And a hardware predicttor needs access to fallback timing

By: Linus Torvalds (torvalds.delete@this.linux-foundation.org), April 3, 2021 11:11 am
Room: Moderated Discussions
Linus Torvalds (torvalds.delete@this.linux-foundation.org) on April 3, 2021 10:41 am wrote:
>
> Look here: if you know your abort cost, and you have some rough idea of how much you expect to win from doing
> the operation as a transaction, you should now have a rough idea how hard you should avoid aborts.

Put another way: just a fairly simple rule of thumb might be "for a transaction to make sense, it has to succeed 99% of the time, because the abort costs are generally 100x the win".

And the actual failure predictor doesn't have to really count to 100 to get that. The actual failure predictor can be just a couple of bits - exactly like a branch predictor, and make it be exactly that same kind of "weak predict" (failed once) and "strong predict" (failed twice, or failed with a capacity error") logic.

And once you have a "strongly predict not to do this transaction", you basically treat it as a branch prediction to go to the fallback case, and you can do the aging with just a global counter. When the counter hits some predetermined value, you clear the prediction data (for that predictor), and reset the counter.

The above is obviously just a suggested starting point. As mentioned, you have to do a lot of actual simulation of real loads to get there, but you may be able to do a lot of it with real hardware (implement the prediction in microcode).

The above is the kind of thing that hardware can do. But software cannot. Because doing it in software will eat into that very precious - and not very big - "this is how many cycles you can win by doing it as a transaction".

Because please realize that the cost of locking is not very big at all in the common case. The cost of an uncontended lock is literally on the order of "ten cycles", and most of those ten cycles are because of memory ordering constraints, not "instruction costs".

And so that "on the order of ten cycles" is what you have to at least match with a transaction for it to really make sense in general, because the contended case is not the common case.

If you have to do the prediction in software, you already ate up all the wins.

So your transaction cost should match the cost of that uncontended lock at the good end ("because common case"), and then scale better than a contended lock at the bad end ("because this is why we do transactions").

And that is fundamental. Because otherwise, why would you ever want that transaction in the first place?

