Nehalem Architecture: Improvements Detailed

By: Linus Torvalds (torvalds.delete@this.linux-foundation.org), March 25, 2008 8:52 am
Room: Moderated Discussions
Philip Honermann (Philip.Honermann@gmail.com) on 3/25/08 wrote:
>
>No, I'm not talking about the ABA-problem.

I think you're a troll. If you cannot even bother to say
what you're blathering about, why don't you just go away?

>You can't write a usual Memory-Allocator with
>malloc()-semantics with CMPXCHG because of the deallocation
>issue; with LL/SC you could.

Google for SMR. Or other alternatives. Yeah, yeah, it isn't
trivial, and yeah, people always end up using locking in
practice, because locking is just so much easier to think
about and validate.

But more importantly, you seem to be totally ignoring that
while LL/SC could be nice in general, it's not actually all
that nice in reality: actual real implementations of
LL/SC disallow "fancy" things like memory operations or
even taken conditional branches in between the LL and the
SC because the SC may never complete successfully.

In fact, you seem to be entirely arguing from some LL/SC
implementation that simply does not exist. If you
can't actually do anything interesting in between the LL/SC,
and you almost invariably end up having to essentially
just make the LL/SC act as just a CAS.

For example, for a linked list operation, you'd like to do
something trivial like

p = LL (HEAD)
next = p->next;
if (SC(next, HEAD))
goto repeat

but the thing is, real-world implementations of LL/SC do
not allow that code-sequence to work, because the memory
load in between the LL and SC is against the rules (it can
cause HEAD to be evicted out of the cache, and thus make
the SC fail - think direct-mapped caches with 'p' and HEAD
in the same way).

(Yeah, I didn't actually validate the above code-sequence,
maybe it's just terminally broken for other reasons, I'm
just writing it as an example as I go).

My point is, you may think that LL/SC makes it very
trivial to make arbitrary atomic ops like the above, but
no it does not. Generally the only thing you can actually
rely on with LL/SC is the exact same semantics that you
get from x86-style ops
. IOW, what you can do in between
the LL and the SC are trivial arithmetic ops and "compare
and abort".

So it seems that from a practical perspective, real people
who work with real-life lockless algorithms either

- accept the fact that general LL/SC isn't there (ie the
hardware doesn't do LL/SC at all, or if it does, it does
the weak form that severly restricts what you can do in
between the LL/SC)

- would actually prefer DCAS instead if they could dream
about hardware extensions, knowing that DCAS can actually
be done with well-defined semantics over arbitrary code.

and I don't see why you're being so married to LL/SC, and
so unable to actually try to even explain why, or
unable to actually face the arguments I've presented.

But while past performance doesn't guarantee future returns,
I have this strong suspicion that it's not worth continuing
this discussion.

