Messages in this thread | | | Date | Tue, 17 Jan 2017 16:51:43 -0800 (PST) | From | Shivappa Vikas <> | Subject | Re: [PATCH 1/8] Documentation, x86: Documentation for Intel Mem b/w allocation user interface |
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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 > > > >
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