Linus
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TopicPosted ByDate
Armv9 officially announcedJon Masters2021/03/30 11:41 AM
  Armv9 officially announcedGabriele Svelto2021/03/30 01:27 PM
    HTM and TLEFoo_2021/03/30 01:31 PM
      HTM and TLEdmcq2021/03/30 03:03 PM
      Intel RTM and HLE is a successGanon2021/03/30 05:22 PM
        Intel RTM and HLE is a success (is it?)Foo_2021/03/31 01:16 AM
          Intel RTM and HLE was an abject failureanonymou52021/03/31 03:04 AM
          Intel RTM and HLE is a success (is it?)Andrey2021/03/31 05:27 AM
            Intel RTM and HLE is a success (is it?)Foo_2021/03/31 05:58 AM
              Intel RTM and HLE is a success (is it?)Andrey2021/03/31 07:45 AM
                Intel RTM and HLE is a success (is it?)Foo_2021/03/31 09:32 AM
                  Intel RTM and HLE is a success (is it?)Andrey2021/03/31 09:57 AM
                    Intel RTM and HLE is a success (is it?)anonymou52021/03/31 10:39 AM
                      ^ feel free to delete this one -- broken HTML there (NT)anonymou52021/03/31 10:40 AM
                      Intel RTM and HLE is a success (is it?)Andrey2021/03/31 10:47 AM
                    Intel RTM and HLE is a success (is it?)anonymou52021/03/31 10:40 AM
                Intel RTM and HLE is a success (is it?)Ganon2021/03/31 09:58 AM
                  Intel RTM and HLE is a success (is it?)anonymou52021/03/31 10:42 AM
                  Intel RTM and HLE is a success (is it?)Linus Torvalds2021/03/31 11:54 AM
                    Intel RTM and HLE is a success (is it?)Linus Torvalds2021/03/31 12:00 PM
                      Any idea about IBM?Mark Roulo2021/03/31 12:15 PM
                        Any idea about IBM?Linus Torvalds2021/03/31 12:37 PM
                          Any idea about IBM?dmcq2021/03/31 03:04 PM
                            Any idea about IBM?Linus Torvalds2021/03/31 04:44 PM
                              A non straw man view of hardware transactional memoryGanon2021/03/31 07:52 PM
                                A non straw man view of hardware transactional memoryanon22021/03/31 11:03 PM
                                  A non straw man view of hardware transactional memoryCarson2021/04/02 01:11 AM
                                    A non straw man view of hardware transactional memoryanon22021/04/02 05:28 AM
                              IBM zArch TM - guaranteed progressDavid Kanter2021/03/31 08:37 PM
                              Any idea about IBM?Andrey2021/03/31 11:31 PM
                                Any idea about IBM?Linus Torvalds2021/04/01 10:54 AM
                                  Any idea about IBM?Andrey2021/04/02 12:50 PM
                        Any idea about IBM?someone2021/04/01 12:02 AM
                      Intel RTM and HLE is a success (is it?)anon22021/03/31 03:46 PM
                        Intel RTM and HLE is a success (is it?)Linus Torvalds2021/03/31 05:08 PM
                          Leaving it to software is tricky!David Kanter2021/03/31 08:41 PM
                            And a hardware predicttor needs access to fallback timingCarson2021/04/01 11:13 PM
                              Hardware fallback pathAnon2021/04/02 10:51 AM
                              And a hardware predicttor needs access to fallback timingLinus Torvalds2021/04/03 10:41 AM
                                And a hardware predicttor needs access to fallback timingLinus Torvalds2021/04/03 11:11 AM
                                  And a hardware predicttor needs access to fallback timingsr2021/04/03 11:30 AM
                                    And a hardware predicttor needs access to fallback timingLinus Torvalds2021/04/03 12:14 PM
                                      And a hardware predicttor needs access to fallback timingsr2021/04/03 12:39 PM
                                        And a hardware predicttor needs access to fallback timingAnon2021/04/03 02:08 PM
                                          And a hardware predicttor needs access to fallback timingsr2021/04/03 02:33 PM
                                            And a hardware predicttor needs access to fallback timingdmcq2021/04/04 05:35 AM
                                        And a hardware predicttor needs access to fallback timingLinus Torvalds2021/04/03 02:22 PM
                                          Transactional memory isn't exclusive to lockingsr2021/04/04 12:17 AM
                                            Transactional memory isn't exclusive to lockingAspect of Anonimity2021/04/04 03:49 AM
                                            Transactional memory isn't exclusive to lockingAndrey2021/04/04 04:58 AM
                                              Transactional memory isn't exclusive to lockingsr2021/04/04 10:10 AM
                                                Transactional memory isn't exclusive to lockingAndrey2021/04/04 10:33 AM
                                                  Transactional memory isn't exclusive to lockingsr2021/04/05 02:41 AM
                                      And a hardware predicttor needs access to fallback timingGeertB2021/04/04 07:08 PM
                            Leaving it to software is tricky!Andrey2021/04/02 03:00 PM
                          Intel RTM and HLE is a success (is it?)@never_released2021/04/01 08:21 AM
                            Intel RTM and HLE is a success (is it?)@never_released2021/04/01 08:30 AM
                            Intel RTM and HLE is a success (is it?)Linus Torvalds2021/04/01 10:00 AM
                              Intel RTM and HLE is a success (is it?)dmcq2021/04/01 10:35 AM
                                Intel RTM and HLE is a success (is it?)Linus Torvalds2021/04/01 10:59 AM
                                  Is HTM actually in ARMv9?dncq2021/04/01 11:26 AM
                                    Is HTM actually in ARMv9?Linus Torvalds2021/04/01 12:13 PM
                              Intel RTM and HLE is a success (is it?)Jörn Engel2021/04/01 09:15 PM
