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SubjectRe: [PATCH] In-kernel module loader 3/7
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In message <Pine.LNX.4.44.0209190042370.8911-100000@serv> you write:
> Hi,
>
> On Wed, 18 Sep 2002, Rusty Russell wrote:
>
> > +/* Stopping interrupts faster than atomics on many archs (and more
> > + easily optimized if they're not) */
> > +static inline void bigref_inc(struct bigref *ref)
> > +{
> > + unsigned long flags;
> > + struct bigref_percpu *cpu;
> > +
> > + local_irq_save(flags);
> > + cpu = &ref->ref[smp_processor_id()];
> > + if (likely(!cpu->slow_mode))
> > + cpu->counter++;
>
> Did you benchmark this? On most UP machines an inc/dec should be cheaper
> than irq enable/disable.

Oops, I forgot to test that: I had both implementations.

Doing a million loop (so there's loop overhead):

350MHz dual Pentium II, atomic is almost twice as fast
PPC 500MHz G4: atomic is 3.5 times as fast.
Power3 4-way 375MHz machine: atomic is 10% faster.
Power4 4-way 1.3GHz machine: atomic was 20% slower (depending
on version if irq_restore).

I suspect a P4 might get similar pro-irq results (code below,
anyone?)

Ideally we'd have a "local_inc()" and "local_dec()". Architectures
which do soft interrupts enabling should see a real win from the
save/restore version.

Meanwhile, I'll revert to atomics, since it's sometimes dramatically
faster.

Thanks!
Rusty.
--
Anyone who quotes me in their sig is an idiot. -- Rusty Russell.

static void test(void)
{
struct timeval start, end;
atomic_t x;
unsigned int tmp, i, diff;
unsigned long flags;

/* Atomic test. */
atomic_set(&x, 0);
do_gettimeofday(&start);
for (i = 0; i < 1000000; i++)
atomic_dec(&x);
do_gettimeofday(&end);

diff = (end.tv_sec - start.tv_sec) * 1000000
+ (end.tv_usec - start.tv_usec);

printk("Atomic test: %u usec\n", diff);

/* Interrupt test (interrupts enabled) */
tmp = 0;
do_gettimeofday(&start);
for (i = 0; i < 1000000; i++) {
local_irq_save(flags);
tmp++;
local_irq_restore(flags);
}
do_gettimeofday(&end);

diff = (end.tv_sec - start.tv_sec) * 1000000
+ (end.tv_usec - start.tv_usec);

printk("Interrupt test: %u usec\n", diff);

/* Interrupt test (interrupts disabled) */
tmp = 0;
local_irq_disable();
do_gettimeofday(&start);
for (i = 0; i < 1000000; i++) {
local_irq_save(flags);
tmp++;
local_irq_restore(flags);
}
do_gettimeofday(&end);
local_irq_enable();

diff = (end.tv_sec - start.tv_sec) * 1000000
+ (end.tv_usec - start.tv_usec);

printk("Interrupt test (interrupts disabled): %u usec\n", diff);
}
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