Overclocking the Pi3b+ GPU (Results)

Rion

@Brunnis I have also noticed the slowdowns happening in certain games using ondemand CPU governor.

I never bothered with Overclocking but just changed to performance instead.

But if would be interesting to see if there is anyway to optimize cpu governor ondemand.

Brunnis

no, i'm just articulating what i mean when i say that the data presented is not the "smoking gun", but your personal observations of a stutter is.

Fair enough.

i think the sampling_down_factor might be the one we would tweak:

sampling_down_factor:

This parameter controls the rate at which the kernel makes a decision

on when to decrease the frequency while running at top speed. When set

to 1 (the default) decisions to reevaluate load are made at the same

interval regardless of current clock speed. But when set to greater

than 1 (e.g. 100) it acts as a multiplier for the scheduling interval

for reevaluating load when the CPU is at its top speed due to high

load. This improves performance by reducing the overhead of load

evaluation and helping the CPU stay at its top speed when truly busy,

rather than shifting back and forth in speed. This tunable has no

effect on behavior at lower speeds/lower CPU loads.

I don't think that will work either. The problem is, again, that the average load is too low. Whether we stretch out the sample period over 1, 2, 10 frames, the average load will be close to the same and far below the required 95% that's needed to stay at the highest speed.

The way I see it, rapid highly periodic loads like these are hard to handle. The same issue occurs when running RetroArch on Windows 10 machines with modern Core processors, so it's not isolated to the Raspberry Pi. The only possible solutions I've been able to come up with so far are to:

Decrease the sample period, so that reactions to load changes can be carried out faster. If the default sample period is really 10 ms, that means more than half the execution time of a frame can be spent at the lower frequency before the CPU is instructed to increase clocks. The sample period would need to be drastically reduced in order to minimize the time spent down clocked after beginning actual timing critical work.
Use the "performance" governor. This completely eliminates the inefficiency of needing to sample CPU load before reacting.

That's it for me on the topic. I'm fine with using the run command settings to control this, like I always have before. Sometimes it's just fun to try to understand the mechanics behind a behavior. :-)

@Rion said in Overclocking the Pi3b+ GPU (Results):

I have also noticed the slowdowns happening in certain games using ondemand CPU governor.

Even without using video_max_swapchain_images=2?

@Rion said in Overclocking the Pi3b+ GPU (Results):

I never bothered with Overclocking but just changed to performance instead.

Yeah, that's the correct approach. Starting out with overclocking would be bad, since you're then just working against a mechanism that's now even more prone to try to lower the frequency. So, first change the governor, then overclock if performance still isn't good enough. :-)

@Rion said in Overclocking the Pi3b+ GPU (Results):

But if would be interesting to see if there is anyway to optimize cpu governor ondemand.

I think the nature of the load makes it hard. It will never be as performant as simply using the "performance" governor. Well, if you tweak the "ondemand" governor so that it considers the emulator load to be high enough to not down clock inbetween frames, then it will perform the same as the "performance" governor. But then there's no point in doing the optimization in the first place, since it won't save you any power consumption over the "performance" governor anyway!

Riverstorm

Performance is one the first things I turn on with a new build for the past few years. There's several MAME titles that you can feel the difference between on-demand vs performance when gaming. I don't know if it's coincidental but the more demanding titles seem to really show which almost seems contradictory but maybe another component is bottlenecking it.

If you open SSH and use watch you can see it constantly yo-yo while playing almost any game. It never stays at maximum performance like when using the performance setting.

It takes a minute to turn it on, done! :)

pi2user

Another tool you can use to check up on cpu performance is nmon (nigel's monitor). It is a standard debian package that was originally developed for monitoring enterprise level POWER systems.
It shows performance on a per-core basis, so you can see how much any individual core is being used. It is interesting to see how load is spread out over all the cores even in a single-core task like retroarch. The system appears to frequently reassign load to different cores, so each core gets a turn at running fully loaded.
Install and run nmon, then press c for cpu display per-core, and then t for top procs. There's also l for long-term cpu stats but this is overall and not per-core. q quits out of nmon.

dankcushions

@Brunnis said in Overclocking the Pi3b+ GPU (Results):

@dankcushions

i think the sampling_down_factor might be the one we would tweak:

sampling_down_factor:

This parameter controls the rate at which the kernel makes a decision

on when to decrease the frequency while running at top speed. When set

to 1 (the default) decisions to reevaluate load are made at the same

interval regardless of current clock speed. But when set to greater

than 1 (e.g. 100) it acts as a multiplier for the scheduling interval

for reevaluating load when the CPU is at its top speed due to high

load. This improves performance by reducing the overhead of load

evaluation and helping the CPU stay at its top speed when truly busy,

rather than shifting back and forth in speed. This tunable has no

effect on behavior at lower speeds/lower CPU loads.

