Hi Johannes,
On Tue, Jul 31, 2018 at 05:21:50PM -0400, Johannes Weiner wrote:
> Hi Dennis,
>
> this generally looks good to me. Just two small nit picks:
>
> On Tue, Jul 31, 2018 at 01:36:47PM -0700, Dennis Zhou wrote:
> > @@ -135,6 +135,24 @@ struct iolatency_grp {
> > struct child_latency_info child_lat;
> > };
> >
> > +#define BLKIOLATENCY_MIN_WIN_SIZE (100 * NSEC_PER_MSEC)
> > +#define BLKIOLATENCY_MAX_WIN_SIZE NSEC_PER_SEC
> > +/*
> > + * These are the constants used to fake the fixed-point moving average
> > + * calculation just like load average. The latency window is bucketed to
> > + * try to approximately calculate average latency for the last 1 minute.
> > + */
> > +#define BLKIOLATENCY_NR_EXP_FACTORS 5
> > +#define BLKIOLATENCY_EXP_BUCKET_SIZE (BLKIOLATENCY_MAX_WIN_SIZE / \
> > + (BLKIOLATENCY_NR_EXP_FACTORS - 1))
> > +static const u64 iolatency_exp_factors[BLKIOLATENCY_NR_EXP_FACTORS] = {
> > + 2045, // exp(1/600) - 600 samples
> > + 2039, // exp(1/240) - 240 samples
> > + 2031, // exp(1/120) - 120 samples
> > + 2023, // exp(1/80) - 80 samples
> > + 2014, // exp(1/60) - 60 samples
>
> Might be useful to drop the FIXED_1 name in a comment here. It says
> "fixed-point", and "load average", but since the numbers are directly
> in relationship to that constant, it'd be good to name it I think.
>
I've added a comment in v2 that points out FIXED_1 and mentions its
value is 2048. I also explained a little more about the samples and
binding the 1/exp window.
> > + exp_idx = min_t(int, BLKIOLATENCY_NR_EXP_FACTORS - 1,
> > + iolat->cur_win_nsec / BLKIOLATENCY_EXP_BUCKET_SIZE);
> > + CALC_LOAD(iolat->total_lat_avg, iolatency_exp_factors[exp_idx],
> > + stat.mean);
>
> The load average keeps the running value in fixed point presentation
> to avoid rounding errors. I guess because this is IO time in ns, the
> values are so much higher than the FIXED_1 denominator (2048) that
> rounding errors are negligible, and we don't need to bother with it.
>
> Can you mention that in a comment, please?
I've added what you said here in a comment above this.
Thanks,
Dennis
