dd copy, context switch overhead

[Colin Ian King]

On 27/09/13 16:58, Tim Gardner wrote:
> 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 ?
> 
I guess the answer is no, perhaps I should run a set of rigorous
benchmarking next week to find out.

Colin