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SubjectRe: [RFC PATCH] PM / QoS: Introduce new classes: DMA-Throughput and DVFS-Latency
On Sun, Feb 26, 2012 at 8:43 AM, Rafael J. Wysocki <> wrote:
> On Tuesday, February 14, 2012, MyungJoo Ham wrote:
>> On Tue, Feb 14, 2012 at 11:26 AM, MyungJoo Ham <> wrote:
>> >
>> > This might look simliar to CPU_DMA_LATENCY. However, there are H/W
>> > blocks that creates QoS requirement based on DMA throughput, not
>> > latency, while their (those QoS requester H/W blocks) services are
>> > short-term bursts that cannot be effectively responsed by DVFS
>> > mechanisms (CPUFreq and Devfreq).
>> >
>> > In the Exynos4412 systems that are being tested, such H/W blocks include
>> > MFC (multi-function codec)'s decoding and enconding features, TV-out
>> > (including HDMI), and Cameras. When the display is operated at 60Hz,
>> > each chunk of task should be done within 16ms and the workload on DMA is
>> > not well spread and fluctuates between frames; some frame requires more
>> > and some do not and within a frame, the workload also fluctuates
>> > heavily and the tasks within a frame are usually not parallelized; they
>> > are processed through specific H/W blocks, not CPU cores. They often
>> > have PPMU capabilities; however, they need to be polled very frequently
>> > in order to let DVFS mechanisms react properly. (less than 5ms).
>> >
>> > For such specific tasks, allowing them to request QoS requirements seems
>> > adequete because DVFS mechanisms (as long as the polling rate is 5ms or
>> > longer) cannot follow up with them. Besides, the device drivers know
>> > when to request and cancel QoS exactly.
>> >
>> >
>> > Both CPUFreq and Devfreq have response latency to a sudden workload
>> > increase. With near-100% (e.g., 95%) up-threshold, the average response
>> > latency is approximately 1.5 x polling-rate.
>> >
>> > A specific polling rate (e.g., 100ms) may generally fit for its system;
>> > however, there could be exceptions for that. For example,
>> > - When a user input suddenly starts: typing, clicking, moving cursors, and
>> >  such, the user might need the full performance immediately. However,
>> >  we do not know whether the full performance is actually needed or not
>> >  until we calculate the utilization; thus, we need to calculate it
>> >  faster with user inputs or any similar events. Specifying QoS on CPU
>> >  processing power or Memory bandwidth at every user input is an
>> >  overkill because there are many cases where such speed-up isn't
>> >  necessary.
>> > - When a device driver needs a faster performance response from DVFS
>> >  mechanism. This could be addressed by simply putting QoS requests.
>> >  However, such QoS requests may keep the system running fast
>> >  unnecessary in some cases, especially if a) the device's resource
>> >  usage bursts with some duration (e.g., 100ms-long bursts) and
>> >  b) the driver doesn't know when such burst come. MMC/WiFi often had
>> >  such behaviors although there are possibilities that part (b) might
>> >  be addressed with further efforts.
>> >
>> > The cases shown above can be tackled with putting QoS requests on the
>> > response time or latency of DVFS mechanism, which is directly related to
>> > its polling interval (if the DVFS mechanism is polling based).
>> >
>> > Signed-off-by: MyungJoo Ham <>
>> > Signed-off-by: Kyungmin Park <>
>> In this PM-QoS patch, register_pm_qos_misc() for the new classes in
>> pm_qos_power_init() is missing.
>> Those will be included in the next version of the patch.
> Has the new version been posted already?  I seem to have missed it if so.
> Thanks,
> Rafael

No. I've not posted the new version, yet, as I was waiting for other
comments for this patch. I'll send a V2 patch with the omitted part
included today.



MyungJoo Ham, Ph.D.
System S/W Lab, S/W Center, Samsung Electronics
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