Re: [RFC PATCH] blk-iocost: introduce 'linear-max' cost model for cloud disk

[Jialin Wang]

On Fri, Feb 13, 2026 at 8:14 PM Yu Kuai <yukuai@xxxxxxxxx> wrote:
>
> Hi,
>
> 在 2026/2/13 15:38, Jialin Wang 写道:
> > In public cloud environments, block devices usually enforce performance
> > limits based on two independent token buckets: IOPS and BPS. The device
> > is throttled when either the IOPS limit or the BPS limit is reached.
> >
> > To effectively manage "noisy neighbor" problems, we configure iocost
> > model parameters (or vrate max) to approximately 95% of the cloud
> > provider's provisioned limits. The goal is to strictly avoid hitting
> > the storage backend's hard BPS/IOPS limits. By saturating the virtual
> > budget before the physical limit, iocost engages throttling first.
> > Unlike the indiscriminate throttling applied by cloud storage backends,
> > iocost selectively penalizes low-weight cgroups or heavy-traffic
> > perpetrators. Consequently, IO-latency-sensitive critical workloads
> > remain entirely unaffected by the congestion. Extensive testing has
> > verified that this approach yields excellent isolation results.
> >
> > However, the existing 'linear' cost model leads to significant
> > performance loss in this specific configuration due to its additive
> > nature.
> >
> > Using tools/cgroup/iocost_coef_gen.py, we measured the following
> > performance data on a typical cloud disk:
> >
> > 8:16 rbps=173471131 rseqiops=3566 rrandiops=3566 wbps=173333269 wseqiops=3566 wrandiops=3559
>
> Feels like a model similar to blk-throttle will work fine with your IO workload,
> what you really want is blk-throttle absolute threshold and blk-iocost relative
> throttling, correct?

Yes, that is exactly what I am trying to achieve. In cloud environments, we
need the absolute limits to avoid hitting the cloud provider's throttling wall,
while still benefiting from iocost's proportional sharing and weight donation.

Do you have any specific thoughts or suggestions on the best way to combine
these two mechanisms? I would really value your advice on the implementation
direction.

