iMac Pro second impressions

Mon, 12 Feb 2018 17:44:36 +0000

More thoughts in no particular order:

The fan control algorithm is a bit amateur. It oscillates back and forth in frequency at a high enough frequency (0.1-0.2 Hz) – and with significant range – that it’s quite noticeable, audibly, and sometimes distracting.

For some reason the full-load CPU frequency on my machine has dropped from 3.6 GHz to 3.4 GHz. I have no idea why. The inlet temperature to the case is actually lower now, too. The workload isn’t identical, so perhaps this is a natural result of subtle workload differences (e.g. use of AVX512 vs not, or something like that – most Skylake Xeons have significantly lower clock frequencies when the AVX512 unit is in use, though AFAIK the relevant data-sheets aren’t available publicly for the specific SKUs Apple use in the 8- and 10-core iMac Pros, so I can’t be sure).

Note the 4500 (Hz) limit on the Y axis. Still merely aspirational, as far as I can tell. Even under the lightest loads I’m still yet to see it exceed 4.2 GHz, according to iStat Menus (or Intel’s Power Gadget). Though I don’t put too much stock in those, as alluded to in my first impressions post – I doubt they’re relying on actual turbo bin residency counters, but rather just an average over a relatively large period (e.g. using MPERF & APERF). (I’m not actually sure, off-hand, if Skylake has proper residency counters for this purpose)

‘System’ power draw as reported by the the machine’s own “Total Power” sensor maxes out at about 350W, but (including some external hard drives and other such devices) my UPS says 550W is being drawn. For comparison my prior 2014 Retina iMac reported just over 200W for the approximately same sensor (though alas I never checked what the UPS reported). In any case it clearly, and as expected, generates significantly more heat than the non-Pro iMacs, as immediately evident by its much improved ability to heat the room. 😄

The login screen at boot is super sluggish and buggy – it lags behind keyboard input by up to several seconds, and often after you select a user and it transitions to showing just their picture + the password field, it’ll then inexplicably go back to showing all the users’ pictures – but with the password field still visible. The first ten keystrokes into the password field are almost always ignored & lost. And sometimes, upon hitting return in the password field, it just obtusely removes the field and goes back to showing just the initial list of users, requiring you to select your user account again and start over. It’s a plausible hypothesis that whatever is implementing this under the hood is significantly different from on prior Macs. Perhaps due to integration with the T2 SoC for security & flash access. And it’s not implemented well.

Whatever the cause, it’s kind of infuriating and baffling, that such an obvious & egregious flaw exists, given this is literally the first thing you experience every time you turn an iMac Pro on.

login screen – Wade Tregaskis