From: Jan Kara <jack@suse.cz>
To: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Jan Kara <jack@suse.cz>,
willy@infradead.org, akpm@linux-foundation.org, tj@kernel.org,
hcochran@kernelspring.com, axboe@kernel.dk, mszeredi@redhat.com,
linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 2/4] mm: correct calculation of wb's bg_thresh in cgroup domain
Date: Tue, 7 May 2024 15:28:51 +0200 [thread overview]
Message-ID: <20240507132851.rck2mc4sywaav67f@quack3> (raw)
In-Reply-To: <12bf104e-aeac-67a5-6e5a-bc7bdbfe4d79@huaweicloud.com>
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
next prev parent reply other threads:[~2024-05-07 13:28 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-25 13:17 [PATCH v2 0/4] Fix and cleanups to page-writeback Kemeng Shi
2024-04-25 13:17 ` [PATCH v2 1/4] mm: enable __wb_calc_thresh to calculate dirty background threshold Kemeng Shi
2024-05-03 9:11 ` Jan Kara
2024-04-25 13:17 ` [PATCH v2 2/4] mm: correct calculation of wb's bg_thresh in cgroup domain Kemeng Shi
2024-05-03 9:30 ` Jan Kara
2024-05-07 1:16 ` Kemeng Shi
2024-05-07 13:28 ` Jan Kara [this message]
2024-04-25 13:17 ` [PATCH v2 3/4] mm: call __wb_calc_thresh instead of wb_calc_thresh in wb_over_bg_thresh Kemeng Shi
2024-05-03 9:31 ` Jan Kara
2024-04-25 13:17 ` [PATCH v2 4/4] mm: remove stale comment __folio_mark_dirty Kemeng Shi
2024-05-03 9:31 ` Jan Kara
2024-05-01 16:16 ` [PATCH v2 0/4] Fix and cleanups to page-writeback Tejun Heo
2024-05-06 1:25 ` Kemeng Shi
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