[gambit-list] Marc, Q:s re sequential consistency and critical sections in SMP Gambit

Adam adam.mlmb at gmail.com
Sun Jun 2 14:37:59 EDT 2019

 Hi Marc,

Below some questions about sequential consistency and critical sections in
SMP Gambit.

The Sutter slides below illustrate the problematique addressed beautifully.

 * Please have a look at the below and confirm that this is what Gambit

 * In thread-local code does Gambit internally do any optimizations that
alters the execution order? In C-compiled code I presume the C compiler can
make such optimizations, and that in itself pulls Gambit to need to address
this problem domain.

 * The baseline constraint on Gambit's as well as any C compiler's
optimizations are that optimizations that alter order will not span
module-external calls, isn't it?

 * Does Gambit offer any low level primitives for (critical sections for)
doing low level in-module sequentially consistent operations between cores?

   I.e. anything lower level than thread-send/receive and mutex-lock/unlock.

   What about fence, acquire/read fence, release/write fence.

 * Does Gambit guarantee that newly allocated objects have some
initialization when accessed from any CPU core, or if malloc() gave trash
then may a Gambit thread see trash?


   Context: (define x #f) (##fence!)
   Core A: (set! x (vector 1 2))
   Core B: (let loop () (let ((v x)) (if v (print (##vector-ref v 0)))

   Which are all possible values that may be printed?

C/C++ standardized SMP/memory model in the C11/C++11 specs. It's great to
see Gambit spec the same now.


Re critical sections, do you have primitives in SMP Gambit to force the
compiler to respect them, and then in the code output produce assembly
instructions that honor them?

Today in SMP Gambit, I presume thread-send/receive and mutex-lock/unlock
will abstract away the underlying architecture's memory model, and ensure
that thread-receive will have access to the whole structure that
thread-send sent.


GVM processor A:

(thread-send B (list 1 2))

(define l (list 3 4))
(set-car! l 5)
(thread-send B (list 1 2))

GVM processor B:
(let loop () (for-each print (thread-receive) (loop))

This is to illustrate that thread-send/receive ensures that the list
elements will be accessible (= read correctly) at the receiving site. In
this case on any architecture, four print calls are done, each with the
respectove argument: 1, 2, 5, 4.

I guess that design feature is helped by that the Gambit compiler will not
do optimizations that garble the code order, over module-external procedure
calls, and thread-send/receive & mutex-lock/unlock count as procedures.

I presume on strongly ordered architectures the following will make 2 3 go

GVM processor A:
(define l (list 1 2))
(thread-send B l)
(set-car! l 3)

GVM processor B:
(define m (thread-receive))
(let loop ((i 0)) (if (not (eqv? (expt 3 12) i)) (loop (+ i 1)))
(for-each print m)

On a weakly ordered architecture the code would not SIGSEGV nor throw
exception, but the first value could be 1 or 3.

Importantly on a weakly ordered architecture the code would never cause a
SIGSEGV, because Gambit's memory management ensures that pending memory bus
transactions are flushed before it initiates collection of a dead object.

Similarly any allocated object's space is immediately accessible on all

Common workup:
(define s #f)

GVM processor A:
(set! s (make-u8vector 5))

GVM processor B:
(let loop () (let ((v s)) (if v (##u8vector-ref v 1)) (loop))

The promise is limited to that the content is accessible only though - on a
weakly ordered architecture, it may not have been initialized yet and
therefore the type and range check in (u8vector-ref v 1) could fail,
(##u8vector-ref v 1) do not do that though and while it may return junk, at
least it will not crash.

The above two described behaviors are is in symmetry with any host OS'

Important references:

Sutter slides:https://1drv.ms/b/s!Aq0V7yDPsIZOgcI0y2P8R-VifbnTtw

Sutter videos:
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