On Mon, Nov 5, 2018 at 11:41 AM Paul E. McKenney <paulmck@xxxxxxxxxxxxx> wrote:
>
> On Mon, Nov 05, 2018 at 11:08:15AM +0800, Junchang Wang wrote:
> > On Sun, Nov 4, 2018 at 10:21 PM Akira Yokosawa <akiyks@xxxxxxxxx> wrote:
> > >
> > > On 2018/11/04 20:14:50 +0800, Junchang Wang wrote:
> > > > Hi Akira,
> > > >
> > > > Thanks for your email about litmus tests a few weeks ago. I can
> > > > successfully run litmus tests on all three servers I can touch (The
> > > > configurations of the servers are listed below).
> > >
> > > Hi Junchang,
> > >
> > > Glad to know it helped you!
> > >
> > > > Everything goes well
> > > > except litmus tests C-CCIRIW+o+o+o-o+o-o (Listing 15.15) and
> > > > C-WRC+o+o-data-o+o-rmb-o (Listing 15.16). The "exists" assertions
> > > > never trigger on all of my servers. I understand that these assertions
> > > > trigger only if the speeds of propagating writing x to different CPU
> > > > cores varies a lot. So I have tried different litmus thread placement
> > > > strategies by adjusting the affinity setting of litmus7. For example:
> > > > $ litmus7 -affinity incr1 ... ...
> > > > $ ./C-WRC+o+o-data-o+o-rmb-o.exe +ra -p 0,1,6
> > >
> > > By klitmus7, I have seen a low probability "Sometimes" on POWER8 system.
> > >
> > > One of the results reads:
> > >
> > > ----------------------------------------------------------
> > > Test C-WRC+o+o-data-o+o-rmb-o Allowed
> > > Histogram (6 states)
> > > 60327103:>1:r1=0; 2:r2=0; 2:r3=0;
> > > 22740026:>1:r1=1; 2:r2=0; 2:r3=0;
> > > 5 *>1:r1=1; 2:r2=1; 2:r3=0;
> > > 51117838:>1:r1=0; 2:r2=0; 2:r3=1;
> > > 55552319:>1:r1=1; 2:r2=0; 2:r3=1;
> > > 10262709:>1:r1=1; 2:r2=1; 2:r3=1;
> > > Ok
> > >
> > > Witnesses
> > > Positive: 5, Negative: 199999995
> > > Condition exists (1:r1=1 /\ 2:r2=1 /\ 2:r3=0) is validated
> > > Hash=43057833028631b2f87eb65fe95c0ba2
> > > Observation C-WRC+o+o-data-o+o-rmb-o Sometimes 5 199999995
> > > Time C-WRC+o+o-data-o+o-rmb-o 68.34
> > > ----------------------------------------------------------
> > >
> > > In theory, litmus7 can also trigger the "exists" clause, but it would
> > > require a very long runtime. Also please refer to the Answer to
> > > Quick Quiz 15.24. (You will find my name is mentioned there.)
> > >
> >
> > Hi Akira,
> >
> > I will try a long runtime. If that doesn't work, I suppose I need to
> > wait until I get another PPC server which I can physically touch.
> > Thanks a lot.
> >
> > > As Armv8 architecture is other-multicopy-atomic, this "exists" clause
> > > should never trigger, I suppose.
> > >
> >
> > I'm a bit confused here. Perfbook says "Therefore, a
> > non-multicopy-atomic platform can have a store reach different threads
> > at different times." (Page 277 the first paragraph). So my
> > understanding is that for Listing C-WRC+o+o-data-o+o-rmb-o, the
> > message of writing y could reach P2 before the message of writing x
> > could reach there, and hence the "exists" clause could trigger even if
> > on an ARM server. Is that correct? Or did I misunderstand something?
> >
> > Perfbook says "Most CPU vendors interested in providing multicopy
> > atomicity have therefore instead provided the slightly weaker
> > other-multicopy atomicity".(Page 277 the first paragraph). My
> > understanding is that "Most CPU vendors" here implies PPC, ARM, and
> > x86. In other words, all of the servers I can touch adopt
> > other-multicopy-atomic? Is that correct?
>
> ARM and x86 provide other-multicopy atomicity. IBM mainframe provides
> fully multicopy atomicity. PPC doesn't provide multicopy atomicity
> at all.
