linux-kernel - Re: NFS: 82ms wakeup latency 4.14-rc4

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Date:   Mon, 18 Dec 2017 18:17:36 +0100
From:   Mike Galbraith <efault@....de>
To:     "J. Bruce Fields" <bfields@...ldses.org>
Cc:     lkml <linux-kernel@...r.kernel.org>,
        Jeff Layton <jlayton@...nel.org>, linux-nfs@...r.kernel.org
Subject: Re: NFS: 82ms wakeup latency 4.14-rc4

On Mon, 2017-12-18 at 18:00 +0100, Mike Galbraith wrote:
> On Mon, 2017-12-18 at 11:35 -0500, J. Bruce Fields wrote:
> > 
> > Like I say, I don't really understand the issues here, so it's more a
> > question than an objection....  (I don't know any reason a
> > cond_resched() would be bad there.)
> 
> Think of it this way: what all can be queued up behind that kworker
> that is hogging CPU for huge swaths of time?  It's not only userspace
> that suffers.

Bah, I'm gonna sound like a damn Baptist preacher, but I gotta say,
latency matters just as much to an enterprise NOPREEMPT kernel and its
users as it does to a desktop kernel and its users.  For max
throughput, you don't want to do work in _tiny_ quantum, because you
then lose throughput due to massive cache trashing and scheduling
overhead, but latency still does matter, and not just a little.

	-Mike