linux-kernel - Re: [RFC PATCH] getvalues(2) prototype

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Date:   Wed, 23 Mar 2022 12:42:46 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Bernd Schubert <bschubert@....com>
Cc:     Miklos Szeredi <mszeredi@...hat.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org, linux-man@...r.kernel.org,
        linux-security-module@...r.kernel.org, Karel Zak <kzak@...hat.com>,
        Ian Kent <raven@...maw.net>,
        David Howells <dhowells@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <christian@...uner.io>,
        Amir Goldstein <amir73il@...il.com>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        Dharmendra Singh <dsingh@....com>
Subject: Re: [RFC PATCH] getvalues(2) prototype

On Wed, Mar 23, 2022 at 11:26:11AM +0100, Bernd Schubert wrote:
> On 3/23/22 08:16, Greg KH wrote:
> > On Tue, Mar 22, 2022 at 08:27:12PM +0100, Miklos Szeredi wrote:
> > > Add a new userspace API that allows getting multiple short values in a
> > > single syscall.
> > > 
> > > This would be useful for the following reasons:
> > > 
> > > - Calling open/read/close for many small files is inefficient.  E.g. on my
> > >    desktop invoking lsof(1) results in ~60k open + read + close calls under
> > >    /proc and 90% of those are 128 bytes or less.
> > 
> > As I found out in testing readfile():
> > 	https://lore.kernel.org/r/20200704140250.423345-1-gregkh@linuxfoundation.org
> > 
> > microbenchmarks do show a tiny improvement in doing something like this,
> > but that's not a real-world application.
> > 
> > Do you have anything real that can use this that shows a speedup?
> 
> Add in network file systems. Demonstrating that this is useful locally and
> with micro benchmarks - yeah, helps a bit to make it locally faster. But the
> real case is when thousands of clients are handled by a few network servers.
> Even reducing wire latency for a single client would make a difference here.

I think I tried running readfile on NFS.  Didn't see any improvements.
But please, try it again.  Also note that this proposal isn't for NFS,
or any other "real" filesystem :)

> There is a bit of chicken-egg problem - it is a bit of work to add to file
> systems like NFS (or others that are not the kernel), but the work won't be
> made there before there is no syscall for it. To demonstrate it on NFS one
> also needs a an official protocol change first. And then applications also
> need to support that new syscall first.
> I had a hard time explaining weather physicist back in 2009 that it is not a
> good idea to have millions of 512B files on  Lustre. With recent AI workload
> this gets even worse.

Can you try using the readfile() patch to see if that helps you all out
on Lustre?  If so, that's a good reason to consider it.  But again, has
nothing to do with this getvalues(2) api.

thanks,

greg k-h