Re: [PATCH 1/8] Documentation, x86: Documentation for Intel Mem b/w allocation user interface

[Shivappa Vikas]

On Mon, 16 Jan 2017, Thomas Gleixner wrote:

> On Tue, 10 Jan 2017, Vikas Shivappa wrote:
>
> > Memory b/w allocation is part of Intel RDT(resource director technology)
> > which lets user control the amount of memory b/w (L2 external b/w) per
> > thread. This is done programming MSR interfaces like cache allocation
> > technology and other RDT features.
> > This patch adds documentation for Memory b/w allocation interface usage.
>
> Sigh. I told you how often that 'This patch' is crap. We already know that
> this is a patch. Read and finally act according to
> Documentation/process/SubmittingPatches
>
> > +Memory b/w throttle
>
> Can we please spell out Bandwidth at least once? b/w can mean anything
> (black/white, both ways ...)
>
> > +-------------------
> > +For Memory b/w resource, the portion of total memory b/w the user can
> > +restrict or 'throttle by' is indicated by the thrtl_by values.
> > +
> > +Throttle by values could be linear scale or non-linear scale.  In linear
> > +scale a thrtl_by value of say 20 would throttle the memory b/w by 20%
> > +allowing only 80% max b/w. In nonlinear scale currently SDM specifies
> > +throttle values in 2^n values. However the h/w does not guarantee a
> > +specific curve for the amount of memory b/w that is actually throttled.
> > +But for any thrtl_by value x > y, its guaranteed that x would throttle
> > +more b/w than y.  The info directory specifies the max thrtl_by value
> > +and thrtl_by granularity.
>
> This interface is really crap. The natural way to express it is:
>
>     Requested Bandwidth = X %

I wanted to do it this way which did seem more intuitive but the issue is with 
the non-linear scale which the hardware does not guarantee a particular 
percentage for a particular value. Or we don't know the curve for delay value vs. 
actual b/w throttled.

ex: in non linear scale , the granularity is 2^n.
Max : 512

Say a value of 256 is not guaranteed to have 50% or even follow a curve where we 
can calculate the corresponding percentage.

> i.e. 100% is unthrottled.
>
> The info file should tell the minimum bandwidth,the granularity value and
> the scale mode.
>
> The actual programming should just take a bandwidth percentage value
> between 0 and 100. The written value is adjusted by the write function to
> the granularity and minimum bandwidth, so a subsequent readout will tell
> the effective value.
>
> That's important because that allows scripts to work independent of the
> actual hardware implementation with default bandwidth configurations and
> then allows the user/admin to readout the effective values on a particular
> machine. If someone wants to adjust them machine specific, that's possible
> as well.
>
> Aside of that this documentation should contain some information about the
> limitations of that bandwidth control, i.e. the fact that this is a core
> specific mechanism and using a high bandwidth and a low bandwidth setting
> on two threads sharing a core will throttle the high bandwidth thread
> inadvertently.

Ok , will do.

Thanks,
Vikas

That's really important to mention in the documentation
> because that's going to bring interesting surprises for users.
>
> Thanks,
>
> 	tglx