Re: Rep:[f-cpu] Hot issue : external LSU ?

["Christophe Avoinne" <christophe.avoinne@laposte.net>]

Ok take this example : i want to increase a memory value atomically

CAS version :

void atomic_inc (int *slot) {
    int old;
    do {
        old = *slot;
    } while (cas(slot,old,old+1) != old);
}

or better to avoid reading *slot twice :

void atomic_inc (int *slot) {
    int old1,old2 = *slot;
    do {
        old1 = old2;
    } while ((old1 = cas(slot,old1,old1+1)) != old2);
}

ll/sc version :

void atomic_inc (int *slot) {
    int old;
    do {
        old = ll(slot);
    } while (!sc(slot,old+1));
}

as you can see : ll/sc looks much more "elegant" and simpler for a
programmer.

you want to apply a function in a "atomical" way :

void atomic_apply (int *slot,int (*f) (int)) {
    int old;
    do {
        old = ll(slot);
    } while (!sc(slot,f(old)));
}

in C++ :) :

template< typename f > void apply< f >(int &slot) {
    int old;
    do {
        old = ll(slot);
    } while (!sc(slot,f(old)));
}

Don't forget :
*ll read a value in the matching LSU entry and set the lock bit.
*sc doesn't need to read a value in the matching LSU entry and compare with
an expected value unlike CAS, it just needs to check the lock bit, so it
should be faster.

----- Original Message -----
From: "nico" <nicolas.boulay@ifrance.com>
To: <f-cpu@seul.org>
Sent: Saturday, August 31, 2002 4:40 PM
Subject: Re: Rep:[f-cpu] Hot issue : external LSU ?


> I don't refind the precise behavior of ll/sc. But i don't like too much
> to lock a precise adress line. This become to be a very limited
> ressource and raise many problem.
>
> And what about distributed memory where pages are duplicated ? But maybe
> ll/sc seems to be much more low level but it's seen by user. So what do
> you think about ?


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