From mboxrd@z Thu Jan  1 00:00:00 1970
From: rcpilot2010@gmail.com (Abu Rasheda)
Date: Wed, 30 May 2012 14:44:04 -0700
Subject: Module vs Kernel main performacne
In-Reply-To: <CAGdaadazeyRAgfSxFTXCbTB5Jv_1RTtz=7gzADvxKUih3ueiMQ@mail.gmail.com>
References: <CACYKDtgS5nROPz+LrgX8SX2aYyjjfonvOWuwRDhTHMQd49G_7Q@mail.gmail.com>
	<CAGdaadaPj7e7T0uus1Y57D=iAOHoq0o=cQuD2Pi1ZpsSKSD54g@mail.gmail.com>
	<CACYKDtgbDPJdYHXNw=yF==-w5BNaQGBiZ_y+jKU4N9BYWxS7LA@mail.gmail.com>
	<CAGdaadazeyRAgfSxFTXCbTB5Jv_1RTtz=7gzADvxKUih3ueiMQ@mail.gmail.com>
Message-ID: <CACYKDtg6b4Ec4NLPJsWxK7zzhMyk-h-tgwoetLQX+p7Ck4gSnw@mail.gmail.com>
To: kernelnewbies@lists.kernelnewbies.org
List-Id: kernelnewbies.lists.kernelnewbies.org

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 ?