> 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. <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
