Iteration times in i5 and i7

[cuBerBruce]

Originally Posted by Jud McCranie

Yes, but will the increased performance make up for having it off for a while?

Since the downtime is only a few seconds, the time is made up for very quickly.

[TheMawn]

Laptops (and the aluminum macbooks in particular) have a very low rate of cooling, usually because the CPU doesn't happen to be situated next to a vent and the heat must be moved with a heatpipe to a set of very small fins cooled by a very small fan over a very small vent which often has just a few millimeters of clearance above whatever surface it's on. Worse yet is when the vent is entirely covered by a deformable surface, such as when it is placed on TOP of a LAP.

The thing about the macbooks now is because they vent off heat by using the metal casing (in some cases) as a MASSIVE heatsink, they have an insanely high heat capacity which means it takes a good deal of time for the temperatures to change very much.

This can be a bit problematic because if the processor is running a full load at a maxed frequency, one is tempted to say "yeah it's fine look it's still way below TJMax after a minute" but it might actually take ten or twenty or a hundred minutes for the CPU to reach a equilibrium which is already well above the max temperature you want it at.

If you have a metal case, then blowing any kind of air over the case itself (setting the laptop upside down will help a lot too) will probably solve all your problems and them some.

[mdettweiler]

Originally Posted by TheMawn

Laptops (and the aluminum macbooks in particular) have a very low rate of cooling, usually because the CPU doesn't happen to be situated next to a vent and the heat must be moved with a heatpipe to a set of very small fins cooled by a very small fan over a very small vent which often has just a few millimeters of clearance above whatever surface it's on. Worse yet is when the vent is entirely covered by a deformable surface, such as when it is placed on TOP of a LAP.

The thing about the macbooks now is because they vent off heat by using the metal casing (in some cases) as a MASSIVE heatsink, they have an insanely high heat capacity which means it takes a good deal of time for the temperatures to change very much.

This can be a bit problematic because if the processor is running a full load at a maxed frequency, one is tempted to say "yeah it's fine look it's still way below TJMax after a minute" but it might actually take ten or twenty or a hundred minutes for the CPU to reach a equilibrium which is already well above the max temperature you want it at.

If you have a metal case, then blowing any kind of air over the case itself (setting the laptop upside down will help a lot too) will probably solve all your problems and them some.

Ah, that makes sense now! The case must be absorbing enough heat during the first 4+ hours to significantly improve performance. After that, the i7 adjusts its throttling cycle for its new (reduced) thermal dissipation capabilities, hence the increase in iteration times. Putting the computer to sleep briefly would allow the case "heatsink" to cool off rapidly and give you some fresh absorption capacity.

That's very interesting that they're using the case as a heatsink like that. I guess they figure that most people don't use their laptops that heavily for that long, so pumping the excess heat into the case allows them to skimp on more traditional cooling hardware. Incidentally, having that much external heat-sink surface area opens up a lot of possibilities for "manually" helping it along, which is not something you can readily do very well with many laptops. In addition to running air over the case helping a lot, the ice pack trick should be extremely effective (albeit temporary). A good-sized cold pack with flat contact to the bottom of the case would increase the heat absorption capacity by a ton. Even after it's melted, the pack will continue to absorb heat in the same manner as the case itself.

I would note, with regard to putting the laptop upside down, that you do want to be careful not to leave the laptop lid closed while it's crunching full bore like this. Laptops dissipate a fair amount of their heat through the keyboard, and if all of that's going straight into the LCD screen it might cause long-term damage. What I usually do is crack the lid open at about a 30-45 degree angle, and stand the laptop up on its side. This will probably not work for all laptops (some can't stand up stably like that; and the power cable placement may interfere), but it's a quick and handy way to set it up to get some good airflow around the case. Another way could be to put it on a cookie sheet - though if you're planning to use the computer like this be forewarned that you'll have a "springy" effect when you try to type.

[kladner]

If you were going to put the laptop on a metal sheet for the heatsinking ability, I suggest that it be aluminum like a pizza pan. And just to mention it once more, a cooling rack such as you put a hot baked item on (cake, pie) would give you plenty of clearance for air. It might not even be too unstable.

[mdettweiler]

Originally Posted by kladner

If you were going to put the laptop on a metal sheet for the heatsinking ability, I suggest that it be aluminum like a pizza pan. And just to mention it once more, a cooling rack such as you put a hot baked item on (cake, pie) would give you plenty of clearance for air. It might not even be too unstable.

Oh, right...cooling rack, that was the word I was looking for, not cookie sheet. That would make much more sense...(hence my reference to it being "springy" when you try to type on it).

