is:

main()
{
struct timespec ts;
double x,y;
int i;
long long n;
n=15000000;
n *= 10000;
fprintf(stderr,"LONG %Ld\n",n);
/*
clock_gettime(CLOCK_THREAD_CPUTIME_ID, &ts);
*/
printf(" _POSIX_THREAD_CPUTIME _POSIX_CPUTIME %d %d\n",
_POSIX_THREAD_CPUTIME
,_POSIX_CPUTIME);
clock_gettime(CLOCK_THREAD_CPUTIME_ID, &ts);
n = ts.tv_nsec;
fprintf(stderr,"Before %d sec %d nsec\n",ts.tv_sec,ts.tv_nsec);
fprintf(stderr,"Before %d sec %Ld nsec\n",ts.tv_sec,ts.tv_nsec);
y=3.3;
for(i=0;i<111100000;i++) {
x=sqrt(y);
y += 1.0;
}
clock_gettime(CLOCK_THREAD_CPUTIME_ID, &ts);
fprintf(stderr,"After %d sec %d nsec\n",ts.tv_sec,ts.tv_nsec);
fprintf(stderr,"After %d sec %Ld nsec\n",ts.tv_sec,ts.tv_nsec-n);
}

It shows considerable improvement with -O3.

I think the problem is something less esoteric than the cache,
wordsize, etc. One thing I didn't say, I have multi threading loaded,
though no new threads are created by these runs. I have tried a newer
redhat, have not tried Intel compilers.

Dick
Mar 1 '08 #2
I think I misspoke on my timer program. That one was used to attempt
to measure thread time. You can remove the references to the timers
and run it. It only shows about a 2x improvement on optimization.

The large difference I have actually seen is 32bit compile on another
machine, run on 64bit machine (12sec) versus 64bit code compiled on
64bit machine (70sec).

Sorry for the confusion.

Dick
Mar 1 '08 #3
In article <ee**********************************@s19g2000prg. googlegroups.com>,
Dick Dowell <di*********@avagotech.comwrote:
>Thanks for all the hints and thoughts.
>My small program is:
>main()
{
struct timespec ts;
double x,y;
int i;
long long n;
n=15000000;
n *= 10000;
fprintf(stderr,"LONG %Ld\n",n);
/*
clock_gettime(CLOCK_THREAD_CPUTIME_ID, &ts);
*/
printf(" _POSIX_THREAD_CPUTIME _POSIX_CPUTIME %d %d\n",
_POSIX_THREAD_CPUTIME
,_POSIX_CPUTIME);
clock_gettime(CLOCK_THREAD_CPUTIME_ID, &ts);
n = ts.tv_nsec;
fprintf(stderr,"Before %d sec %d nsec\n",ts.tv_sec,ts.tv_nsec);
fprintf(stderr,"Before %d sec %Ld nsec\n",ts.tv_sec,ts.tv_nsec);
y=3.3;
for(i=0;i<111100000;i++) {
x=sqrt(y);
y += 1.0;
}
clock_gettime(CLOCK_THREAD_CPUTIME_ID, &ts);
fprintf(stderr,"After %d sec %d nsec\n",ts.tv_sec,ts.tv_nsec);
fprintf(stderr,"After %d sec %Ld nsec\n",ts.tv_sec,ts.tv_nsec-n);
}
>It shows considerable improvement with -O3.
You do not do anything with x after you compute it. Any good
optimizer would optimize away the x=sqrt(y) statement. Once that
is done, the optimizer could even eliminate the loop completely
and replace it by y += 111100000. Compilers that did the one or
both of these optimizations would result in much faster code than
compilers that did not. Your problem might have nothing to do
with 64 bit integers and everything to do with which optimizations
the compiler performs.
--
"The human mind is so strangely capricious, that, when freed from
the pressure of real misery, it becomes open and sensitive to the
ideal apprehension of ideal calamities." -- Sir Walter Scott
Mar 2 '08 #4
Thanks for all the suggestions. I've discovered the ineffectiveness
of optimization is data dependent. I managed to profile the code and
78% of the runtime is spent in something called

_mul [1] (from gprof output, the [1] just means #1 cpu user)

Here's another line from gprof report
granularity: each sample hit covers 4 byte(s) for 0.01% of 109.71
seconds

index % time self children called name
<spontaneous>
[1] 78.0 85.55 0.00 __mul [1]
-----------------------------------------------

Dick
Mar 10 '08 #5

This thread has been closed and replies have been disabled. Please start a new discussion.

