Re: [RFC PATCH v1 0/6] Introducing `wq_cpu_set` mount option for btrfs

From: Ammar Faizi <ammarfaizi2@gnuweeb.org>
To: Qu Wenruo <quwenruo.btrfs@gmx.com>
Cc: Chris Mason <clm@fb.com>, Josef Bacik <josef@toxicpanda.com>,
	David Sterba <dsterba@suse.com>, Tejun Heo <tj@kernel.org>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	Filipe Manana <fdmanana@suse.com>,
	Linux Btrfs Mailing List <linux-btrfs@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Linux Fsdevel Mailing List <linux-fsdevel@vger.kernel.org>,
	GNU/Weeb Mailing List <gwml@vger.gnuweeb.org>
Subject: Re: [RFC PATCH v1 0/6] Introducing `wq_cpu_set` mount option for btrfs
Date: Mon, 27 Feb 2023 20:42:35 +0700	[thread overview]
Message-ID: <Y/yzS7aQ6PDyFsbm@biznet-home.integral.gnuweeb.org> (raw)
In-Reply-To: <19732428-010d-582c-0aed-9dd09b11d403@gmx.com>

On Mon, Feb 27, 2023 at 06:18:43PM +0800, Qu Wenruo wrote: 
> I'm not sure if pinning the wq is really the best way to your problem.
> 
> Yes, I understand you want to limit the CPU usage of btrfs workqueues, but
> have you tried "thread_pool=" mount option?
> 
> That mount option should limit the max amount of in-flight work items, thus
> at least limit the CPU usage.

I have tried to use the thread_poll=%u mount option previously. But I
didn't observe the effect intensively. I'll try to play with this option
more and see if it can yield the desired behavior.

> For the wq CPU pinning part, I'm not sure if it's really needed, although
> it's known CPU pinning can affect some performance characteristics.

What I like about CPU pinning is that we can dedicate CPUs for specific
workloads so it won't cause scheduling noise to the app we've dedicated
other CPUs for.

-- 
Ammar Faizi