>
Thanks a lot!!! This is the first time I know about this. How about we
add this as a footnote or something to help readers easier to get a
sense of the use of multicopy atomicity in practice? Does the
following look OK?
===
>From 8e07ecd836e5b581d2953938a65a395208fe0af3 Mon Sep 17 00:00:00 2001
From: Junchang Wang <junchangwang@xxxxxxxxx>
Date: Tue, 6 Nov 2018 15:00:31 +0800
Subject: [PATCH] memorder: Clarify implementation details of multicopy
atomicity in mainstream architectures
Reported-by: Junchang Wang <junchangwang@xxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
---
memorder/memorder.tex | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/memorder/memorder.tex b/memorder/memorder.tex
index 5e74a91..5db4971 100644
--- a/memorder/memorder.tex
+++ b/memorder/memorder.tex
@@ -1901,7 +1901,9 @@ Most CPU vendors interested in providing
multicopy atomicity have therefore
instead provided the slightly weaker
\emph{other-multicopy atomicity}~\cite[Section B2.3]{ARMv8A:2017},
which excludes the CPU doing a given store from the requirement that all
-CPUs agree on the order of all stores.
+CPUs agree on the order of all stores \footnote{As of late 2018, ARM and x86
+provide other-multicopy atomicity, IBM mainframe provides fully multicopy
+atomicity, and PPC does not provide multicopy atomicity at all.}.
This means that if only a subset of CPUs are doing stores, the
other CPUs will agree on the order of stores, hence the ``other''
in ``other-multicopy atomicity''.
--
2.7.4
Thanks,
--Junchang
> Thanx, Paul
>
> > Thanks,
> > --Junchang
> >
> > > As for C-CCIRIW+o+o+o-o+o-o, herd7 says
> > >
> > > ----------------------------------------------------------
> > > Test C-CCIRIW+o+o+o-o+o-o Allowed
> > > States 47
> > > 2:r1=0; 2:r2=0; 3:r3=0; 3:r4=0;
> > > 2:r1=0; 2:r2=0; 3:r3=0; 3:r4=1;
> > > 2:r1=0; 2:r2=0; 3:r3=0; 3:r4=2;
> > > 2:r1=0; 2:r2=0; 3:r3=1; 3:r4=1;
> > > [...]
> > > 2:r1=2; 2:r2=2; 3:r3=0; 3:r4=2;
> > > 2:r1=2; 2:r2=2; 3:r3=1; 3:r4=1;
> > > 2:r1=2; 2:r2=2; 3:r3=1; 3:r4=2;
> > > 2:r1=2; 2:r2=2; 3:r3=2; 3:r4=1;
> > > 2:r1=2; 2:r2=2; 3:r3=2; 3:r4=2;
> > > No
> > > Witnesses
> > > Positive: 0 Negative: 72
> > > Condition exists (2:r1=1 /\ 2:r2=2 /\ 3:r3=2 /\ 3:r4=1)
> > > Observation C-CCIRIW+o+o+o-o+o-o Never 0 72
> > > Time C-CCIRIW+o+o+o-o+o-o 0.06
> > > Hash=8e54976d74e1bc1ec5b6ce10eda3cb12
> > > ----------------------------------------------------------
> > >
> > > It says "Never", so if you could ever trigger the condition on a
> > > real machine, it would've meant something wrong in the implementation
> > > on the platform side (cache coherency).
> > >
> > > Hope this helps.
> > >
> > > Thanks, Akira
> > >
> > > >
> > > > But it seems my effort doesn't work; the exist assertions never
> > > > trigger. Did I miss something? What's the right/possible configuration
> > > > to trigger these assertions. I do know you might be very busy. But if
> > > > possible, please give me some hints. These two tests seems very
> > > > interesting :-). Thanks in advance.
> > > >
> > > > Regards,
> > > > --Junchang
> > > >
> > > > ===
> > > > Three servers I can touch:
> > > > 1. One PPC server which consists of 8 cores. I don's know hardware
> > > > details (e.g. which cores share the same LLC or write buffer) because
> > > > tools such as dmidecode don't work.
> > > > 2. One ARM server with two Cavium CPU. 96 cores in total (no Hyper-threading).
> > > > 3. One Intel server with two Xeon E5-2630 CPUs, each of which
> > > > consists of 12 cores (with Hyper-Threading).
> > > >
> > >
> >
>
