Yesterday we reported on an issue where the Terminal's sysctl command incorrectly reports a reduced clock frequency on some PowerBook models until a PMU (power management unit) reset is performed, at which point correct processor speed is reported.
As noted yesterday, this appears to be a cosmetic issue, though some users are seeing a performance boost purely from the restart, while others are actually netting a gain in reduced processor mode performance after the PMU reset.
Benchmark results Benchmark results generated with the utility XBench show that there is no change in actual performance between systems reporting a lower speed using Terminal's sysctl command and those reporting the actual speed.
Eric Merrill writes "Before a PMU reset, it go a report of hw.cpufrequency: 667000000, and an Xbench average (restarted each time, CPU tests only) of 122.1. After I got a report of hw.cpufrequency: 999999997, and Xbench average (same method) of 121.8.
"This basically proves that it is purely cosmetic and has no relation to actual clock speed."
Peter Hillman has some real world test results that confirm the same, and also offers a tip that actually will net PowerBook owners a noticeable speed boost "I have a PowerBook G4 15" 867 MHz model and it has the same issue as reported on today's headlines. I did the terminal command and it reported 667 MHz, which is the reduced CPU mode for this model. I reset the PMU, which is a pain on this model as you have to open the keyboard and push the reset button. After a restart, the Terminal reported 867 MHz.
"So I did some application testing. iTunes ripped a CD at 11-12x (peaked at 12.9x). I did a radial blur at 30 in Photoshop on a large digital image and it took about 13 seconds.
"I restarted the PowerBook and the Terminal reverted back to 667 MHz. I ripped the same tracks in iTunes, did the same radial blur in Photoshop, and both were the same speeds as before.
"To confirm whether or not this is a cosmetic issue, I changed the Energy Setting to 'Reduced Processor' and the Photoshop filter took about 19 seconds, and the iTunes ripping slowed to 7.5-8.1x (peaked at 8.3x).
"Performance Tip: Replace the internal PowerBook drive with a new 7200-RPM drive, and you will really see a nice boost in startup time, application loading, and file opening.
Only reduced speed sees benefit? MacFixIt reader Paul Keller sent in some benchmarks that surprisingly show a boost in reduced processor performance after a PMU reset:
"(With the Terminal reporting) hw.cpufrequency: 667000000, the CPU score in xbench is 101 (average of three runs) at highest processor speed setting in energy saver and 79 at reduced processor speed
"After the PMU reset 'sysctl hw.cpufrequency' reports: hw.cpufrequency: 866666664
"The CPU score in xbench is 105.2 at highest processor speed setting in energy saver and 102.7 at reduced processor speed."
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Speed boost purely from restart, placebo effect, more to PMU reset? For users who have not restarted their PowerBooks in some time, the mere process itself can reap some performance gains in startup time and application launch.
One MacFixIt reader, Bernie, ran a series of three benchmarks: one while the Terminal was reporting a 667 MHz clock speed for his 867 MHz PowerBook G4, another after a simple reset, and another when the Terminal correctly reported 867 MHz after a PMU reset
He found overall CPU performance pegged at 90.19 while the system was reporting the lower speed, CPU performance of 104.12 after the first restart, and 104.62 after the system was correctly reporting 867 Mhz.
He wrote "As you will see there is a dramatic improvement between test 1 and 2 (simple restart) but virtually no improvement between test 2 and 3
"Conclusion: A simple restart speeds up my computer Resetting the PMU doesn't."
The combination of the typical restart speed boost and a placebo effect seem to be the main vectors for systems "feeling faster," though resetting the PMU may, itself, bring some speed gains. Byron Harris writes:
"I'm an owner of a PB 12" 867 and have noticed the same behavior in my laptop. While checking my processor speed in the terminal it reports it as 533. I reset the PMU and checked again and it did indeed report back 867.
"After the reset everything does indeed "feel" faster. My boot time was cut in half and application launch times and opening files also felt quicker."
Problem with some apps/games and lower frequency reading Some applications that require a minimum processor speed, and check speed via the sysctl -a Terminal command, may have problems with PowerBooks that are incorrectly reporting the lower frequency. MacFixIt reader Erik writes
"It remains a problem whenever I try to Launch Halo because it says it does not meet the minimum CPU frequency for launching the game."
In this case, a PMU reset is likely the only option for getting certain applications to recognize the true processor speed.