ISSCC Poulson Preview Online - OoO not unlikely?

Article: New Itanium Microarchitecture at ISSCC 2011
By: anon (anon.delete@this.anon.com), November 17, 2010 9:22 pm
Room: Moderated Discussions
someone (someone@somewhere.com) on 11/17/10 wrote:
---------------------------
>anon (anon@anon.com) on 11/17/10 wrote:
>---------------------------
>>someone (someone@somewhere.com) on 11/17/10 wrote:
>>---------------------------
>>>Richard Cownie (tich@pobox.com) on 11/17/10 wrote:
>>>---------------------------
>>
>>>>But for sure, if Intel are really going to keep on building
>>>>new IA64 cpu's, they should make them as good as they can,
>>>>whether OoO or not. I just don't see any reason to think
>>>>that an ISA contorted to avoid the need for OoO and
>>>>register renaming is magically going to provide benefits
>>>>for an OoO implementation. That would be very weird.
>>>>
>>>
>>>It burns a lot of power using hundreds of thousands of
>>>logic transistors re-discovering something about a scrap
>>>of code every single loop iteration or subroutine call for
>>>every execution of all copies of program that one compiler
>>>need only to discern once. Everything can't be discovered
>>>at compile time but it is stupid to ignore that which can
>>>be and then passed on using a suitable equipped ISA.
>>
>>I don't think you can really call a sane OoOE implementation "heroic" and "burning
>>lots of power" any more.
>
>The OOOE logic in the Pentium M consumes about 26%
>of device power and twice as much power it uses actually
>doing computation (i.e. integer and FP data paths). Going
>wider issue and/or larger window burns disproportionally
>more of the power budget.

Well since the Pentium M and power wall, focus has been continually on energy efficiency, and width and depth has only gotten larger. POWER7 has some pretty big widths and depths too, but it delivers clear twice the throughput of a Xeon at probably no more than twice the power (and that includes a vastly more capable SMP fabric).

Efforts on improving efficiency (like using PRFs in SB) has likewise apparently allowed larger windows within power budget.

So I don't think trends have played out like the naive view of things back when EPIC was being designed. There is no "end game" that says OOOE CPUs must converge on larger and larger windows and widths and eventually undergo heat-death, nor that in-order CPUs are destined to naturally overtake OOOE as transistor size shrinks. ILP and IPC has not been increasing monotonically, there was no final battle between hardware and software to see who can extract more parallelism.

In fact the reality is much more pragmatic and obvious really (if only in hindsight). Most workloads do not have abundant ILP, and so widths will not continue with a spiral of increases. There are pretty hard limits on effectiveness of speculation, so pipelines and windows will not continue to spiral upwards. When branches don't get you, cache misses probably will, and in that arena, MLP is the most effective way to get performance. Some measure of OOOE seems to do a much better job than software there, but returns will diminish rather quickly.

So there may be a very steep curve on efficiency of OOOE, but there is no trend to keep expanding it indefinitely -- there are rapidly diminishing returns even if you can do it efficiently, it won't buy you much. There is no competition from in-order devices for OOOE windows to continue increasing, because in-order are subject to much the same limits.

So I don't see why there should be an ever increasing proportion of energy spend on OOOE. I think it is more like an *appropriate* amount of energy is spent depending on where in the performance and efficiency curve the device is intended. Same can be said of any feature at all. Wide issue and big rfs and forwarding network; fast caches to help static scheduling; predication / prediction and wasted results.

We simply never reached the stage where OOOE got pushed right off the edge of the efficiency graph. Instruction patterns and limits just never saw that happen.

The only place I can really see that happen in high performance CPUs is when there is a _lot_ of parallelism (ie. Niagara or GPU workload), and in that case there does seem to be a lot of merit and efficiency in using multi core and multi threading to get the throughput. But TLP in real world isn't unbounded either, not even for servers.

