Re: Syncing cpus on a multi-cpu, dual core system
In message: [EMAIL PROTECTED] M. L. Dodson [EMAIL PROTECTED] writes: : On a computational chemistry list I subscribe to there is a : current thread about multi-cpu systems needing to have the cpu : frequencies synced (this is in a Linux context). This is : evidently not just having the cpus running at nominally the same : frequency but something else in addition. A posting in the thread : said variations less than 0.1% were not problematic. However, the : poster said it was an issue in a dual cpu, dual core system he had : set up. : : My questions are: : 1. Is this real or an urban legend? : 2. If real, is this a Linuxism or is FreeBSD affected as well? : 3. How do you sync the cpus, if it is needed? : 4. anything else some one wants to expound on along this line. Linux keeps the cpu's frequencies 'synchronized' so that it can use the fast time keeping hardware (TSC). FreeBSD uses different mechanisms for its timekeeping, so doesn't need to keep them in sync at all, and doesn't even try at this point. Maybe this is what they are talking about... Warner ___ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to [EMAIL PROTECTED]
Re: Syncing cpus on a multi-cpu, dual core system
M. Warner Losh wrote: In message: [EMAIL PROTECTED] M. L. Dodson [EMAIL PROTECTED] writes: : On a computational chemistry list I subscribe to there is a : current thread about multi-cpu systems needing to have the cpu : frequencies synced (this is in a Linux context). This is : evidently not just having the cpus running at nominally the same : frequency but something else in addition. A posting in the thread : said variations less than 0.1% were not problematic. However, the : poster said it was an issue in a dual cpu, dual core system he had : set up. : : My questions are: : 1. Is this real or an urban legend? : 2. If real, is this a Linuxism or is FreeBSD affected as well? : 3. How do you sync the cpus, if it is needed? : 4. anything else some one wants to expound on along this line. Linux keeps the cpu's frequencies 'synchronized' so that it can use the fast time keeping hardware (TSC). FreeBSD uses different mechanisms for its timekeeping, so doesn't need to keep them in sync at all, and doesn't even try at this point. Maybe this is what they are talking about... Warner One thing I have noticed with FreeBSD is that if I am running a program that multithreads and creates and destroys threads a lot (e.g. the fftw3 port), then top underreports significantly - that is on a 4 processor system it might report 60% (or even 0%) cpu usage, when it is clear from the TIME field that it is closer to 250% cpu usage. The other thing I have noticed is that when I split jobs using threads so that I can use several processors, the speed up to the program is far less than one might expect - indeed sometimes it even gets slower. Stephen ___ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to [EMAIL PROTECTED]
Re: Syncing cpus on a multi-cpu, dual core system
- Original Message From: Stephen Montgomery-Smith [EMAIL PROTECTED] ...snip snip... The other thing I have noticed is that when I split jobs using threads so that I can use several processors, the speed up to the program is far less than one might expect - indeed sometimes it even gets slower. Stephen - This is probably Amdahl's law. Simon __ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com ___ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to [EMAIL PROTECTED]
Re: Syncing cpus on a multi-cpu, dual core system
On Saturday 16 December 2006 12:30, M. Warner Losh wrote: In message: [EMAIL PROTECTED] M. L. Dodson [EMAIL PROTECTED] writes: : On a computational chemistry list I subscribe to there is a : current thread about multi-cpu systems needing to have the cpu : frequencies synced (this is in a Linux context). This is : evidently not just having the cpus running at nominally the same : frequency but something else in addition. A posting in the thread : said variations less than 0.1% were not problematic. However, the : poster said it was an issue in a dual cpu, dual core system he had : set up. : : My questions are: : 1. Is this real or an urban legend? : 2. If real, is this a Linuxism or is FreeBSD affected as well? : 3. How do you sync the cpus, if it is needed? : 4. anything else some one wants to expound on along this line. Linux keeps the cpu's frequencies 'synchronized' so that it can use the fast time keeping hardware (TSC). FreeBSD uses different mechanisms for its timekeeping, so doesn't need to keep them in sync at all, and doesn't even try at this point. Maybe this is what they are talking about... Warner That is very much in line with the context of the postings on the computational chemistry list I mentioned. This is the likely source of the issue for people on that list (very few use *BSD). They just assume if you are not using Solaris or Irix, then you must be using Linux of some flavor. And this means I don't have to bother with that particular piece of nonsense. Thanks to everyone who answered. Bud Dodson -- M. L. Dodson Email: mldodson-at-houston-dot-rr-dot-com Phone: eight_three_two-56_three-386_one ___ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to [EMAIL PROTECTED]
