On Thu, Mar 21, 2019 at 07:07:38PM +0200, Maxim Levitsky wrote:
> On Thu, 2019-03-21 at 16:13 +0000, Stefan Hajnoczi wrote:
> > On Tue, Mar 19, 2019 at 04:41:07PM +0200, Maxim Levitsky wrote:
> > > Date: Tue, 19 Mar 2019 14:45:45 +0200
> > > Subject: [PATCH 0/9] RFC: NVME VFIO mediated device
> > > 
> > > Hi everyone!
> > > 
> > > In this patch series, I would like to introduce my take on the problem of
> > > doing 
> > > as fast as possible virtualization of storage with emphasis on low latency.
> > > 
> > > In this patch series I implemented a kernel vfio based, mediated device
> > > that 
> > > allows the user to pass through a partition and/or whole namespace to a
> > > guest.
> > > 
> > > The idea behind this driver is based on paper you can find at
> > > https://www.usenix.org/conference/atc18/presentation/peng,
> > > 
> > > Although note that I stared the development prior to reading this paper, 
> > > independently.
> > > 
> > > In addition to that implementation is not based on code used in the paper
> > > as 
> > > I wasn't being able at that time to make the source available to me.
> > > 
> > > ***Key points about the implementation:***
> > > 
> > > * Polling kernel thread is used. The polling is stopped after a 
> > > predefined timeout (1/2 sec by default).
> > > Support for all interrupt driven mode is planned, and it shows promising
> > > results.
> > > 
> > > * Guest sees a standard NVME device - this allows to run guest with 
> > > unmodified drivers, for example windows guests.
> > > 
> > > * The NVMe device is shared between host and guest.
> > > That means that even a single namespace can be split between host 
> > > and guest based on different partitions.
> > > 
> > > * Simple configuration
> > > 
> > > *** Performance ***
> > > 
> > > Performance was tested on Intel DC P3700, With Xeon E5-2620 v2 
> > > and both latency and throughput is very similar to SPDK.
> > > 
> > > Soon I will test this on a better server and nvme device and provide
> > > more formal performance numbers.
> > > 
> > > Latency numbers:
> > > ~80ms - spdk with fio plugin on the host.
> > > ~84ms - nvme driver on the host
> > > ~87ms - mdev-nvme + nvme driver in the guest
> > 
> > You mentioned the spdk numbers are with vhost-user-nvme.  Have you
> > measured SPDK's vhost-user-blk?
> 
> I had lot of measuments of vhost-user-blk vs vhost-user-nvme.
> vhost-user-nvme was always a bit faster but only a bit.
> Thus I don't think it makes sense to benchamrk against vhost-user-blk.

It's interesting because mdev-nvme is closest to the hardware while
vhost-user-blk is closest to software.  Doing things at the NVMe level
isn't buying much performance because it's still going through a
software path comparable to vhost-user-blk.

From what you say it sounds like there isn't much to optimize away :(.

Stefan