All in all, I can't see OOOE becoming extinct for desktop or general purpose sever CPUs any time soon. POWER6 shows that it is continually being evaluated, but they had to jump through hoops to get MLP, and evidently decided it was not worthwhile to keep going down that road.
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ISSCC Poulson Preview OnlineDavid Kanter2010/11/17 03:40 AM
  ISSCC Poulson Preview Onlinesavantu2010/11/17 05:25 AM
  ISSCC Poulson Preview Onlinesomeone2010/11/17 08:51 AM
    ISSCC Poulson Preview OnlineLinus Torvalds2010/11/17 09:27 AM
      ISSCC Poulson Preview Onlinesomeone2010/11/17 10:31 AM
        ISSCC Poulson Preview OnlineRichard Cownie2010/11/17 10:55 AM
          ISSCC Poulson Preview OnlineLinus Torvalds2010/11/17 11:30 AM
          ISSCC Poulson Preview Onlinesomeone2010/11/17 11:42 AM
            ISSCC Poulson Preview Onlinea reader2010/11/17 02:07 PM
        ISSCC Poulson Preview OnlineGabriele Svelto2010/11/19 02:52 AM
          ISSCC Poulson Preview Onlinesavantu2010/11/19 05:37 AM
    ISSCC Poulson Preview OnlineDavid Kanter2010/11/17 12:12 PM
      ISSCC Poulson Preview Onlinesomeone2010/11/17 12:40 PM
        ISSCC Poulson Preview OnlineDavid Kanter2010/11/17 02:08 PM
          ISSCC Poulson Preview Onlineajensen2010/11/18 01:24 AM
  ISSCC Poulson Preview Onlinelubemark2010/11/17 09:52 AM
  ISSCC Poulson Preview Online - OoO not unlikely?anon2010/11/17 11:41 AM
    ISSCC Poulson Preview Online - OoO not unlikely?Linus Torvalds2010/11/17 12:12 PM
      ISSCC Poulson Preview Online - OoO not unlikely?anon2010/11/17 05:38 PM
        ISSCC Poulson Preview Online - OoO not unlikely?Linus Torvalds2010/11/17 08:03 PM
    ISSCC Poulson Preview Online - OoO not unlikely?someone2010/11/17 12:14 PM
      ISSCC Poulson Preview Online - OoO not unlikely?Richard Cownie2010/11/17 02:07 PM
        ISSCC Poulson Preview Online - OoO not unlikely?someone2010/11/17 02:59 PM
          ISSCC Poulson Preview Online - OoO not unlikely?sylt2010/11/17 03:26 PM
          ISSCC Poulson Preview Online - OoO not unlikely?anon2010/11/17 05:53 PM
            ISSCC Poulson Preview Online - OoO not unlikely?someone2010/11/17 08:34 PM
              ISSCC Poulson Preview Online - OoO not unlikely?anon2010/11/17 09:22 PM
              ISSCC Poulson Preview Online - OoO not unlikely?Martin Høyer Kristiansen2010/11/18 12:35 AM
                ISSCC Poulson Preview Online - OoO not unlikely?anon2010/11/18 01:14 AM
                  ISSCC Poulson Preview Online - OoO not unlikely?Martin Høyer Kristiansen2010/11/18 02:00 AM
          ISSCC Poulson Preview Online - OoO not unlikely?Groo2010/11/18 03:28 PM
            ISSCC Poulson Preview Online - OoO not unlikely?someone2010/11/18 09:30 PM
              ISSCC Poulson Preview Online - OoO not unlikely?Doug Siebert2010/11/19 12:06 AM
                ISSCC Poulson Preview Online - OoO not unlikely?someone2010/11/19 07:52 AM
                  Negativity and ignorancePaul A. Clayton2010/11/19 06:11 PM
                  rhetoricshobold2010/11/20 05:01 AM
              ISSCC Poulson Preview Online - OoO not unlikely?anonymous2010/11/19 02:17 AM
                ISSCC Poulson Preview Online - OoO not unlikely?someone2010/11/19 08:22 AM
                  ISSCC Poulson Preview Online - OoO not unlikely?Linus Torvalds2010/11/19 09:26 AM
                    ISSCC Poulson Preview Online - OoO not unlikely?someone2010/11/19 11:04 AM
