stack alignment issues (was: unbelievable benchmark output)
* Michal Mertl [EMAIL PROTECTED] [020203 08:17] wrote: I wrote a simple program which does this: gettimeofday something (takes several seconds) gettimeofday print time elapsed Several runs of the program take about the same time but the time changes wildly when the executable is called differently. --- ./xx/xxx 5 s xx/xxx 9 s and similar. It holds true on vastly different machines with current and stable. The only thing which I can think of that can be causing this is some memory alignment issue. It sure looks like an alignment issue. If you print the address of 'i' and 'j' in the attached program you can see for the fast case they are aligned to 8 byte boundries, but when it's slow they are at an address that is a multiple of 4 but not 8. Not really sure what to make of this, anyone else know how we ought to fix this? -Alfred To Unsubscribe: send mail to [EMAIL PROTECTED] with unsubscribe freebsd-hackers in the body of the message
Re: stack alignment issues (was: unbelievable benchmark output)
On Sun, 3 Feb 2002 08:59:41 -0800 Alfred Perlstein [EMAIL PROTECTED] wrote: Hi, It sure looks like an alignment issue. If you print the address of 'i' and 'j' in the attached program you can see for the fast case they are aligned to 8 byte boundries, but when it's slow they are at an address that is a multiple of 4 but not 8. Agreed, my bet is on data alignment. Not really sure what to make of this, anyone else know how we ought to fix this? Well, you could always malloc() some memory and make sure your data is in an address that is multiple of 8. You'll waste some mem but will gain performance. I actually haven't tried it on FreeBSD but it's a trick I used to do on the Amiga some years ago. Cheers, -- Miguel Mendez - [EMAIL PROTECTED] Public Key :: http://energyhq.homeip.net/files/pubkey.txt EnergyHQ :: http://energyhq.homeip.net FreeBSD - The power to serve! To Unsubscribe: send mail to [EMAIL PROTECTED] with unsubscribe freebsd-hackers in the body of the message
Re: stack alignment issues (was: unbelievable benchmark output)
In the last episode (Feb 03), Alfred Perlstein said: * Michal Mertl [EMAIL PROTECTED] [020203 08:17] wrote: Several runs of the program take about the same time but the time changes wildly when the executable is called differently. The only thing which I can think of that can be causing this is some memory alignment issue. It sure looks like an alignment issue. If you print the address of 'i' and 'j' in the attached program you can see for the fast case they are aligned to 8 byte boundries, but when it's slow they are at an address that is a multiple of 4 but not 8. Not really sure what to make of this, anyone else know how we ought to fix this? This has actually been an issue for ages, most commonly seen with doubles. take a look at the thread at http://www.freebsd.org/cgi/getmsg.cgi?fetch=393691+0+/usr/local/www/db/text/2000/freebsd-current/2507.freebsd-current or, easier to read the entire thread: http://groups.yahoo.com/group/freebsd-current/messages/39583?threaded=1 -- Dan Nelson [EMAIL PROTECTED] To Unsubscribe: send mail to [EMAIL PROTECTED] with unsubscribe freebsd-hackers in the body of the message
Re: stack alignment issues (was: unbelievable benchmark output)
In message [EMAIL PROTECTED], Dan Nelson cleopede: In the last episode (Feb 03), Alfred Perlstein said: * Michal Mertl [EMAIL PROTECTED] [020203 08:17] wrote: Not really sure what to make of this, anyone else know how we ought to fix this? This has actually been an issue for ages, most commonly seen with doubles. take a look at the thread at Has any real world program ever been significantly affected by this problem? Greg Shenaut To Unsubscribe: send mail to [EMAIL PROTECTED] with unsubscribe freebsd-hackers in the body of the message