                          Intel RTM and HLE is a success (is it?)---2021/04/02 10:00 AM
                      Intel RTM and HLE is a success (is it?)Jon Masters2021/04/01 10:56 AM
                        RockMichael S2021/04/01 12:29 PM
            Intel RTM and HLE is a success (is it?)Linus Torvalds2021/03/31 11:50 AM
              Intel RTM and HLE is a success (is it?)anon22021/03/31 03:57 PM
                Intel RTM and HLE is a success (is it?)anon32021/03/31 04:09 PM
          Intel RTM and HLE is a success (is it?)someone2021/03/31 11:56 PM
            Intel RTM and HLE is a success (is it?)someone2021/04/01 12:21 AM
              Intel RTM and HLE is a success (is it?)none2021/04/01 04:31 AM
                Intel RTM and HLE is a success (is it?)anonymou52021/04/01 09:24 AM
                  Intel RTM and HLE is a success (is it?)anony2021/04/01 10:26 AM
                  Intel RTM and HLE is a success (is it?)none2021/04/01 11:20 AM
                Intel RTM and HLE is a success (is it?)Brendan2021/04/01 05:23 PM
                  Intel RTM and HLE is a success (is it?)Adrian2021/04/02 01:03 AM
      Transactional memory similarity to garbage collectionPaul A. Clayton2021/04/05 01:53 PM
        Maybe notMark Roulo2021/04/05 03:07 PM
        Transactional memory similarity to garbage collectionAnon2021/04/05 03:14 PM
        No conflict between theory and practiceAspect of Anonimity2021/04/05 07:57 PM
          No conflict between theory and practiceNoSpammer2021/04/05 09:38 PM
            No conflict between theory and practicedmcq2021/04/06 07:05 AM
              No conflict between theory and practicesr2021/04/06 09:22 AM
                No conflict between theory and practiceBen LaHaise2021/04/06 03:43 PM
                  No conflict between theory and practicesr2021/04/07 09:42 AM
                    No conflict between theory and practiceAnon2021/04/07 10:06 AM
                    No conflict between theory and practiceLinus Torvalds2021/04/07 10:35 AM
                      No conflict between theory and practicesr2021/04/07 12:34 PM
                        If HTM were well implemented, nobody would complain (NT)Anon2021/04/07 12:54 PM
                          If HTM were well implemented, nobody would complaindmcq2021/04/07 03:36 PM
                            If HTM were well implemented, nobody would complainAnon2021/04/07 04:04 PM
                          If HTM were well implemented, nobody would complain---2021/04/08 09:37 AM
                        No conflict between theory and practiceLinus Torvalds2021/04/07 01:20 PM
                          No conflict between theory and practiceAndrey2021/04/07 02:32 PM
                            No conflict between theory and practicedmcq2021/04/07 03:32 PM
                              No conflict between theory and practiceanonymou52021/04/07 04:26 PM
                              No conflict between theory and practiceAndrey2021/04/07 05:54 PM
                                No conflict between theory and practiceLinus Torvalds2021/04/08 08:41 AM
                                  No conflict between theory and practiceAndrey2021/04/08 09:12 AM
                                  No conflict between theory and practiceRobert Williams2021/04/08 09:15 AM
                                    No conflict between theory and practiceLinus Torvalds2021/04/08 09:56 AM
                                      No conflict between theory and practiceRobert Williams2021/04/08 07:50 PM
                                        No conflict between theory and practiceLinus Torvalds2021/04/09 09:25 AM
                                          TSX for all?Robert Williams2021/04/09 12:46 PM
                                            It helps adoption when developers can run the code an their machines.Mark Roulo2021/04/09 12:54 PM
                                              It helps adoption when developers can run the code an their machines.me2021/04/09 02:21 PM
                                                It helps adoption when developers can run the code an their machines.Andrey2021/04/10 07:08 AM
                                                  It helps adoption when developers can run the code an their machines.me2021/04/10 12:43 PM
                                                    It helps adoption when developers can run the code an their machines.Robert Williams2021/04/10 07:05 PM
                                                      It helps adoption when developers can run the code an their machines.Andrey2021/04/11 01:42 AM
                                                        It helps adoption when developers can run the code an their machines.Michael S2021/04/11 04:23 AM
                                              It helps adoption when developers can run the code an their machines.Robert Williams2021/04/10 08:24 AM
                                                It helps adoption when developers can run the code an their machines.Andrey2021/04/10 10:36 AM
                                                  It helps adoption when developers can run the code an their machines.Michael S2021/04/10 10:58 AM
                                                    It helps adoption when developers can run the code an their machines.Robert Williams2021/04/10 11:42 AM
                                                    It helps adoption when developers can run the code an their machines.Brendan2021/04/10 11:27 PM
                                                      It helps adoption when developers can run the code an their machines.Michael S2021/04/11 03:34 AM
                                                        It helps adoption when developers can run the code an their machines.Brendan2021/04/11 04:18 PM
                                            TSX for all?wumpus2021/04/09 01:10 PM
                                            TSX for all?Linus Torvalds2021/04/09 03:03 PM
                                              [CLICK BAIT?] Torvalds to recommend arm64!anonymous22021/04/09 03:17 PM
