On Fri, Feb 02, 2018 at 11:09:03AM -0500, Chuck Lever wrote: > > > > On Feb 2, 2018, at 10:49 AM, bfields@xxxxxxxxxxxx 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 > >
