Re: [PATCH RFC V2 3/3] scsi_mq: enable runtime PM

Alan Stern
2018-07-19T14:35:39Z

On Thu, 19 Jul 2018, Johannes Thumshirn wrote:

> On Wed, Jul 18, 2018 at 11:01:52AM -0400, Alan Stern wrote:
> > On Wed, 18 Jul 2018, Johannes Thumshirn wrote:
> >
> > > On Wed, Jul 18, 2018 at 10:12:37AM -0400, Alan Stern wrote:
> > > > As one extra reminder... People who care about extreme performance can
> > > > perfectly well disable CONFIG_PM in their kernels. That should remove
> > > > any overhead due to runtime PM.
> > >
> > > Yes but how likely is this for people who are running their data
> > > center with a distribution kernel.
> >
> > Strictly speaking, that's a nonsense question. People who run
> > distribution kernels are 100% unlikely to change the kernel
> > configuration, by definition.
> >
> > That's not the point. If people really care about getting the utmost
> > performance, to the extent that they resent the minimal overhead
> > required for runtime PM, then they should not be running distribution
> > kernels in the first place.
>
> Can we agree to disagree here?
>
> Some of the people I know in HPC, HFT, etc... run stock SLE/RHEL and
> they do file bugs for every single 3-5% performance regression. This
> is not something I made up, this is the kind of things I have to deal
> with on my day job.

The performance affect of runtime PM ought to be less than 1%. If it
is larger, we'll figure out a different approach.

Alan Stern