High CPU load during file copy to SD card

Question

Copying large files to micro SD card (via MTP mode or USB thetering and FTP) causes a high CPU load (around 90%) with the result of my phone rebooting from overheating.

Is this a normal behavior? Any suggestions?

My system is Cyanogenmod 11-M8 on a Galaxy S4 mini with a SanDisk Ultra microSDXC 64GB.

EDIT

Here are the last messages before reboot according to adb logcat

I/ThermalDaemon(  322): Sensor 'tsens_tz_sensor0' - alarm cleared 1 at 47.0 degC
E/MP-Decision( 1785): num online cores: 2 reqd : 1 available : 2 rq_depth:0.000000 hotplug_avg_load_dw: 29
E/MP-Decision( 1785): DOWN cpu:1 core_idx:1 Ns:1.100000 Ts:190 rq:0.000000 seq:1069.000000
E/MP-Decision( 1785): num online cores: 1 reqd : 2 available : 2 rq_depth:2.500000 hotplug_avg_load_dw: 58
E/MP-Decision( 1785): UP cpu:1 core_idx:1 Nw:1.900000 Tw:140 rq:2.500000 seq:182.000000
I/ThermalDaemon(  322): Sensor 'tsens_tz_sensor0' - alarm raised 1 at 50.0 degC
E/NetdConnector(  832): NDC Command {4878 bandwidth gettetherstats} took too long (1006ms)
D/MobileDataStateTracker(  832): default: setPolicyDataEnable(enabled=true)

Answer 1

No, it's not. File transfer can cause some cpu load but not nearly as much. And then high cpu load can heat up a phone but should not cause a thermal protection cutoff. The problem also happens when you move files with the file manager with the phone detached from the pc?

Answer 2

I do not have a Samsung device but I had the similar issue in my OPO. The average temperature it works on is in between 45-52deg Celsius. While copying over FTP using USB tethering, the temperatures reached beyond 70deg sometimes. Since it isn't normal for me, I found a solution as:

Note: - Device needs to be rooted, bootloader unlocked

The goal is simple. Your device is overheating because it is using too much CPU at its peak. I underclocked it and was able to lessen the resource usage.

Things I needed:

• Busybox
• A kernel that supports Universal Kernel Manager (UKM). AK already has UKM which I used for flashing over my stock kernel
• Synapse -- app to modify kernel settings

The flashing varies from device to device, so you've to search how to root and flash the kernel, and install the aforementioned apps.

Once you've everything at its place, launch Synapse → CPU → change the clock speed using slider. (I changed it from 2.4GHz to 1.3GHz.) Additionally, you can also set a custom limit for thermal protection cutoff. Go to Thermal in the same app and use the slider. I have it set on 74deg.

You can monitor CPU temperature and clock speed using CPU Temp.

Since then, I didn't see any overheating in my phone while transferring the data. However, the obvious and the biggest downside is that the data transfer speed will take a huge hit. Mine was decreased from ~24MB/s to 8MB/s for Internal SD-card (type emmc5).