On Tue, 21 Mar 2023 19:10:04 +0000,
Colton Lewis <coltonlewis@xxxxxxxxxx> wrote:
>
> Marc Zyngier <maz@xxxxxxxxxx> writes:
>
> >> +#define MEASURE_CYCLES(x) \
> >> + ({ \
> >> + uint64_t start; \
> >> + start = cycles_read(); \
> >> + x; \
>
> > You insert memory accesses inside a sequence that has no dependency
> > with it. On a weakly ordered memory system, there is absolutely no
> > reason why the memory access shouldn't be moved around. What do you
> > exactly measure in that case?
>
> cycles_read is built on another function timer_get_cntct which includes
> its own barriers. Stripped of some abstraction, the sequence is:
>
> timer_get_cntct (isb+read timer)
> whatever is being measured
> timer_get_cntct
>
> I hadn't looked at it too closely before but on review of the manual
> I think you are correct. Borrowing from example D7-2 in the manual, it
> should be:
>
> timer_get_cntct
> isb
> whatever is being measured
> dsb
> timer_get_cntct
That's better, but also very heavy handed. You'd be better off
constructing an address dependency from the timer value, and feed that
into a load-acquire/store-release pair wrapping your payload.
>
> >> + cycles_read() - start; \
>
> > I also question the usefulness of this exercise. You're comparing the
> > time it takes for a multi-GHz system to put a write in a store buffer
> > (assuming it didn't miss in the TLBs) vs a counter that gets updated
> > at a frequency of a few tens of MHz.
>
> > My guts feeling is that this results in a big fat zero most of the
> > time, but I'm happy to be explained otherwise.
>
>
> In context, I'm trying to measure the time it takes to write to a buffer
> *with dirty memory logging enabled*. What do you mean by zero? I can
> confirm from running this code I am not measuring zero time.
See my earlier point: the counter tick is a few MHz, and the CPU
multiple GHz. So unless "whatever" is something that takes a
significant time (several thousands of CPU cycles), you'll measure
nothing using the counter. Page faults will probably show, but not a
normal access.
The right tool for this job is to use PMU events, as they count at the
CPU frequency.
Thanks,
M.
--
Without deviation from the norm, progress is not possible.