                      ISSCC Poulson Preview Online - OoO not unlikely?Linus Torvalds2010/11/19 11:24 AM
                        ISSCC Poulson Preview Online - OoO not unlikely?someone2010/11/19 12:05 PM
                          ISSCC Poulson Preview Online - OoO not unlikely?Linus Torvalds2010/11/19 01:08 PM
                            ISSCC Poulson Preview Online - OoO not unlikely?someone2010/11/20 11:02 AM
                              ISSCC Poulson Preview Online - OoO not unlikely?dc2010/11/20 12:59 PM
                              ISSCC Poulson Preview Online - OoO not unlikely?dc2010/11/20 01:01 PM
                                ISSCC Poulson Preview Online - OoO not unlikely?rwessel2010/11/20 01:27 PM
                                  ISSCC Poulson Preview Online - OoO not unlikely?dc2010/11/21 05:00 PM
                                    single-system single-location != RASanonymous2010/11/21 09:33 PM
                                      single-system single-location != RASdc2010/11/25 12:06 PM
                                        single-system single-location != RASDavid Kanter2010/11/25 01:58 PM
                    CS:SNAREPslacker2010/11/20 04:07 AM
                      CS:SNAREPnone2010/11/20 04:53 AM
                        CS:SNAREPS. Rao2010/11/20 11:20 AM
                      CS:SNAREPGabriele Svelto2010/11/22 07:11 AM
                  ISSCC Poulson Preview Online - OoO not unlikely?anon2010/11/19 11:08 AM
                    ISSCC Poulson Preview Online - OoO not unlikely?someone2010/11/19 06:51 PM
                      ISSCC Poulson Preview Online - OoO not unlikely?rwessel2010/11/20 01:53 AM
                        ISSCC Poulson Preview Online - OoO not unlikely?dc2010/11/20 12:43 PM
                          ISSCC Poulson Preview Online - OoO not unlikely?anon2010/11/20 05:43 PM
                            ISSCC Poulson Preview Online - OoO not unlikely?Foo_2010/11/20 05:56 PM
              ISSCC Poulson Preview Online - OoO not unlikely?Groo2010/11/20 12:59 PM
                P = E/tslacker2010/11/20 11:31 PM
                  P = E/tAndi Kleen2010/11/21 03:02 AM
                ISSCC Poulson Preview Online - OoO not unlikely?someone2010/11/21 10:24 AM
                  ISSCC Poulson Preview Online - OoO not unlikely?Richard Cownie2010/11/21 01:07 PM
                    ISSCC Poulson Preview Online - OoO not unlikely?someone2010/11/21 04:50 PM
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                        ISSCC Poulson Preview Online - OoO not unlikely?someone2010/11/21 06:27 PM
                          ISSCC Poulson Preview Online - OoO not unlikely?Richard Cownie2010/11/22 08:37 AM
                            ISSCC Poulson Preview Online - OoO not unlikely?someone2010/11/22 09:07 AM
                              ISSCC Poulson Preview Online - OoO not unlikely?Richard Cownie2010/11/22 09:25 AM
                                ISSCC Poulson Preview Online - OoO not unlikely?dc2010/11/25 12:22 PM
                                  ISSCC Poulson Preview Online - OoO not unlikely?Richard Cownie2010/11/26 10:44 AM
                              perfAM2010/11/24 03:28 AM
                                perfajensen2010/11/24 03:46 AM
                                perfsomeone2010/11/24 09:59 AM
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                            OOOE and energy efficiencysomeone2010/11/23 12:42 PM
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