Re: [RFC PATCH] getvalues(2) prototype

[Bernd Schubert]

On 3/23/22 12:42, Greg KH wrote:
> 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@xxxxxxxxxxxxxxxxxxx
> > >
> > > 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 :)

How did you run it on NFS? To get real benefit you would need to add a 
READ_FILE rpc to the NFS protocol and code? Just having it locally won't 
avoid the expensive wire calls?

> > 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.

I don't have a Lustre system to easily play with (I'm working on another 
network file system). But unless Lustre would implement aggressive 
prefetch of data on stat, I don't see how either approach would work 
without a protocol addition. For Lustre it probably would be helpful 
only when small data are inlined into the inode.
In end this is exactly the chicken-egg problem - Lustre (or anything 
else) won't implement it before the kernel does not support it. But then 
the new syscall won't be added before it is proven that it helps.


 - Bernd