Writing a single buffer in kernel ring buffer take 6-8 ms

[Subramaniam Appadodharana]

On Thu, May 3, 2012 at 7:21 AM, Arun KS <getarunks at gmail.com> wrote:

> Hi Chetan,
>
> On Thu, May 3, 2012 at 3:35 PM, Chetan Nanda <chetannanda at gmail.com>
> wrote:
> > Hi,
> >
> > We are working on ARM based SOC.
> > Currently we are implementing a efficient tracing mechanism to logs
> messages
> > from a dedicated MCU.
> >
> > For this, we share a circular buffer between ARM and MCU and messages
> stored
> > in the buffer will be put into kernel ring-buffer and then output via
> dmesg
> > call.
> > Strangely wringing a single log message in the kernel ring-buffer cost
> > around 6-8ms.
>
> Consider this situvation, if your do a printk message of 100
> characters at a baud rate of 115200 bps on your UART console.
> Now 1 bit it will take 8.68 us(ie.. 1/115200).
>
> So 100 character ie.. 800 bits(exculuding parity, stop bits and all
> other over heads) will take 8.68 us * 800 = 6.94 ms.
>
> How are you writing to the kernel log buffer?
> Are you using UART as console? If yes what is your baud rate?
>
> If possible give more details about your design.
>
> I think Arun's assumptions are valid here. I am not aware of how the
kernel ring buffer is used, but I would think it is better to have the logs
printed out from a user space application. That way you are not blocking
the kernel to spend time printing each message that gets put into the
buffer. Does that make sense?

Thanks,
> Arun
>
> >
> > Please help to confirm this, or we are doing something wrong.
> >
> > Thanks,
> > Chetan Nanda
> >
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