* Joseph Schuchart <[email protected]> wrote:
> Sorry for my delayed reply, it's been a busy week and I really wanted to
> give Ingo's idea below some thought. Please find my comments below.
>
> >>> If done that way then AFAICS we could even eliminate the
> >>> ->max_timestamps[NR_CPUS] array.
> >>
> >> I can understand your performance concerns. However, I am not
> >> sure how we can determine the minimal max_timestamp of all cpus
> >> without storing the information on a per-cpu basis first.
> >> Accumulating it on the fly would only lead to a global
> >> max_timestamp. [...]
> >
> > Ok. So this:
> >
> > +static inline void set_next_flush(struct perf_session *session)
> > +{
> > + int i;
> > + u64 min_max_timestamp = session->ordered_samples.max_timestamps[0];
> > + for (i = 1; i < MAX_NR_CPUS; i++) {
> > + if (min_max_timestamp >
> > session->ordered_samples.max_timestamps[i])
> > + min_max_timestamp =
> > session->ordered_samples.max_timestamps[i];
> > + }
> > + session->ordered_samples.next_flush = min_max_timestamp;
> > +}
> >
> > which should IMHO be written in a bit clearer form as:
> >
> > static inline void set_next_flush(struct perf_session *session)
> > {
> > u64 *timestamps = session->ordered_samples.max_timestamps;
> > u64 min_timestamp = timestamps[0];
> > int i;
> >
> > for (i = 1; i < MAX_NR_CPUS; i++) {
> > if (min_timestamp > timestamps[i])
> > min_timestamp = timestamps[i];
> > }
> >
> > session->ordered_samples.next_flush = min_timestamp;
> > }
>
> Agreed.
>
> >
> > calculates the minimum of the max_timestamps[] array, right?
> >
> > Now, the max_timestamps[] array gets modified only in a single
> > place, from the sample timestamps, via:
> >
> > os->max_timestamps[sample->cpu] = timestamp;
> >
> > My suggestion was an identity transformation: to calculate the
> > minimum of the array when the max_timestamps[] array is modified.
> > A new minimum happens if the freshly written value is smaller
> > than the current minimum.
>
> I am really not sure how this would work since I don't see where we
> could make progress while flushing if the max_timestamp is only
> replaced with a smaller one but is never increased. IMO, it is
> necessary to distinguish between timestamps of different cpus to
> determine the next_flush timestamp across all cpus in each pass. I
> have tried to come up with a working implementation of your idea but
> do not see a way to safely increase the value of max_timestamp
> (without making assumptions about the order of timestamps between
> cpus and passes).
Mine isn't really an 'idea' - I did to the code what I see an identity
transformation, a change that does not change the principle or the
working of the code.
And after the identity transformation your code seems to have
simplified down significantly - at which point I was wondering.
If what I did is _not_ an identity transformation then please point
out where I break your logic. (it might easily be some really simple
misundestanding on my part.)
Thanks,
Ingo
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