Similar topics

0
by: Wen Jiang | last post by:
Hi, Has anyone been able to get pymat work on a 64bit linux system? I compiled the CVS version of pymat with python2.3/matlab7.01 on Mandrake10.1 64bit system, it can start the engine and use...
8
by: Vijay | last post by:
Hi all, Im using gcc version 3.2.3 20030502 (Red Hat Linux 3.2.3-20) on 64bit linux server im trying to compile following code --------------------sam.cpp--------------------- #include...
8
by: dwelch91 | last post by:
I need to detect whether the operating system I am running on (not the Python version) is 64bit or 32bit. One requirement is that I need to include support for non-Intel/AMD architectures. The 2...
19
by: llothar | last post by:
I must say i didn't expect this. I just did some measures on FreeBSD 6.2 with gcc 3.4.6 and there is absolutely no significant difference between 32 and 64 bit mode - neither in compilation speed,...
1
by: Iros | last post by:
Could anyone recommend debuggers for 64bit Linux platforms, SLES 10 in particular? I'm a Firebug addict but had no luck getting it up and running. Thanks!
18
by: cman | last post by:
Hi guys, why does this fail raising bad_alloc int *v = new int ; if this succeeds int *v = (int *) malloc((unsigned)6000000000) both on the same machine, same compiler g++, 64bit linux red...
0
by: Leo Park | last post by:
Hello, guys. I want to compile 32bit *.c and *.cc files On 64bit x86_64 systems. But, it is not work. ...
5
by: Neetu | last post by:
Hi Everybody, Our reports works well on every machine but not on 64bit systems. We can view the report but when we press print button it does nothing, it just stay there and prompt nothing, even...
9
by: Aditi | last post by:
I am working around a problem called Y2038 bug....
1
by: CloudSolutions | last post by:
Introduction: For many beginners and individual users, requiring a credit card and email registration may pose a barrier when starting to use cloud servers. However, some cloud server providers now...
0
by: Faith0G | last post by:
I am starting a new it consulting business and it's been a while since I setup a new website. Is wordpress still the best web based software for hosting a 5 page website? The webpages will be...
0
by: ryjfgjl | last post by:
In our work, we often need to import Excel data into databases (such as MySQL, SQL Server, Oracle) for data analysis and processing. Usually, we use database tools like Navicat or the Excel import...
0
by: ryjfgjl | last post by:
If we have dozens or hundreds of excel to import into the database, if we use the excel import function provided by database editors such as navicat, it will be extremely tedious and time-consuming...
0
by: ryjfgjl | last post by:
In our work, we often receive Excel tables with data in the same format. If we want to analyze these data, it can be difficult to analyze them because the data is spread across multiple Excel files...
0
by: emmanuelkatto | last post by:
Hi All, I am Emmanuel katto from Uganda. I want to ask what challenges you've faced while migrating a website to cloud. Please let me know. Thanks! Emmanuel
1
by: nemocccc | last post by:
hello, everyone, I want to develop a software for my android phone for daily needs, any suggestions?
1
by: Sonnysonu | last post by:
This is the data of csv file 1 2 3 1 2 3 1 2 3 1 2 3 2 3 2 3 3 the lengths should be different i have to store the data by column-wise with in the specific length. suppose the i have to...
0
by: Hystou | last post by:
There are some requirements for setting up RAID: 1. The motherboard and BIOS support RAID configuration. 2. The motherboard has 2 or more available SATA protocol SSD/HDD slots (including MSATA, M.2...

By using Bytes.com and it's services, you agree to our Privacy Policy and Terms of Use.

To disable or enable advertisements and analytics tracking please visit the manage ads & tracking page.