linux-kernel - Re: [PATCH 2/2] bcachefs: set rebalance thread to SCHED

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Message-ID: <20250114143214.GT5388@noisy.programming.kicks-ass.net>
Date: Tue, 14 Jan 2025 15:32:14 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Florian Schmaus <flo@...kplace.eu>
Cc: Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Kent Overstreet <kent.overstreet@...ux.dev>,
	linux-bcachefs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] bcachefs: set rebalance thread to SCHED_BATCH and
 nice 19

On Tue, Jan 14, 2025 at 01:47:28PM +0100, Florian Schmaus wrote:
> While the rebalance thread is isually not compute bound, it does cause
> a considerable amount of I/O. Since "reducing" the nice level from 0
> to 19, also implicitly reduces the threads best-effort I/O scheduling
> class level from 4 to 7, the reblance thread's I/O will be depriotized
> over normal I/O.
> 
> Furthermore, we set the rebalance thread's scheduling class to BATCH,
> which means that it will potentially receive a higher scheduling
> latency. Making room for threads that need a low
> schedulinglatency (e.g., interactive onces).

sorta.. what worries me most about these patches are the claims without
backing numbers.

Supposedly there is a problem, and this here fixes it, but it doesn't
really get quantified much here.