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SubjectRe: [PATCH v2 2/4] mm: correct calculation of wb's bg_thresh in cgroup domain
On Tue 07-05-24 09:16:39, Kemeng Shi wrote:
>
> Hi Jan,
> on 5/3/2024 5:30 PM, Jan Kara wrote:
> > On Thu 25-04-24 21:17:22, Kemeng Shi wrote:
> >> The wb_calc_thresh is supposed to calculate wb's share of bg_thresh in
> >> global domain. To calculate wb's share of bg_thresh in cgroup domain,
> >> it's more reasonable to use __wb_calc_thresh in which way we calculate
> >> dirty_thresh in cgroup domain in balance_dirty_pages().
> >>
> >> Consider following domain hierarchy:
> >> global domain (> 20G)
> >> / \
> >> cgroup domain1(10G) cgroup domain2(10G)
> >> | |
> >> bdi wb1 wb2
> >> Assume wb1 and wb2 has the same bandwidth.
> >> We have global domain bg_thresh > 2G, cgroup domain bg_thresh 1G.
> >> Then we have:
> >> wb's thresh in global domain = 2G * (wb bandwidth) / (system bandwidth)
> >> = 2G * 1/2 = 1G
> >> wb's thresh in cgroup domain = 1G * (wb bandwidth) / (system bandwidth)
> >> = 1G * 1/2 = 0.5G
> >> At last, wb1 and wb2 will be limited at 0.5G, the system will be limited
> >> at 1G which is less than global domain bg_thresh 2G.
> >
> > This was a bit hard to understand for me so I'd rephrase it as:
> >
> > wb_calc_thresh() is calculating wb's share of bg_thresh in the global
> > domain. However in case of cgroup writeback this is not the right thing to
> > do. Consider the following domain hierarchy:
> >
> > global domain (> 20G)
> > / \
> > cgroup1 (10G) cgroup2 (10G)
> > | |
> > bdi wb1 wb2
> >
> > and assume wb1 and wb2 have the same bandwidth and the background threshold
> > is set at 10%. The bg_thresh of cgroup1 and cgroup2 is going to be 1G. Now
> > because wb_calc_thresh(mdtc->wb, mdtc->bg_thresh) calculates per-wb
> > threshold in the global domain as (wb bandwidth) / (domain bandwidth) it
> > returns bg_thresh for wb1 as 0.5G although it has nobody to compete against
> > in cgroup1.
> >
> > Fix the problem by calculating wb's share of bg_thresh in the cgroup
> > domain.
> Thanks for improving the changelog. As this was merged into -mm and
> mm-unstable tree, I'm not sure if a new patch is needed. If there is
> anything I should do, please let me konw. Thanks.

No need to do anything here. Andrew has picked up these updates.

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR

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