                                              TSX for all?Linus Torvalds2021/04/09 03:22 PM
                                                amen! (NT)anonymou52021/04/09 05:42 PM
                                            TSX for all?Emil Briggs2021/04/10 05:52 AM
                                              TSX for all?Michael S2021/04/10 11:13 AM
                                                TSX for all?Brendan2021/04/10 11:05 PM
                                          No conflict between theory and practiceanonymouse2021/04/09 03:02 PM
                            No conflict between theory and practiceLinus Torvalds2021/04/07 05:12 PM
                              No conflict between theory and practiceAndrey2021/04/07 05:33 PM
                                No conflict between theory and practiceanon22021/04/07 08:12 PM
                                  No conflict between theory and practiceAndrey2021/04/08 12:29 AM
                                    No conflict between theory and practiceAnon2021/04/08 12:50 AM
                                    No conflict between theory and practiceanon22021/04/08 12:52 AM
                                      No conflict between theory and practiceAndrey2021/04/08 02:41 AM
                                        No conflict between theory and practiceAdrian2021/04/08 03:09 AM
                                          No conflict between theory and practiceAndrey2021/04/08 03:50 AM
                                            No conflict between theory and practiceAdrian2021/04/08 05:07 AM
                                            No conflict between theory and practiceanon22021/04/08 05:18 AM
                                              No conflict between theory and practiceAndrey2021/04/08 08:18 AM
                                                No conflict between theory and practiceanon22021/04/08 09:11 AM
                                                  No conflict between theory and practiceAndrey2021/04/08 09:48 AM
                                                    No conflict between theory and practiceanon22021/04/08 04:10 PM
                                            No conflict between theory and practice---2021/04/08 08:30 PM
                                              No conflict between theory and practicedmcq2021/04/09 02:25 AM
                                                No conflict between theory and practiceLinus Torvalds2021/04/09 09:44 AM
                                        No conflict between theory and practiceanon22021/04/08 04:09 AM
                                          No conflict between theory and practicesr2021/04/10 01:22 AM
                                            No conflict between theory and practiceAnon2021/04/10 04:00 AM
                                              No conflict between theory and practiceEtienne Lorrain2021/04/12 12:56 AM
                                                No conflict between theory and practiceAnon2021/04/12 01:54 AM
                                                  No conflict between theory and practicedmcq2021/04/12 01:44 PM
                              No conflict between theory and practiceGabriele Svelto2021/04/08 01:03 AM
                                No conflict between theory and practicedmcq2021/04/12 01:53 PM
                                  No conflict between theory and practicedmcq2021/04/14 04:50 AM
                      transactional memory = memory lock/unlocksr2021/04/10 12:56 AM
                        transactional memory = memory lock/unlockAnon2021/04/10 04:05 AM
                    No conflict between theory and practiceanon22021/04/07 05:19 PM
                No conflict between theory and practicedmcq2021/04/07 01:16 PM
            No conflict between theory and practiceAnon2021/04/06 10:46 AM
            No conflict between theory and practiceAspect of Anonimity2021/04/06 02:16 PM
              No conflict between theory and practiceNoSpammer2021/04/10 12:02 AM
                No conflict between theory and practicesr2021/04/10 01:47 AM
                No conflict between theory and practiceAspect of Anonimity2021/04/10 11:30 PM
                  No conflict between theory and practiceAndrey2021/04/11 05:05 AM
                    HmAspect of Anonimity2021/04/12 07:29 PM
                      Special internal SRAM to store Mutex?Etienne Lorrain2021/04/13 01:01 AM
                        Special internal SRAM to store Mutex?Anon2021/04/13 01:26 AM
                        Special internal SRAM to store Mutex?Linus Torvalds2021/04/13 09:53 AM
                        Special internal SRAM to store Mutex?Dan Fay2021/04/13 12:03 PM
                          Special internal SRAM to store Mutex?Ben LaHaise2021/04/13 03:32 PM
                          Special internal SRAM to store Mutex?Gabriele Svelto2021/04/13 11:43 PM
                        Special internal SRAM to store Mutex?Carson2021/04/13 10:19 PM
          No conflict between theory and practicesr2021/04/06 07:12 AM
            No conflict between theory and practiceAnon2021/04/06 10:43 AM
            No conflict between theory and practiceAspect of Anonimity2021/04/06 03:20 PM
              No conflict between theory and practicesr2021/04/07 10:09 AM
                No conflict between theory and practiceanon22021/04/07 08:53 PM
  Armv9 officially announceddmcq2021/03/30 03:28 PM
    Totally backwads logic on compatibilityHeikki Kultala2021/03/30 03:44 PM
      Totally backwads logic on compatibilityDoug S2021/03/31 12:09 PM
        Totally backwads logic on compatibilitydmcq2021/03/31 03:15 PM
  Armv9 officially announcedv92021/04/12 03:57 PM
    Armv9 officially announcedj2021/04/13 05:07 AM
      Armv9 officially announcedDoug S2021/04/13 10:21 AM
        Armv9 officially announcedanonymou52021/04/13 11:06 AM
          Armv9 officially announcedDoug S2021/04/13 01:01 PM
        Armv9 officially announceddmcq2021/04/13 03:20 PM
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