[cuBerBruce]

I thought I would provide a little update.

As the main heatsinks had been running a bit warm, I decided to decrease the throttle setting from 85% to 75%. I think when I had raised it up to 85%, it generally increased thermal stresses in my system, and that resulted in my system showing degraded levels of performance more often and and to a higher level.

I have done some cleaning. Spraying compressed air at the keyboard didn't seem to have much affect. I took the bottom cover off. Overall, it looked fairly clean, but there was a couple of little dust bunnies by one fan and the inner side of the bottom cover was a little dusty/dirty. (I also discovered what was causing my DVD drive not to work, and fixed it. So taking the cover off was worth it.)

I also picked up a Targus Lap Chill Mat with dual fans to set the computer on.

Now the system seems to be running for long periods without going into the modes of lower levels of performance that I was seeing before. Of course, the "potential" throughput is somewhat reduced because of the lower throttle setting. (The displayed per iteration times do not take into account the rest time between iterations.)

Several days ago I started a double check assignment on my MacBook Pro. This is part of a plan to have at least one double check run on all my hardware since a successful double-check will give some validation that the hardware is working properly. This assignment is now over half done. When complete, this should give confirmation that despite the thermal throttling behavior and changes to performance that I'd been seeing, that the computer still functioned correctly. (I don't really expect any problem, but I still think this added confirmation will be reassuring.)

In summary, I think that the dropping in the performance level to a different relatively stable value was basically the hardware reacting to and dealing with thermal issues. While Prime95 appears to simply generate heat too fast for my system to dissipate adequately, it can still manage to maintain a good level of performance while going through short duration thermal cycles. But if the hardware fails to maintain the same level of performance after running awhile, then it seems to be an indication that Prime95 is generating too much heat overall and the throttle setting should probably be lowered a little.

Off topic: Early this month, I achieved a ranking of being in the top 1000 producers (overall, over last 365 days). My MacBook Pro contributes about half of my overall productivity.

[mdettweiler]

Originally Posted by cuBerBruce

I have done some cleaning. Spraying compressed air at the keyboard didn't seem to have much affect. I took the bottom cover off. Overall, it looked fairly clean, but there was a couple of little dust bunnies by one fan and the inner side of the bottom cover was a little dusty/dirty. (I also discovered what was causing my DVD drive not to work, and fixed it. So taking the cover off was worth it.)

Ah...I may have misled you a bit on that. Reading your post just now, I remembered that I had it backwards - you're supposed to blow the air in through the vents and intakes, and the dust comes out through the keyboard area. Sorry for wasting your time (and compressed air) there. It may be that your computer wasn't too dusty to begin with - the laptop I usually do that trick on picks up quite a lot of dust since it sits near the floor hooked up to a KVM switch, instead of being moved around like a typical laptop. My other laptop (which I carry around frequently) tends to produce much lest dust when I blow it out. But, if you're still looking to improve your thermal performance a bit, it might help some if there's some more dust yet in there.

[cuBerBruce]

This should be my final post regarding the slowdowns I was seeing on my MacBook Pro (except to respond to someone else who might keep this discussion going).

My double-check assignment completed with successful verification of the original LL test. So my MacBook Pro at least seems to run reliably despite the thermal cycling and the observed changes in performance level.

Since my previous post, I decided to raise Prime95's throttle setting from 75 to 80 (but still less than 85 that was the setting when the issue of very significant slowing down was observed). The displayed iteration times remained essentially the same for the next 45+ hours. Then a definite but relatively minor slowdown occurred on all workers. After noticing the slowdown about 4 hours later, and being the heatsink temperatures were relatively low, I set the computer into sleep mode for a couple seconds, after which Prime95 performance went back to the normal level. It has stayed at that level now for another day and half.

The performance slowdowns have seemed to be related to coping with heat that is generated. The lack of big slowdowns over the last few days seem to be due to the lower throttle setting that means Prime95 rests more between iterations, thus generating less heat per unit of time. The chill mat and generally more mild ambient temperatures over the last few days probably have also helped.

It might be nice to further understand exactly what mechanism is causing this slowdown, and also why after a brief period of sleep mode, the system doesn't tend to go quickly go back to this slowdown mode when Prime95 resumes.

But anyway, it appears that measures to improve heat dissipation, and using Prime95's throttle feature to limit heat generation (while also obviously limiting the possible performance that can be reached) does seem to help a lot in preventing these slowdowns from occurring. So I feel that I have the issue effectively under control.

