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I have been reading the cpuinfo file on my Samsung Galaxy (sgh-i897) to retrive the bogoMIPS value. And just now learning how to interpret such information.

Initially I did this under the main activity in a loading thread, and ALWAYS got a value of 997.59. I then moved the file reading method into a Service since I didn't need it in the UI until much later anyways. Once I did this, the value I read became quite different, and seems to change for each application start, always much slower, like in the 300 to 500 range.

My questions are:

  1. Does the Android OS / Linux cause a measurement of the bogoMIPS value and modify the cpuinfo file periodically?

  2. If so, is there a programatic way to force it?

  3. Why would it vary depending on if I am reading the file from a Service verses the main activity thread?

  4. Ultimately, if one wants the most correct number is it best to simply write our own timing loops to get it ourselves at the moment we need it? (assuming it's changing, due to cpu load, all the time anyhow?)

artless noise
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Tam
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    My android phone dynamically sets the CPU speed to save power when the CPU isn't in demand. I would imagine most (maybe all) android phones do this. So this would change the bogoMIPS value, as well... – Jonathan Hall Jul 13 '12 at 02:56
  • Good to know. Then the OS rewrites the cpuinfo file each time this happens(?) and possibly my retrieval from the Service is in a slower "state" of cpu since the activity is fully loaded and basically waiting (and then the Service is started). That makes sense, I just didn't expect the OS to update the cpuinfo file except maybe once after bootup. – Tam Jul 13 '12 at 03:06
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    Well, keep in mind the cpuinfo file isn't really a file at all... it's just a file-like interface to internal kernel info. I think at one time in the past, bogomips was calculated once on boot... with the advent of dynamic CPU speeds, I imagine that was changed long ago (at least for such platforms). But that's speculation. Google probably knows more. – Jonathan Hall Jul 13 '12 at 03:08
  • Doh!, I always assumed it was a file writen onto the disk(main memory chip) by the OS. Thank you! – Tam Jul 13 '12 at 03:11

2 Answers2

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Short answer: don't bother with BogoMIPS if you're not hacking the kernel.

What is this BogoMIPS thing?

Most tasks performed by an operating system kernel involve waiting for hardware, or more recently, processes running on other processor cores. The kernel has various methods to wait on different time scales. For the shortest periods of time, it will often simply 'run around in circles', more technically known as a "busy wait loop".

The BogoMIPS benchmark is just a busy wait loop. The kernel runs such a loop it, times it using a pretty reliable wall clock, and determines how long it needs to run a busy wait loop to wait for a specific amount of time.

Why does the BogoMIPS value change?

When a computer (smartphone, in this case) isn't performing a lot of work, it rarely has much need for a very fast processor. When the operating system notices it can do with less than the maximum frequency, it may throttle down the processor to a lower frequency. This is one of the ways the OS can reduce energy usage (relevant for battery-powered computers) and heat production (relevant for pretty much everything).

Of course, when the maximum frequency is lowered, the result of a BogoMIPS benchmark isn't relevant anymore, because every iteration in a busy wait loop will take longer. The kernel will rerun the benchmark and update the BogoMIPS value, so that busy waiting remains a reliable way of waiting for short periods of time.

Something similar holds when the need for processing rises, and the processor needs to be throttled up again.

How should I use this BogoMIPS value?

Unless you are hacking some part of the kernel that deals with timing, you shouldn't be bothered by the BogoMIPS value. In fact, you should use anything but the BogoMIPS value. There might be some way to determine the current CPU frequency using the Android SDK, or using information provided by /sys filesystem.

  • Thank you. That also explains why multi processors would have different bogoMIPS on the different cores. I am getting a better feel for this. I am now under the impression that my constant value of EXACTLY 997.59 during the main activity startup is the max (ramped up) possible value, while the Service is a more quiet time, and that can vary by what else is lightly going on in the phone. (no other apps are runing on this phone except those that are required by the telco.) I know this hasn't had much to do with code, but it's been very enlightening on how I will proceed with my coding now... – Tam Jul 13 '12 at 03:42
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You cannot force to change the bogoMIPs as that is rather temperamental under Android under the varying factors, initially at the start, (same as any other Linux kernel), the jiffies used on initial boot (arch/arm/boot/smp.c and arch/arm/boot/vmlinux.lds.S within the kernel source), is what determines the bogoMIPS (It is not to be taken for granted, that's partially the reason why its 'bogus' per se).

The factors that influences the skewing of the bogoMIPS is down to

  • How many services/widgets/apps running?
  • Is the CPU under/over-clocked? What frequency?
  • What sort of scheduler is in place (there's a few of them, deadline/ondemand/interactive to name but a few)
  • How is the kernel built? Is it optimized? What configuration used to build it?

The data used to "store" the value of the cpu's info is stored in the virtual directory created on boot within the /proc filesystem, it is there, visible and most, if not all, files in there are zero bytes and yet can read from it. It is created dynamically at boot time.

t0mm13b
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  • Thank you. I am not actually interested to change it (hmmm, but now that I think about it ... maybe), but rather getting the most updated value. Your commnmets along with Flimzy's have made it more clear. – Tam Jul 13 '12 at 03:22
  • @t0mm13b `You cannot force to change the bogoMIP` Wrong ! It’s possible to change it through hibernate to disk – user2284570 Jan 06 '19 at 15:21