I did another experiment.
Wrote a stand alone module and user program which does ioctl and pass
buffer to kernel module.
User program passes a buffer through ioctl and kernel module does
kmalloc on it and calls copy_from_user, kfree and return. Test program
send 120 gigabyte data to module.
If I pass 1k buffer per call, I get
115,396,349,819 instructions # 0.90 insns per cycle
[95.00%]
as I increase size of buffer, insns per cycle keep decreasing. Here is the data:
1k 0.90 insns per cycle
8k 0.43 insns per cycle
43k 0.18 insns per cycle
100k 0.08 insns per cycle
Showing that cop_from_user is more efficient when copy data is small,
why it is so ?
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