I don't think that will work either. The problem is, again, that the average load is too low. Whether we stretch out the sample period over 1, 2, 10 frames, the average load will be close to the same and far below the required 95% that's needed to stay at the highest speed.

agree i think you’d also have to raise that threshold also (which is possible)

Well, if you tweak the "ondemand" governor so that it considers the emulator load to be high enough to not down clock inbetween frames, then it will perform the same as the "performance" governor. But then there's no point in doing the optimization in the first place, since it won't save you any power consumption over the "performance" governor anyway!

for those situations, absolutely, but the issue is that using the performance governer puts ALL applications launched via run command at full speed, which includes multithreaded or otherwise low-load applications (kodi, pixel desktop (??), maybe even some emulators like gambette, etc). i still like the idea of finding a way to make the ondemand governer work for us.

Brunnis

@dankcushions said in Overclocking the Pi3b+ GPU (Results):

@Brunnis said in Overclocking the Pi3b+ GPU (Results):

@dankcushions

i think the sampling_down_factor might be the one we would tweak:

sampling_down_factor:

This parameter controls the rate at which the kernel makes a decision

on when to decrease the frequency while running at top speed. When set

to 1 (the default) decisions to reevaluate load are made at the same

interval regardless of current clock speed. But when set to greater

than 1 (e.g. 100) it acts as a multiplier for the scheduling interval

for reevaluating load when the CPU is at its top speed due to high

load. This improves performance by reducing the overhead of load

evaluation and helping the CPU stay at its top speed when truly busy,

rather than shifting back and forth in speed. This tunable has no

effect on behavior at lower speeds/lower CPU loads.

I don't think that will work either. The problem is, again, that the average load is too low. Whether we stretch out the sample period over 1, 2, 10 frames, the average load will be close to the same and far below the required 95% that's needed to stay at the highest speed.

agree i think you’d also have to raise that threshold also (which is possible)

Well, if you tweak the "ondemand" governor so that it considers the emulator load to be high enough to not down clock inbetween frames, then it will perform the same as the "performance" governor. But then there's no point in doing the optimization in the first place, since it won't save you any power consumption over the "performance" governor anyway!

for those situations, absolutely, but the issue is that using the performance governer puts ALL applications launched via run command at full speed, which includes multithreaded or otherwise low-load applications (kodi, pixel desktop (??), maybe even some emulators like gambette, etc). i still like the idea of finding a way to make the ondemand governer work for us.

I was still stuck in thinking emulation (for which I’m not convinced ondemand is a great idea). I agree for Kodi and the likes.

As for trying out an optimization: I’d start out by either testing half as long sampling_rate OR decreasing the up_threshold to something like 50%. It won’t fix all possible performance issues, but there’s a good chance it’s much better than the defaults for the RetroPie use case.

Rascas

Here is the default settings of Libreelec:
https://github.com/LibreELEC/LibreELEC.tv/blob/libreelec-9.0/projects/RPi/initramfs/platform_init
I believe they are the same now on Raspbian/RetroPie. The only difference is the io_is_busy, which improves performance when in heavy reading/writing to the sdcard, for example streaming a torrent. But probably not so usefull for emulation.

hhromic

@Rascas said in Overclocking the Pi3b+ GPU (Results):

I believe they are the same now on Raspbian/RetroPie.

Can confirm from one of my Raspbian devices:

/sys/devices/system/cpu/cpufreq/ondemand/io_is_busy: 0
/sys/devices/system/cpu/cpufreq/ondemand/up_threshold: 50
/sys/devices/system/cpu/cpufreq/ondemand/sampling_rate: 100000
/sys/devices/system/cpu/cpufreq/ondemand/sampling_down_factor: 50

Silent

I really like the snippet with setting individual emulators to performance governor! I'd love to have that set eg. for PSX but I wouldn't necessarily want it for GBC or Kodi. Onstart sounds like a good way to optimally utilize the option!

quicksilver

So after a lot of testing it looks like the two pi3b+'s are stable at 590mhz core_freq and 615mhz respectively. Compared to my 3b which was stable up to 565mhz core_freq this is a pretty good jump. My old pi2's were only stable up to 525 and 535mhz core_freq. And they are all supposed to have the same GPU which means there must be some improvements with the manufacturing process from each generation of pi or perhaps power regulation is better too. I know that my testing sample is fairly small but there is a clear trend in the 6-7 different pis I have tested that shows that even though the GPU is the same in all models, later models definitely seem to have more GPU overclocking headroom. While this may not mean much to most people, it makes a noticeable difference when trying to run N64 or Dreamcast games that are right on the edge of being playable, in some cases an overclock is just enough to push it into the playable zone.

I should also note that while its often discussed that the CPU is not the bottleneck for N64 emulation on the pi, this is only true of the pi3b and 3b+. Going back and testing my pi2's there were quite a few N64 games that were pushing its cpu past 100% usage, even when overclocked to 1075mhz.

mitu

@quicksilver said in Overclocking the Pi3b+ GPU (Results):

I should also note that while its often discussed that the CPU is not the bottleneck for N64 emulation on the pi, this is only true of the pi3b and 3b+.

