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

From: Jens Axboe
Date: Thu Sep 22 2016 - 10:24:57 EST

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.

Additionally, it should not be on by default.

Jens Axboe