Throttlestop fixed my slow laptop, but I can't understand how it slowed down!

[cloudybeer]

Hi everyone,

This post might seem a bit backwards. Usually people would present a problem and ask for advice how to fix it. Mine is a bit weird in the sense that using Throttlestop (just to change the Intel PL-1/PL-2 power-limit settings) has fixed my laptop suddenly going really slow about a week ago, but I still don't understand the underlying cause despite somehow having fixed/masked the problem.

The details: Lenovo Yoga 3 14 laptop; processor is an i7-5500u; 8GB RAM; 120GB SSD; Windows 10. No hardware modifications or overclocking/undervolting (sorry to be boring!). Been very reliable since I bought it four years ago. About a week ago, it suddenly started going stupid slow. After a bit of digging, finally stumbled upon Throttlestop--which has been an absolute lifeline!--and discovered that it was regularly breaching its power limits (PL 1 / long term default was 15W with 28s hold-off; PL 2 / short-term/turbo was 25W), and getting stuck at its lowest speed of ~500MHz, with a VID of around 0.63V. Perversely, in its low-speed state, its power was still above 15W most of the time--usually around 23W or 24W--probably because the CPU was still busy with apps still demanding from it, so it would never recover from its low-speed state. Or, if it did, it got back into it pretty quickly.

Using Throttlestop, I've changed is for the PL 1 limit to be 25W (28s clamp), and left the PL 2 limit at 25W with no time clamp. This has sorted it. And, a bit weirdly, the CPU power now sits well below 15W most of the time, because applications/services now *can* get the CPU resource it wants when it needs it

So, I guess the question is, how did the laptop get into this state where it needed CPU tweaks? What changed? Do CPUs get less power-efficient over time? I did remove the bottom cover and clear out fan dust, etc (wasn't too bad for four years' frequent usage). My worry is that the battery or PSU has degraded (no reports of it though--battery life still seems as good as it ever was), but I'm not convinced by this either.

Grateful for any advice or info. Might just be interesting if anyone's seen anything similar elsewhere!

Thanks!!

 

[unclewebb]

When your computer is idle with only ThrottleStop open on the desktop, what are you seeing for power consumption? What percentage of time is your CPU spending in the C0 state processing background tasks? If something was running in the background, chewing up CPU cycles, this could lead to higher power consumption and trigger power limit throttling sooner. Some background task might have been stuck in an endless loop.

I get a little OCD about making sure my computers are nice and lean. The fewer background tasks the better. This allows me to run my CPUs fast when idle without any power consumption penalty. As you learned, a fast CPU is efficient and can get background tasks done quickly.

https://imgur.com/Sjo8mtn

Also keep an eye on C State usage. This should be fairly consistent when idle from day to day. Sudden changes are a good sign that something is running in the background that you might not know about.

https://imgur.com/k59lB8K

If you do not see anything unusual in the above data on your computer then also keep in mind that some 4th and 5th Gen processors have bugs within the CPU itself. Power consumption is approximated using a formula that calculates power consumption internally. Sometimes, things go wrong. Here is an example of the opposite to your problem. Power consumption is being reported at 2.8W which is way too low when this CPU is fully loaded.

https://imgur.com/NFDwdA5

 

[cloudybeer]

Firstly, a massive thank-you! That's an amazingly thorough reply!!

I'm re-tracing my steps a bit here, as I didn't have Throttlestop installed until after the problem started, so I don't have a very good set of numbers to compare as a before/after.

I did happen to take a screenshot from my phone when it was running very slow (I would have taken PC screenshot, but it was so slow it was just about unusable!).

...but it does show that the C0% was around 29% for all CPU threads, but with a very low temperature (36 Celsius), 23.1W package power (exceeding PL 1).

I don't have data from the full C10 states table. Since this is quite easy to replicate the problem--a reboot or power-cycle seems to bring it back around again, I think it's pretty easy to gather more info.

Thanks again for your detailed reply. Really appreciate your time! Thanks so much.

 

[unclewebb]

a reboot or power-cycle seems to bring it back around again

Hopefully you do not have to do that too often.

It appears to be a bug within the CPU where reported power consumption is way higher than actual power consumption. That is the exact opposite of the example that I posted. If actual power consumption was at 23.1W, you would expect the CPU core temperature to be much higher than 36°C.

The only fix is through a BIOS update which contains an updated microcode for the CPU. You have an older laptop so you might already be using the latest BIOS. Check the manufacturers website to see if any BIOS updates are available. This is a CPU level bug. As far as I know, it was never properly fixed by Intel. Most people do not realize that all Intel CPUs have a long list of known bugs. For many of these bugs, Intel specifically states that they have no plans for a fix. I am sure there are quite a few undocumented bugs as well.

Do not be afraid to set the power limits WAY higher. Set these to 50W or 100W if it helps solve your throttling problem. My desktop board sets both turbo power limits to 4095W so they do not interfere with maximum performance. CPUs automatically slow down if they ever get too hot. The power limits are a redundant throttling method. Avoid checking the Clamp option.