[mdettweiler]

Originally Posted by cuBerBruce

Since my previous post, I decided to raise Prime95's throttle setting from 75 to 80 (but still less than 85 that was the setting when the issue of very significant slowing down was observed). The displayed iteration times remained essentially the same for the next 45+ hours. Then a definite but relatively minor slowdown occurred on all workers. After noticing the slowdown about 4 hours later, and being the heatsink temperatures were relatively low, I set the computer into sleep mode for a couple seconds, after which Prime95 performance went back to the normal level. It has stayed at that level now for another day and half.

The performance slowdowns have seemed to be related to coping with heat that is generated. The lack of big slowdowns over the last few days seem to be due to the lower throttle setting that means Prime95 rests more between iterations, thus generating less heat per unit of time. The chill mat and generally more mild ambient temperatures over the last few days probably have also helped.

It might be nice to further understand exactly what mechanism is causing this slowdown, and also why after a brief period of sleep mode, the system doesn't tend to go quickly go back to this slowdown mode when Prime95 resumes.

I think you can chalk it up to the effect TheMawn described back in post #46 - the MacBook's large aluminum case is acting as a heatsink with a very large thermal capacity, and thus is "soaking up" heat when the system first comes on (that is, the non-slowed-down mode); after a while it "fills up", and the CPU can only dissipate heat as fast as it actually vents off the case, so it dials back performance to match.

It sounds like the chill mat has helped a lot - as TheMawn had also said earlier, blowing air over that case-acting-as-very-large-heatsink boosts its dissipation capabilities quite a bit. Any time you have that kind of heatsink surface area to work with, airflow makes a huge difference. Hence, why now the system can stay in its "higher performance mode" for 45+ hours instead of 4+ as before - a very significant improvement.

As for the brief period of sleep mode letting it "reset", it seems that the case is able to quickly dissipate much of that "stored" heat when the computer is off (or in sleep mode, which is basically the same in terms of heat). I've noticed a similar thing with my desktop computer; if I simply turn off Prime95 (or similar program) and leave it running idle for a while, or even restart the computer without turning it off, the CPU temp never drops below a certain point, but if I actually turn it off, even briefly, it can cool down to a lower level and idle at a lower temperature before I start up Prime95 again. Again, just like with your laptop, this is likely due to the computer's heatsink (in this case a real heatsink on top of the CPU, not a large metal case, but the principle is the same) being able to much more rapidly dissipate its "stored heat" while there's no new heat being pumped in.

[cuBerBruce]

Originally Posted by mdettweiler

I think you can chalk it up to the effect TheMawn described back in post #46 - the MacBook's large aluminum case is acting as a heatsink with a very large thermal capacity, and thus is "soaking up" heat when the system first comes on (that is, the non-slowed-down mode); after a while it "fills up", and the CPU can only dissipate heat as fast as it actually vents off the case, so it dials back performance to match.

It sounds like the chill mat has helped a lot - as TheMawn had also said earlier, blowing air over that case-acting-as-very-large-heatsink boosts its dissipation capabilities quite a bit. Any time you have that kind of heatsink surface area to work with, airflow makes a huge difference. Hence, why now the system can stay in its "higher performance mode" for 45+ hours instead of 4+ as before - a very significant improvement.

As for the brief period of sleep mode letting it "reset", it seems that the case is able to quickly dissipate much of that "stored" heat when the computer is off (or in sleep mode, which is basically the same in terms of heat). I've noticed a similar thing with my desktop computer; if I simply turn off Prime95 (or similar program) and leave it running idle for a while, or even restart the computer without turning it off, the CPU temp never drops below a certain point, but if I actually turn it off, even briefly, it can cool down to a lower level and idle at a lower temperature before I start up Prime95 again. Again, just like with your laptop, this is likely due to the computer's heatsink (in this case a real heatsink on top of the CPU, not a large metal case, but the principle is the same) being able to much more rapidly dissipate its "stored heat" while there's no new heat being pumped in.

So far, I don't have any hard data to conclude how much of the stability is due to decreased throttle setting versus the use of the chill mat. I definitely do think the chill mat is helping to keep heatsink temperatures down, so you may be right in that it's making a big difference in keeping the Prime95 performance stable.

Thanks again (and to TheMawn for the earlier post) for your thoughts on this. I don't consider myself to be any sort of expert on thermal issues. I'll keep your posts in mind as I continue to monitor the performance I get out of Prime95. I'll soon experiment with adding a 4th worker while reducing the throttle setting to 60% to start with. With the same number of workers as actual cores, maybe this will help distribute the heat within the CPU better. But with 3 workers, I assume the OS has been bouncing the workers around the four available cores to get better distribution of the heat.