dd copy, context switch overhead

[Tim Gardner]

On 09/27/2013 08:04 AM, Colin Ian King wrote:
> Hi,
> 
> I've observed that CONFIG_CONTEXT_TRACKING_FORCE is being turned on when
> we have CONFIG_NO_HZ_FULL enabled.  The CONTEXT_TRACKING_FORCE is
> causing some noticeable overhead, for example:
> 
> dd if=/dev/zero bs=$DD_BS count=$DD_COUNT | cat | cat | cat | dd
> of=/dev/null
> 
> ..this takes about 145 seconds with CONTEXT_TRACKING_FORCE enabled and
> about 83 seconds with it disabled.
> 
> I suggest that we should revert back to CONFIG_NO_HZ_IDLE and make sure
> CONFIG_CONTEXT_TRACKING_FORCE is disabled.
> 
> I believe this explains the huge increase in duration of the dd_copy
> test in the QA tests, see the dd_copy graph and the blue line that shows
> the duration (in seconds) of the copy:
> http://reports.qa.ubuntu.com/power/hardware/arch/amd64/?size=big
> 
> Colin
> 

So it looks like we ought to fall back to NO_HZ_IDLE. Do you know of any
adverse impacts this change might incur with regard to power
consumption, latency, or performance with other workloads ?

rtg
-- 
Tim Gardner tim.gardner at canonical.com