linux-kernel - Re: [RFC PATCH v1 0/6] Introducing `wq_cpu

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Date:   Mon, 27 Feb 2023 20:42:35 +0700
From:   Ammar Faizi <ammarfaizi2@...weeb.org>
To:     Qu Wenruo <quwenruo.btrfs@....com>
Cc:     Chris Mason <clm@...com>, Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>, Tejun Heo <tj@...nel.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Filipe Manana <fdmanana@...e.com>,
        Linux Btrfs Mailing List <linux-btrfs@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Fsdevel Mailing List <linux-fsdevel@...r.kernel.org>,
        GNU/Weeb Mailing List <gwml@...r.gnuweeb.org>
Subject: Re: [RFC PATCH v1 0/6] Introducing `wq_cpu_set` mount option for
 btrfs

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