> >
> > Dividing BPS by IOPS (173471131 / 3566) yields approximately 48607
> > bytes. When running fio with bs=48607, we observed a 50% drop in
> > throughput compared to running without iocost enabled.
> >
> > The reason is that the current 'linear' model calculates cost as:
> >
> >    Cost = BaseCost + (Pages * PerPageCost)
> >
> > Expanding the internal variables relative to IOPS and BPS, this is
> > effectively:
> >
> >    Cost = VTIME_PER_SEC * ((1 / IOPS - 4096 / BPS) + size / BPS)
> >
> > When the I/O size is such that the IOPS cost component roughly equals
> > the BPS cost component (as in the bs=48607 case above), the linear
> > model sums them up. Since cloud disks throttle based on *either* IOPS
> > *or* BPS (whichever is exhausted first), summing them effectively
> > doubles the calculated cost. This causes iocost to drain virtual time
> > twice as fast as necessary, throttling the device to 50% utilization.
> >
> > To solve this, this patch introduces a new 'linear-max' cost model.
> > Instead of adding the components, it takes the maximum:
> >
> >    Cost = VTIME_PER_SEC * max(1 / IOPS, size / BPS)
> >
> > Which translates to:
> >
> >    Cost = max(BaseCost + PerPageCost, Pages * PerPageCost)
> >
> > This formula correctly models the dual-bucket behavior of cloud disks.
> > It ensures that for any block size, the calculated cost aligns with the
> > actual bottleneck (IOPS or BPS). This allows the system to reach close
> > to the provisioned BPS/IOPS limits without premature throttling, while
> > still maintaining the latency protection benefits of iocost.
> >
> > Signed-off-by: Jialin Wang <wjl.linux@xxxxxxxxx>
> > ---
> >   block/blk-iocost.c | 21 ++++++++++++++++++---
> >   1 file changed, 18 insertions(+), 3 deletions(-)
> >
> > diff --git a/block/blk-iocost.c b/block/blk-iocost.c
> > index ef543d163d46..ead478d8e5bc 100644
> > --- a/block/blk-iocost.c
> > +++ b/block/blk-iocost.c
> > @@ -445,6 +445,7 @@ struct ioc {
> >       int                             autop_idx;
> >       bool                            user_qos_params:1;
> >       bool                            user_cost_model:1;
> > +     bool                            cost_model_linear_max:1;
> >   };
> >
> >   struct iocg_pcpu_stat {
> > @@ -2565,7 +2566,12 @@ static void calc_vtime_cost_builtin(struct bio *bio, struct ioc_gq *iocg,
> >                       cost += coef_seqio;
> >               }
> >       }
> > -     cost += pages * coef_page;
> > +
> > +     if (ioc->cost_model_linear_max)
> > +             cost = max(cost + coef_page, pages * coef_page);
> > +     else
> > +             cost += pages * coef_page;
> > +
> >   out:
> >       *costp = cost;
> >   }
> > @@ -3368,10 +3374,11 @@ static u64 ioc_cost_model_prfill(struct seq_file *sf,
> >               return 0;
> >
> >       spin_lock(&ioc->lock);
> > -     seq_printf(sf, "%s ctrl=%s model=linear "
> > +     seq_printf(sf, "%s ctrl=%s model=%s "
> >                  "rbps=%llu rseqiops=%llu rrandiops=%llu "
> >                  "wbps=%llu wseqiops=%llu wrandiops=%llu\n",
> >                  dname, ioc->user_cost_model ? "user" : "auto",
> > +                ioc->cost_model_linear_max ? "linear-max" : "linear",
> >                  u[I_LCOEF_RBPS], u[I_LCOEF_RSEQIOPS], u[I_LCOEF_RRANDIOPS],
> >                  u[I_LCOEF_WBPS], u[I_LCOEF_WSEQIOPS], u[I_LCOEF_WRANDIOPS]);
> >       spin_unlock(&ioc->lock);
> > @@ -3412,6 +3419,7 @@ static ssize_t ioc_cost_model_write(struct kernfs_open_file *of, char *input,
> >       struct ioc *ioc;
> >       u64 u[NR_I_LCOEFS];
> >       bool user;
> > +     bool linear_max;
> >       char *body, *p;
> >       int ret;
> >
> > @@ -3442,6 +3450,7 @@ static ssize_t ioc_cost_model_write(struct kernfs_open_file *of, char *input,
> >       spin_lock_irq(&ioc->lock);
> >       memcpy(u, ioc->params.i_lcoefs, sizeof(u));
> >       user = ioc->user_cost_model;
> > +     linear_max = ioc->cost_model_linear_max;
> >
> >       while ((p = strsep(&body, " \t\n"))) {
> >               substring_t args[MAX_OPT_ARGS];
> > @@ -3464,7 +3473,11 @@ static ssize_t ioc_cost_model_write(struct kernfs_open_file *of, char *input,
> >                       continue;
> >               case COST_MODEL:
> >                       match_strlcpy(buf, &args[0], sizeof(buf));
> > -                     if (strcmp(buf, "linear"))
> > +                     if (!strcmp(buf, "linear"))
> > +                             linear_max = false;
> > +                     else if (!strcmp(buf, "linear-max"))
> > +                             linear_max = true;
> > +                     else
> >                               goto einval;
> >                       continue;
> >               }
> > @@ -3481,8 +3494,10 @@ static ssize_t ioc_cost_model_write(struct kernfs_open_file *of, char *input,
> >       if (user) {
> >               memcpy(ioc->params.i_lcoefs, u, sizeof(u));
> >               ioc->user_cost_model = true;
> > +             ioc->cost_model_linear_max = linear_max;
> >       } else {
> >               ioc->user_cost_model = false;
> > +             ioc->cost_model_linear_max = false;
> >       }
> >       ioc_refresh_params(ioc, true);
> >       spin_unlock_irq(&ioc->lock);
>
> --
> Thansk,
> Kuai