Re: [PATCH] blk-wbt: use fast inverse square root to optimize window size calculation

[Meng Shao Liu <sau525@gmail.com>]

On Mon, Jul 28, 2025 at 02:06:52AM +0800, Yizhou Tang wrote:
> On Sun, Jul 27, 2025 at 7:22 PM Meng Shao Liu <sau525@gmail.com> wrote:
> >
> > Optimize the computation of cur_win_nsec = win_nsec / sqrt(scale_step + 1)
> > in blk-wbt by introducing a fast inverse square root algorithm.
> > This approach replaces the original use of int_sqrt and division with a
> > more efficient and accurate approximation method.
> >
> > Signed-off-by: Meng Shao Liu <sau525@gmail.com>
> > ---
> > Since this fast inverse square root algorithm now appears in three locations
> > (blk-wbt, sch_cake, codel), it might be worth considering refactoring
> > the implementation into a shared helper to reduce duplication and ensure consistency.
> > However, this patch focuses solely on introducing the optimization in blk-wbt.
> >
> >  block/blk-wbt.c | 60 +++++++++++++++++++++++++++++++++++++++++--------
> >  1 file changed, 51 insertions(+), 9 deletions(-)
> >
> > diff --git a/block/blk-wbt.c b/block/blk-wbt.c
> > index a50d4cd55..1fd5af3ba 100644
> > --- a/block/blk-wbt.c
> > +++ b/block/blk-wbt.c
> > @@ -80,6 +80,8 @@ struct rq_wb {
> >         u64 win_nsec;                           /* default window size */
> >         u64 cur_win_nsec;                       /* current window size */
> >
> > +       u32 rec_inv_sqrt;   /* reciprocal value of sqrt(scaling step + 1) */
> >         struct blk_stat_callback *cb;
> >
> >         u64 sync_issue;
> > @@ -130,6 +132,11 @@ enum {
> >          */
> >         RWB_WINDOW_NSEC         = 100 * 1000 * 1000ULL,
> >
> > +       /*
> > +        * Initial reciprocal value of sqrt(scaling step + 1)
> > +        */
> > +       RWB_REC_INV_SQRT    = 0,
> 
> Hi Meng,
> 
> As the initial value of scale_step is 0, then sqrt(0 + 1) = 1, which is not 0.
> 
Thanks for pointing out the problem.
> > +
> >         /*
> >          * Disregard stats, if we don't meet this minimum
> >          */
> > @@ -395,20 +402,55 @@ static void scale_down(struct rq_wb *rwb, bool hard_throttle)
> >         rwb_trace_step(rwb, tracepoint_string("scale down"));
> >  }
> >
> > +#define REC_INV_SQRT_CACHE (16)
> > +static const u32 inv_sqrt_cache[REC_INV_SQRT_CACHE] = {
> > +               ~0,         ~0, 3037000500, 2479700525,
> > +       2147483647, 1920767767, 1753413056, 1623345051,
> > +       1518500250, 1431655765, 1358187914, 1294981364,
> > +       1239850263, 1191209601, 1147878294, 1108955788
> > +};
> > +
> > +/* http://en.wikipedia.org/wiki/Methods_of_computing_square_roots
> > + * new_invsqrt = (invsqrt / 2) * (3 - count * invsqrt^2)
> > + *
> > + * Here, invsqrt is a fixed point number (< 1.0), 32bit mantissa, aka Q0.32
> > + */
> > +
> > +static void rwb_newton_step(struct rq_wb *rwb)
> > +{
> > +       struct rq_depth *rqd = &rwb->rq_depth;
> > +       u32 invsqrt, invsqrt2;
> > +       u64 val;
> > +
> > +       invsqrt = rwb->rec_inv_sqrt;
> > +       invsqrt2 = ((u64)invsqrt * invsqrt) >> 32;
> > +       val = (3LL << 32) - ((u64)(rqd->scale_step + 1) * invsqrt2);
> > +
> > +       val >>= 2; /* avoid overflow in following multiply */
> > +       val = (val * invsqrt) >> (32 - 2 + 1);
> > +
> > +       rwb->rec_inv_sqrt = val;
> > +}
> > +
> > +static void rwb_invsqrt(struct rq_wb *rwb)
> > +{
> > +       struct rq_depth *rqd = &rwb->rq_depth;
> > +
> > +       if (rqd->scale_step + 1 < REC_INV_SQRT_CACHE)
> > +               rwb->rec_inv_sqrt = inv_sqrt_cache[rqd->scale_step + 1];
> > +       else
> > +               rwb_newton_step(rwb);
> > +}
> > +
> >  static void rwb_arm_timer(struct rq_wb *rwb)
> >  {
> >         struct rq_depth *rqd = &rwb->rq_depth;
> >
> >         if (rqd->scale_step > 0) {
> > -               /*
> > -                * We should speed this up, using some variant of a fast
> > -                * integer inverse square root calculation. Since we only do
> > -                * this for every window expiration, it's not a huge deal,
> > -                * though.
> > -                */
> > -               rwb->cur_win_nsec = div_u64(rwb->win_nsec << 4,
> > -                                       int_sqrt((rqd->scale_step + 1) << 8));
> > -       } else {
> > +               rwb_invsqrt(rwb);
> > +               rwb->cur_win_nsec = reciprocal_scale(rwb->win_nsec,
> > +                                            rwb->rec_inv_sqrt);
> 
> I think placing the two lines of code involving mathematical formulas
> directly in a core wbt code path is not a good idea. I suggest you
> encapsulate them in a separate function and document its purpose.
> 
> Thanks,
> Yi
> 
Got it!
> > +       } else {
> >                 /*
> >                  * For step < 0, we don't want to increase/decrease the
> >                  * window size.
> > @@ -911,6 +953,7 @@ int wbt_init(struct gendisk *disk)
> >
> >         rwb->last_comp = rwb->last_issue = jiffies;
> >         rwb->win_nsec = RWB_WINDOW_NSEC;
> > +       rwb->rec_inv_sqrt = RWB_REC_INV_SQRT;
> >         rwb->enable_state = WBT_STATE_ON_DEFAULT;
> >         rwb->rq_depth.default_depth = RWB_DEF_DEPTH;
> >         rwb->min_lat_nsec = wbt_default_latency_nsec(q);
> > --
> > 2.50.1
> >
> >