linux-kernel - Re: [PATCH 0/4] kernfs: proposed locking and concurrency improvement

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Date:   Mon, 25 May 2020 08:16:16 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Ian Kent <raven@...maw.net>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>, Tejun Heo <tj@...nel.org>,
        Rick Lindsley <ricklind@...ux.vnet.ibm.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        David Howells <dhowells@...hat.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/4] kernfs: proposed locking and concurrency improvement

On Mon, May 25, 2020 at 01:46:59PM +0800, Ian Kent wrote:
> For very large systems with hundreds of CPUs and TBs of RAM booting can
> take a very long time.
> 
> Initial reports showed that booting a configuration of several hundred
> CPUs and 64TB of RAM would take more than 30 minutes and require kernel
> parameters of udev.children-max=1024 systemd.default_timeout_start_sec=3600
> to prevent dropping into emergency mode.
> 
> Gathering information about what's happening during the boot is a bit
> challenging. But two main issues appeared to be, a large number of path
> lookups for non-existent files, and high lock contention in the VFS during
> path walks particularly in the dentry allocation code path.
> 
> The underlying cause of this was believed to be the sheer number of sysfs
> memory objects, 100,000+ for a 64TB memory configuration.

Independant of your kernfs changes, why do we really need to represent
all of this memory with that many different "memory objects"?  What is
that providing to userspace?

I remember Ben Herrenschmidt did a lot of work on some of the kernfs and
other functions to make large-memory systems boot faster to remove some
of the complexity in our functions, but that too did not look into why
we needed to create so many objects in the first place.

Perhaps you might want to look there instead?

thanks,

greg k-h