Re: [LSF/MM TOPIC ATTEND][RFD] ZUFS - Zero-copy User-mode File System

[Bruce Fields <bfields@fieldses.org>]

On Fri, Feb 02, 2018 at 11:09:03AM -0500, Chuck Lever wrote:
> 
> 
> > On Feb 2, 2018, at 10:49 AM, bfields@fieldses.org wrote:
> > 
> > On Thu, Feb 01, 2018 at 08:59:18PM +0200, Boaz Harrosh wrote:
> >> On 01/02/18 20:34, Chuck Lever wrote: <>
> >>> This work was also presented at the SNIA Persistent Memory Summit
> >>> last week.  The use case of course is providing a user space
> >>> platform for the development and deployment of memory-based file
> >>> systems. The value-add of this kind of file system is ultra-low
> >>> latency, which is a challenge for the current most popular such
> >>> framework, FUSE.
> >>> 
> >>> To start, I can think of three areas where specific questions might
> >>> be entertained by LSF/MM attendees:
> >>> 
> >>> - Spectre mitigations make this whole "user space filesystem"
> >>> arrangement even slower, thanks to additional context switches
> >>> between user space and the kernel.
> > 
> > I think you're referring to the KPTI patches, which address Meltdown,
> > not Spectre.
> 
> I enabled KPTI on my NFS client and server systems in early
> v4.15-rc, and didn't measure a change in latency or throughput.
> 
> But with v4.15 final, which includes some Spectre mitigations,
> write(2) on NFS files, for example, takes about 15us longer.
> Since the RPC round-trip times did not increase, I presume this
> extra latency is incurred on the client, where the user-kernel
> boundary transitions occur.

OK, that's interesting, thanks.

--b.

> 
> <shrug>
> 
> Anyway there's more latency in the user space-kernel transition
> now. Thus any stack that adds more such transitions will need
> attention. That would include FUSE, user-space file servers,
> ZUFS, any activity that requires upcalls, and so on.
> 
> 
> --
> Chuck Lever
> 
>