Well, the PI3B / 3B+ have a slightly different CPU than the Pi2:

PI2 - Broadcom BCM2836 SoC, with quad-core ARM Cortex-A7 900 MHz processor
PI3B/3B+ - Broadcom BCM2837 SoC, with quad-core ARM Cortex-A53 1200 MHz processor

quicksilver

@mitu correct, just the GPU has remained the same. I just felt it was important distinction to make, especially for those using older pi devices.

hhromic

Also don't forget that the RPI2 has two revisions, one with BCM2836 and another with BCM2837 (underclocked).

Ref: https://www.raspberrypi.org/documentation/hardware/raspberrypi/revision-codes/README.md
Ref: https://elinux.org/RPi_HardwareHistory

quicksilver

@hhromic Interesting! I think I remember hearing that, though I had forgotten about it. Something to the effect that they took the BCM2837's that couldnt handle being clocked at 1200 mhz and used them on the revised pi2's.

Brunnis

@quicksilver Thanks for the results! Out of curiosity, how does the instability manifest itself when you push the core_freq too far? I'm asking because I just changed from using a large heatsink sandwich to a Flirc case and I'm having issues. With the previous heatsink, I had run stability tests for days in total and saw no issues at all. With the Flirc case, I quickly ran into a few issues:

Crash (black screen) while at Raspbian desktop
Floating point error while running sysbench (within 30 minutes)
Tainted kernel (memory paging issue) while idling in RetroPie over night. EmulationStation still running, but not responding to input.

My current quess is that these issues have mainly cropped up due to a temperature difference. The Flirc case is not as good at cooling the Pi as the heatsink sandwich and this probably pushes the system over the edge. It still doesn't explain the tainted kernel issue at idle, though. Maybe that issue was there before changing case.

I'm going to try three things:

Increase CPU voltage one step
Lower core_freq overclock from 600 to 550 MHz
Increase SDRAM voltage one step

Brunnis

A small FYI (I think we've touched on this before in this thread):

On my Pi 3B+, only over_voltage=1 has an effect. Increasing over_voltage setting to 2 or more has no effect at all. Maximum allowed CPU voltage is 1.4V and since my Pi already runs at 1.375V, it makes sense that any setting higher than 1 (which adds 0.025V) would have no effect.
Default SDRAM voltages on my Pi are: sdram_c=1.25V, sdram_i=1.25V, sdram_p=1.225V. The over_voltage_sdram setting assumes 1.20V is default. So, using over_voltage_sdram=1 has no effect on my board (since it's 1.225V, which is the same or lower than the defaults). Using over_voltage_sdram=2 only affects sdram_p (it's raised to 1.25V). Using over_voltage_sdram=3 finally increases voltage on all three SDRAM rails to 1.275V.

I don't know if the defaults are the same for all Pi 3B+ boards. Check your voltages with:

for id in core sdram_c sdram_i sdram_p ; do echo -e "$id:\t$(vcgencmd measure_volts $id)" ; done

Please note that you should put a load on the CPU before running this command, otherwise you'll just see the default 1.2V used in downclocked state.

quicksilver

@Brunnis instability from having core freq too high has always shown up for me as a freeze. Quake 3 is running but suddenly locks up, though I can still ssh in and reboot my pi. I should note that my max overclock is only achievable with maximum overvoltage (1.397v, not sure why your pi won't go this high), anything less will freeze very quickly at these speeds. If I overclock v3d frequency too high I get visual artifacts onscreen followed by a freeze.

Twice I encountered an issue where I left my pi idle in emulation station to find that it had frozen hours later. One of those times my pi was overheating and my case fan which is programed to turn on at 65C was not on. Not entirely sure what would cause the pi to overheat when idle. But after removing my sdram and CPU overclocks I have not had that issue since. I haven't had time to thoroughly test them yet so I'll just leave them off for now.

Brunnis

@quicksilver No, you misunderstood me. My Pi also goes up to 1.39V. However, since default voltage is 1.375V, the only over_voltage setting that does anything is over_voltage=1 (which will give 1.39V). The Pi is not allowed to go over 1.4V, so setting over_voltage to 2 or higher will still result in the same 1.39V as over_voltage=1.

Thanks for sharing your experience. That over heating condition sounds...weird. I have modified a few of my overclock settings and will see if that helps (lowered core_freq from 600 to 550, lowered video related clocks from 400 to 367 and increased SDRAM voltages by 0.025V).

arm_freq=1475
core_freq=550
v3d_freq=367
h264_freq=367
isp_freq=367
sdram_freq=550
over_voltage=1
over_voltage_sdram_c=3
over_voltage_sdram_i=3
over_voltage_sdram_p=2
temp_soft_limit=70

quicksilver

@Brunnis are you sure about the over voltage values? I just did a test and this is what I got on my pi3b+:

No over voltage= 1.344V
Over_voltage 1= 1.369V
Over_voltage 2= 1.388V
Over_voltage 3= 1.394V (not 1.397v like I said earlier)

So for me over_voltage 3 is max setting. Are you sure yours is different?

Brunnis

@quicksilver Yep, absolutely sure. Default voltage for mine is 1.375V. They probably bin the chips with different default voltages during production, just like AMD and Intel do.

So, you're lucky and have received a chip that's two bins lower on the voltage scale than mine!