Re: Syncing cpus on a multi-cpu, dual core system
The TSCs for each individual cpu core can drift relative to each other, even on multi-core chips like AMD X2s. This only effects code which uses the TSC, which isn't a whole lot. They need to be synchronized with each other (by calculating the drift and correcting for it) when using the TSC to log events, such as KTR logging might do. Since the TSC runs off the PLL, the drift rate will change based on the temperature of each core. I'm not sure whether the LAPIC timers can be depended upon to operate from the same physical clock source or not (i.e. whether they drift relative to each other or not). I haven't tested them for that. -Matt ___ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to [EMAIL PROTECTED]
Re: Syncing cpus on a multi-cpu, dual core system
- Original Message On a computational chemistry list I subscribe to there is a current thread about multi-cpu systems needing to have the cpu frequencies synced (this is in a Linux context). This is evidently not just having the cpus running at nominally the same frequency but something else in addition. A posting in the thread said variations less than 0.1% were not problematic. However, the poster said it was an issue in a dual cpu, dual core system he had set up. My questions are: 1. Is this real or an urban legend? If CPUs use the same FSB (as is the case with dual-core chip), they are already in sync. Right? For system that use multiple FSB clocks [like dual-(dualcore-CPU) systems], it might be possible to vary the clocks (as much as the manufacturer allows without hw modifications: e.g., SpeedStep, or something similar). Why someone would want to have CPUs running at precisely the same frequency is beyond my imagination. My dual core system does speedstep the two CPUs independently, it's clearly visible when running the speed monitor applets. I guess if only one thread is busy, only one CPU needs to work hard. As to why it would matter that they be in sync? My imagination runs to these two possibilities: 1) someone is writing multi-threaded software but doesn't really know how to do this correctly. 2) someone is writing ultra-high performance multi-threaded software that requires two threads on the two CPUs to be able to run in a way that doesn't risk data corruption, but needs to be able to do it without slowing down to check monitors/semaphores/other-threadsafety-devices. To address this, they've counted CPU cycles and are running a producer and consumer type scenario in such a way that the threads are perfectly out of phase with one another, so as to ensure that reads by one are not mixed in with writes by the other. I guess option 2 would be kinda legitimate, but I suspect actually a special case of 1 anyway--mainly because I find it hard to believe it has a snowball's chance in hell of working on systems that have pipelines, instruction look-ahead, etc. Might have worked on a dual 6502 based system when you knew exactly how long an instruction would take, and it always took that long :) Bottom line; I'm inclined to the urban-legend classification. $0.02 Simon __ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com ___ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to [EMAIL PROTECTED]
Re: Syncing cpus on a multi-cpu, dual core system
frequency but something else in addition. A posting in the thread said variations less than 0.1% were not problematic. However, the poster said it was an issue in a dual cpu, dual core system he had set up. Why would application code care about CPU frequencies? Is it trying to measure its 'performance' by subtracting two TSC readings? That won't necessarily work on modern multi-CPU systems where each CPU could be running at a different CPU speed. On FreeBSD, with hwpmc(4), you can allocate a process-mode counting pmc that counts non-sleep cycles (e.g., p4-global-power-events,mask=running on an Intel P4) and then use RDPMC instructions where you would have used RDTSC instructions. This is as cheap as the RDTSC technique and will work on SMP systems. -- FreeBSD Volunteer, http://people.freebsd.org/~jkoshy/ ___ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to [EMAIL PROTECTED]
Re: Syncing cpus on a multi-cpu, dual core system
On a computational chemistry list I subscribe to there is a current thread about multi-cpu systems needing to have the cpu frequencies synced (this is in a Linux context). This is evidently not just having the cpus running at nominally the same frequency but something else in addition. A posting in the thread said variations less than 0.1% were not problematic. However, the poster said it was an issue in a dual cpu, dual core system he had set up. My questions are: 1. Is this real or an urban legend? If CPUs use the same FSB (as is the case with dual-core chip), they are already in sync. Right? For system that use multiple FSB clocks [like dual-(dualcore-CPU) systems], it might be possible to vary the clocks (as much as the manufacturer allows without hw modifications: e.g., SpeedStep, or something similar). Why someone would want to have CPUs running at precisely the same frequency is beyond my imagination. Bud Dodson [SorAlx] ridin' VN1500-B2 ___ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to [EMAIL PROTECTED]
