Re: [PATCH v4 0/3] nvme power saving

[Andy Lutomirski]

On Thu, Sep 22, 2016 at 2:33 PM, J Freyensee
<james_p_freyensee@xxxxxxxxxxxxxxx> wrote:
> On Thu, 2016-09-22 at 14:43 -0600, Jens Axboe wrote:
>> On 09/22/2016 02:11 PM, Andy Lutomirski wrote:
>> >
>> > On Thu, Sep 22, 2016 at 7:23 AM, Jens Axboe <axboe@xxxxxx> wrote:
>> > >
>> > >
>> > > On 09/16/2016 12:16 PM, Andy Lutomirski wrote:
>> > > >
>> > > >
>> > > > Hi all-
>> > > >
>> > > > Here's v4 of the APST patch set.  The biggest bikesheddable
>> > > > thing (I
>> > > > think) is the scaling factor.  I currently have it hardcoded so
>> > > > that
>> > > > we wait 50x the total latency before entering a power saving
>> > > > state.
>> > > > On my Samsung 950, this means we enter state 3 (70mW, 0.5ms
>> > > > entry
>> > > > latency, 5ms exit latency) after 275ms and state 4 (5mW, 2ms
>> > > > entry
>> > > > latency, 22ms exit latency) after 1200ms.  I have the default
>> > > > max
>> > > > latency set to 25ms.
>> > > >
>> > > > FWIW, in practice, the latency this introduces seems to be well
>> > > > under 22ms, but my benchmark is a bit silly and I might have
>> > > > measured it wrong.  I certainly haven't observed a slowdown
>> > > > just
>> > > > using my laptop.
>> > > >
>> > > > This time around, I changed the names of parameters after Jay
>> > > > Frayensee got confused by the first try.  Now they are:
>> > > >
>> > > >  - ps_max_latency_us in sysfs: actually controls it.
>> > > >  - nvme_core.default_ps_max_latency_us: sets the default.
>> > > >
>> > > > Yeah, they're mouthfuls, but they should be clearer now.
>> > >
>> > >
>> > > The only thing I don't like about this is the fact that's it's a
>> > > driver private thing. Similar to ALPM on SATA, it's yet another
>> > > knob that needs to be set. It we put it somewhere generic, then
>> > > at least we could potentially use it in a generic fashion.
>> >
>> > Agreed.  I'm hoping to hear back from Rafael soon about the
>> > dev_pm_qos
>> > thing.
>> >
>> > >
>> > >
>> > > Additionally, it should not be on by default.
>> >
>> > I think I disagree with this.  Since we don't have anything like
>> > laptop-mode AFAIK, I think we do want it on by default.  For the
>> > server workloads that want to consume more idle power for faster
>> > response when idle, I think the servers should be willing to make
>> > this
>> > change, just like they need to disable overly deep C states, etc.
>> > (Admittedly, unifying the configuration would be nice.)
>>
>> I can see two reasons why we don't want it the default:
>>
>> 1) Changes like this has a tendency to cause issues on various types
>> of
>> hardware. How many NVMe devices have you tested this on? ALPM on SATA
>> had a lot of initial problems, where slowed down some SSDs unberably.

I'm reasonably optimistic that the NVMe situation will be a lot better
for a couple of reasons:

1. There's only one player involved.  With ALPM, the controller and
the drive need to cooperate on entering and leaving various idle
states.  With NVMe, the controller *is* the drive, so there's no issue
where a drive manufacturer might not have tested with the relevant
controller or vice versa.

2. Windows appears to use it.  I haven't tested directly, but the
Internet seems to think that Windows uses APST and maybe even manual
state transitions, and that NVMe power states are even mandatory for
Connected Standby logo compliance.

3. The feature is new.  NVMe 1.0 didn't support APST at all, so the
driver is unlikely to cause problems with older drivers.

>
> ...and some SSDs don't even support this feature yet, so the number of
> different NVMe devices available to test initially will most likely be
> small (like the Fultondales I have, all I could check is to see if the
> code broke anything if the device did not have this power-save
> feature).
>
> I agree with Jens, makes a lot of sense to start with this feature
> 'off'.
>
> To 'advertise' the feature, maybe make the feature a new selection in
> Kconfig?  Example, initially make it "EXPERIMENTAL", and later when
> more devices implement this feature it can be integrated more tightly
> into the NVMe solution and default to on.
>

How about having a config option that's "default n" that changes the
default?  I could also add a log message when APST is first enabled on
a device to make it easier to notice a change.

--Andy