No conflict between theory and practice

By: Linus Torvalds (torvalds.delete@this.linux-foundation.org), April 8, 2021 7:41 am
Room: Moderated Discussions
Andrey (andrey.semashev.delete@this.gmail.com) on April 7, 2021 5:54 pm wrote:
>
> Thing is, I don't see the point in defining the limit on the transaction size to begin with. HTM implementation
> has no reason to care about instruction count, the only thing it should care about is memory accesses.

Wrong.

If the transaction hardware is built around the - already existing - rename hardware and out-of-order queues, then the transaction size is limited not only by number of memory accesses, but by everything else too - number of register accesses, number of (possibly split) instructions etc etc.

And if the transaction hardware is not built around that kind of existing machinery, but around some special "checkpoint/restore" mode, then the transaction hardware is slow garbage.

Really.

So that's my argument. I think MTH is a quite reasonable idea, but it needs to be a hell of a lot better than TSX ever was. And the way to make it better is to make it (a) faster and (b) less of a buggy morass. Because it was both slow and buggy.

And my argument is that the way to get there might well be to make it simpler. Not add more complexity, and not make it depend on some checkpointing model that is fundamentally expensive.

Right now TSX is unusable. Not only because it's broken and buggy, but simply because it is too slow.

Doing a (successful) TSX transaction was in my tests slower than just using a lock.

And I will repeat: the default for locking - and the common case - is not contention. So if your transactional hardware is slower than a lock when there is no contention on the lock and the data, then your transactional hardware is a dead end.

Sure, that was only one particular implementation (I only ever had one machine that for a while had TSX enabled), and I didn't do some kind of rigorous big testing. I did enough to see "Oh, this is slower than just doing it by hand, both when uncontended and when contended", and dropped my test-patches for the kernel.

Andrey, I don't understand why you are pushing TSX. You apparently have never used it, you don't seem to care about the fact that it's been buggy and slow. You just try to push a false narrative of "it could work if it was just even bigger and more complex and slower".

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