Linus
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TopicPosted ByDate
Nehalem Architecture: Improvements Detailed Blut Aus Nord2008/03/17 02:52 PM
  Nehalem Architecture: Improvements Detailed bah2008/03/17 04:45 PM
    Nehalem Architecture: Improvements Detailed Linus Torvalds2008/03/17 06:14 PM
      Nehalem Architecture: Improvements Detailed Gabriele Svelto2008/03/18 01:11 AM
        Nehalem Architecture: Improvements Detailed Henrik S2008/03/18 04:23 AM
        Nehalem Architecture: Improvements Detailed Doug Siebert2008/03/18 09:48 PM
          Nehalem Architecture: Improvements Detailed anon2008/03/18 10:37 PM
            Nehalem Architecture: Improvements Detailed Doug Siebert2008/03/19 05:23 PM
          Nehalem Architecture: Improvements Detailed Ian Ollmann2008/03/19 08:15 AM
            SSE 4.2 Michael S2008/03/19 04:13 PM
              SSE 4.2 Ian Ollmann2008/03/20 09:56 AM
              SSE 4.2 anonymous2008/03/20 12:29 PM
                SSE 4.2 David W. Hess2008/03/21 07:24 AM
                  SSE 4.2 anonymous2008/03/22 07:27 AM
      CMPXCHG latencyDavid Kanter2008/03/28 05:59 PM
        CMPXCHG latencyanonymous coward2008/03/28 10:24 PM
          CMPXCHG latencyDavid Kanter2008/03/28 10:26 PM
            CMPXCHG latencyLinus Torvalds2008/03/29 11:43 AM
              CMPXCHG latencyDavid W. Hess2008/03/29 11:56 AM
              CMPXCHG latencyLinus Torvalds2008/03/29 02:17 PM
                CMPXCHG latencyGabriele Svelto2008/03/31 12:25 AM
                  CMPXCHG latencyMichael S2008/03/31 12:38 AM
                    CMPXCHG latencynick2008/03/31 12:52 AM
                      CMPXCHG latencyMichael S2008/03/31 01:51 AM
                        CMPXCHG latencyGabriele Svelto2008/03/31 02:08 AM
                        CMPXCHG latencynick2008/03/31 07:20 PM
                          CMPXCHG latencyMichael S2008/04/01 01:14 AM
                            CMPXCHG latencynick2008/04/01 02:34 AM
                    CMPXCHG latencyLinus Torvalds2008/03/31 10:16 AM
                      CMPXCHG latencyAaron Spink2008/03/31 07:15 PM
                        CMPXCHG latencynick2008/03/31 07:34 PM
                        CMPXCHG latencyLinus Torvalds2008/04/01 08:25 AM
                          CMPXCHG latencyZan2008/04/01 09:54 PM
                          CMPXCHG latencyZan2008/04/02 12:11 AM
                            CMPXCHG latencyLinus Torvalds2008/04/02 08:04 AM
                              CMPXCHG latencyZan2008/04/02 11:02 AM
                                CMPXCHG latencyLinus Torvalds2008/04/02 12:02 PM
                                  CMPXCHG latencyZan2008/04/02 04:15 PM
                      CMPXCHG latencyMichael S2008/04/01 01:26 AM
                        CMPXCHG latencyLinus Torvalds2008/04/01 07:08 AM
                CMPXCHG latency - Intel sourceWouter Tinus2008/04/02 12:36 PM
                  CMPXCHG latency - Intel sourceLinus Torvalds2008/04/02 02:21 PM
                    CMPXCHG latency - Intel sourceDavid Kanter2008/04/02 02:39 PM
    Nehalem Architecture: Improvements Detailed Philip Honermann2008/03/19 01:11 PM
      Nehalem Architecture: Improvements Detailed Linus Torvalds2008/03/19 01:43 PM
        CMPXCHG - all or nothingMichael S2008/03/19 03:49 PM
          multithreading - all or nothingno@thanks.com2008/03/19 05:17 PM
          CMPXCHG - all or nothingLinus Torvalds2008/03/19 05:21 PM
            CMPXCHG - all or nothingMichael S2008/03/20 06:38 AM
              CMPXCHG - all or nothingLinus Torvalds2008/03/20 08:45 AM
                CMPXCHG - all or nothingMichael S2008/03/21 07:08 AM
                  CMPXCHG - all or nothingLinus Torvalds2008/03/21 08:47 AM
            CMPXCHG - all or nothingHenrik S2008/03/20 10:09 AM
              CMPXCHG - all or nothingLinus Torvalds2008/03/20 10:53 AM
                CMPXCHG - all or nothingHenrik S2008/03/20 12:03 PM
                  CMPXCHG - all or nothingLinus Torvalds2008/03/20 01:12 PM
                    CMPXCHG - all or nothingHenrik S2008/03/21 12:13 AM
                      CMPXCHG - all or nothingGabriele Svelto2008/03/21 01:22 AM
        Nehalem Architecture: Improvements Detailed Philip Honermann2008/03/19 06:28 PM
          Nehalem Architecture: Improvements Detailed Linus Torvalds2008/03/19 07:42 PM
            Nehalem Architecture: Improvements Detailed Philip Honermann2008/03/20 06:03 PM
              Nehalem Architecture: Improvements Detailed Linus Torvalds2008/03/20 06:33 PM
                Nehalem Architecture: Improvements Detailed Philip Honermann2008/03/25 06:37 AM
                  Nehalem Architecture: Improvements Detailed Linus Torvalds2008/03/25 08:52 AM
                    What is DCAS? (NT)David Kanter2008/03/25 10:13 AM
                      Double compare-and-exchangeHenrik S2008/03/25 10:57 AM
                        Double compare-and-exchangeLinus Torvalds2008/03/25 11:38 AM
                          Double compare-and-exchangesavantu2008/03/25 01:54 PM
                            Double compare-and-exchangeLinus Torvalds2008/03/25 04:09 PM
                              Double compare-and-exchangeJamie Lucier2008/03/25 08:55 PM
                                Double compare-and-exchangesavantu2008/03/25 09:15 PM
                                  Double compare-and-exchangeHenrik S2008/03/26 08:40 AM
                                    Double compare-and-exchangeArun Ramakrishnan2008/03/27 02:07 AM
                                      Double compare-and-exchangeHenrik S2008/03/27 04:45 AM
                                  Surely GPL applies ?Richard Cownie2008/03/26 10:05 AM
                                    Surely GPL applies ?anon2008/03/26 02:58 PM
                                    Surely GPL applies ?Paul2008/03/26 05:01 PM
                                Double compare-and-exchangesomeone2008/03/25 09:18 PM
                                  Double compare-and-exchangeArun Ramakrishnan2008/03/27 02:03 AM
                                    Double compare-and-exchangesavantu2008/03/27 03:01 AM
                                      Double compare-and-exchangeArun Ramakrishnan2008/03/30 09:09 AM
                                        Double compare-and-exchangesavantu2008/03/30 09:59 AM
                                Double compare-and-exchangeLinus Torvalds2008/03/26 10:50 AM
                                  Double compare-and-exchangeanon2008/03/26 04:47 PM
                                  Double compare-and-exchangePaul2008/03/26 05:07 PM
                          Double compare-and-exchangeHoward Chu2008/03/25 05:18 PM
  Nehalem Architecture: Improvements Detailed Mr. Camel2008/03/17 08:50 PM
    Nehalem Architecture: Improvements Detailed anonymous2008/03/17 09:20 PM
  TFP will finally come :-)Paul A. Clayton2008/03/18 12:56 PM
  Nehalem Architecture: Improvements Detailed IntelUser20002008/03/27 07:46 PM
    Nehalem Architecture: Improvements Detailed David Kanter2008/03/27 10:21 PM
      Nehalem Architecture: Improvements Detailed nick2008/03/27 11:06 PM
        Nehalem Architecture: Improvements Detailed David Kanter2008/03/28 02:45 PM
          Nehalem Architecture: Improvements Detailed nick2008/03/28 07:52 PM
  L1 I-cachepuzzled2008/04/01 07:53 AM
    L1 I-cacheS. Rao2008/04/01 09:47 AM
    L1 I-cacherwessel2008/04/01 12:23 PM
    L1 I-cacheGabriele Svelto2008/04/03 12:30 AM
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