Thinkpads throttle CPUs down when battery is removed

I was pretty sure that the proper way to treat your laptop battery, if you want a long fulfilling life for it, is to remove it from the laptop when you are connected to an AC adapter. After all the lifespan of a laptop battery (or any rechargeable battery) depends largely on the number of charging cycles. Now if you leave the battery in the computer when it’s connected to the AC adapter, it will drain it just a little bit and then recharge it again over and over. This is true at least for the default setting of Lenovo ThinkPads (as you can read here).

Now let me back up here a little: Today I noticed one of my current programming projects performance dropped significantly over night. I ran a benchmark last night and the same this morning without any major changes, but got much worse results. So I looked for the reason in the code, reverting my recent changes (which should not affect performance at all, but just to be sure). Just before I started to believe in black magic and question my sanity, I thought maybe the reason isn’t in the code after all but my computer is just somehow slower. So I did a little research and finally noticed something odd in the output of cpufreq-info for all 4 cores:

  analyzing CPU 0:
  driver: acpi-cpufreq
  CPUs which run at the same hardware frequency: 0 1 2 3
  CPUs which need to have their frequency coordinated by software: 0
  maximum transition latency: 10.0 us.
  hardware limits: 800 MHz - 2.30 GHz
  available frequency steps: 2.30 GHz, 2.30 GHz, 2.00 GHz, 1.80 GHz, 1.60 GHz, 1.40 GHz, 1.20 GHz, 1000 MHz, 800 MHz
  available cpufreq governors: powersave, ondemand, performance
  current policy: frequency should be within 800 MHz and 800 MHz.
                  The governor "ondemand" may decide which speed to use
                  within this range.
  current CPU frequency is 800 MHz.

What? “Frequency should be between 800 MHz and 800 MHz?” That’s a rather tight margin! Of course this would explain the performance drop, but where does this come from? Changing the governor or setting the frequency manually did not change anything. Some web research showed that I was by far not the only one experiencing the problem and that can have multiple reasons. Explanations include kernel bugs and overheating CPUs. I could exclude the overheating and did not want to believe in kernelbugs yet. So I looked on and finally found an issue affecting Lenovo Thinkpads exclusively:

According to this article from ThinkWiki, the included AC adapters 65W are not enough to provide enough power to the system when it is on full load, so the BIOS decides to set the frequency to the lowest value and keep it there, if no battery is found. The article also offers a quick fix to this: Add processor.ignore_ppc=1 to your parameters in /boot/grup/menu.lst and everything will be fine. I tried it and yes, it really fixes the problem, but it’s not supposed to be safe as it doesn’t change the fact that the adapter doesn’t supply enough power. So when on full load the system can crash, I guess. So far it hasn’t for me.

So this leaves me with three options:

  1. Use the fix described above and risk the possibility of crashes.
  2. Leave the battery inserted always and risk decreasing it’s lifetime.
  3. Buy a stronger AC adapter. There are 90W and even more powerful ones available (but the 90W is enough for the T420; see this thread)

I have not decided yet, but am probably going with option three, since the extra adapter costs less than 20€.

Anyway, I’m a little angry at Lenovo now. I mean if they ship an inferior Adapter with my laptop, they could at least tell me. (People in this thread argue, that it is not inferior, because it is smaller and lighter and thus easier to carry.)

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2 Responses to Thinkpads throttle CPUs down when battery is removed

  1. Troy says:

    Thank you for this complete information. However, I do not use GRUP as I use windows. Is there a solution for windows system?

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