Work scheduling for moderating power

Article: Power Delivery in a Modern Processor
By: Travis Downs (travis.downs.delete@this.gmail.com), May 12, 2020 1:25 pm
Room: Moderated Discussions
Paul A. Clayton (paaronclayton.delete@this.gmail.com) on May 12, 2020 10:15 am wrote:
> Even for in-order processors, the execution width could be adjusted to moderate power draw; this has
> been used for thermal throttling, but it might apply to power supply issues. For out-of-order processors,
> there may be more opportunities for scheduling flexibility to temporarily reduce power use.

I don't know if you already saw this, but I find that modern Intel chips do a very coarse grained version of this.

That is, they seem to throttle dispatch for all instructions to 1/4th the normal rate as long as *any* wide (vector) instruction is in the scheduler. This was previously reported by Agner and others to be a warmup period where vector instructions were executed on narrower half-width vector EUs at a reduced rate (but no one knew why it was 1/4 rather than say 1/2), but I believe that is incorrect: the full width is available but instructions dispatch slowly to limit the worst case droop.

The effect is very specific: it basically rounds the latency of every instruction up to the next multiple of 4, so a latency 3 multiply takes 4 cycles now, a 5-cycle load is 8 cycles, etc. Probably implemented by just dispatching every 4th cycle or similar, as well as counting SIMD instructions coming in and out of the scheduler to avoid this penalty if there are no SIMD instructions imminent.

This continues for a while until the new voltage level is achieved and then full speed execution can continue.

Previous RWT thread.
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TopicPosted ByDate
New article: Power Delivery in a Modern ProcessorDavid Kanter2020/05/11 07:37 AM
  New article: Power Delivery in a Modern ProcessorMaynard Handley2020/05/11 10:03 AM
    Buried power rails, super vias, etc.David Kanter2020/05/11 11:44 AM
      Buried power rails, super vias, etc.Maynard Handley2020/05/11 06:06 PM
  InductorsMoritz2020/05/11 12:01 PM
    InductorsDavid Kanter2020/05/11 12:36 PM
  New article: Power Delivery in a Modern ProcessorTravis Downs2020/05/12 07:53 AM
    New article: Power Delivery in a Modern ProcessorJason Creighton2020/05/12 09:15 AM
      New article: Power Delivery in a Modern ProcessorDan Fay2020/05/12 09:35 AM
        New article: Power Delivery in a Modern ProcessorJason Creighton2020/05/13 09:09 AM
          New article: Power Delivery in a Modern ProcessorRicardo B2020/05/13 11:47 PM
            New article: Power Delivery in a Modern ProcessorMichael S2020/05/14 12:29 PM
              New article: Power Delivery in a Modern Processoranon³2020/05/14 05:22 PM
              New article: Power Delivery in a Modern ProcessorRicardo B2020/05/15 05:06 AM
                New article: Power Delivery in a Modern ProcessorMichael S2020/05/15 05:13 AM
      New article: Power Delivery in a Modern ProcessorTravis Downs2020/05/13 03:01 PM
    LDO power savingDavid Kanter2020/05/12 10:16 AM
      LDO power savingYoav2020/05/13 03:56 AM
        LDO power savingTravis Downs2020/05/13 06:44 AM
          LDO power savingDan Fay2020/05/13 07:03 AM
            LDO power savingTravis Downs2020/05/13 03:02 PM
              LDO power savingRicardo B2020/05/14 12:08 AM
    Asynchronous CPU?dmcq2020/05/12 01:05 PM
      Hindered by device testing and design validationPaul A. Clayton2020/05/12 02:13 PM
        Hindered by device testing and design validationMark Roulo2020/05/13 08:22 AM
          Hindered by device testing and design validationAnon32020/05/14 07:06 AM
  Work scheduling for moderating powerPaul A. Clayton2020/05/12 10:15 AM
    Work scheduling for moderating powerDavid Kanter2020/05/12 10:40 AM
      Work scheduling for moderating powerPaul A. Clayton2020/05/14 09:27 AM
    Work scheduling for moderating powerTravis Downs2020/05/12 01:25 PM
      That is certain one real world example (NT)Paul A. Clayton2020/05/14 09:28 AM
  New article: Power Delivery in a Modern ProcessorDanjel McGougan2020/05/13 06:52 AM
    New article: Power Delivery in a Modern ProcessorTravis Downs2020/05/13 03:07 PM
      New article: Power Delivery in a Modern ProcessorRob Thorpe2020/05/15 06:36 PM
        New article: Power Delivery in a Modern ProcessorRicardo B2020/05/15 10:31 PM
  Bypassing capacitors & testingRob Thorpe2020/05/15 06:54 PM
  New article: Power Delivery in a Modern ProcessorRicardo B2020/05/15 10:22 PM
    New article: Power Delivery in a Modern ProcessorDavid Kanter2020/05/16 09:03 AM
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