Re: Elvis upstreaming plan

[Anthony Liguori <anthony@codemonkey.ws>]

Razya Ladelsky <RAZYA@il.ibm.com> writes:

> Hi all,
>
> I am Razya Ladelsky, I work at IBM Haifa virtualization team, which 
> developed Elvis, presented by Abel Gordon at the last KVM forum: 
> ELVIS video:  https://www.youtube.com/watch?v=9EyweibHfEs 
> ELVIS slides: https://drive.google.com/file/d/0BzyAwvVlQckeQmpnOHM5SnB5UVE 
>
>
> According to the discussions that took place at the forum, upstreaming 
> some of the Elvis approaches seems to be a good idea, which we would like 
> to pursue.
>
> Our plan for the first patches is the following: 
>
> 1.Shared vhost thread between mutiple devices 
> This patch creates a worker thread and worker queue shared across multiple 
> virtio devices 
> We would like to modify the patch posted in
> https://github.com/abelg/virtual_io_acceleration/commit/3dc6a3ce7bcbe87363c2df8a6b6fee0c14615766 
> to limit a vhost thread to serve multiple devices only if they belong to 
> the same VM as Paolo suggested to avoid isolation or cgroups concerns.
>
> Another modification is related to the creation and removal of vhost 
> threads, which will be discussed next.

I think this is an exceptionally bad idea.

We shouldn't throw away isolation without exhausting every other
possibility.

We've seen very positive results from adding threads.  We should also
look at scheduling.

Once you are scheduling multiple guests in a single vhost device, you
now create a whole new class of DoS attacks in the best case scenario.

> 2. Sysfs mechanism to add and remove vhost threads 
> This patch allows us to add and remove vhost threads dynamically.
>
> A simpler way to control the creation of vhost threads is statically 
> determining the maximum number of virtio devices per worker via a kernel 
> module parameter (which is the way the previously mentioned patch is 
> currently implemented)
>
> I'd like to ask for advice here about the more preferable way to go:
> Although having the sysfs mechanism provides more flexibility, it may be a 
> good idea to start with a simple static parameter, and have the first 
> patches as simple as possible. What do you think?
>
> 3.Add virtqueue polling mode to vhost 
> Have the vhost thread poll the virtqueues with high I/O rate for new 
> buffers , and avoid asking the guest to kick us.
> https://github.com/abelg/virtual_io_acceleration/commit/26616133fafb7855cc80fac070b0572fd1aaf5d0

Ack on this.

Regards,

Anthony Liguori

> 4. vhost statistics
> This patch introduces a set of statistics to monitor different performance 
> metrics of vhost and our polling and I/O scheduling mechanisms. The 
> statistics are exposed using debugfs and can be easily displayed with a 
> Python script (vhost_stat, based on the old kvm_stats)
> https://github.com/abelg/virtual_io_acceleration/commit/ac14206ea56939ecc3608dc5f978b86fa322e7b0
>
>
> 5. Add heuristics to improve I/O scheduling 
> This patch enhances the round-robin mechanism with a set of heuristics to 
> decide when to leave a virtqueue and proceed to the next.
> https://github.com/abelg/virtual_io_acceleration/commit/f6a4f1a5d6b82dc754e8af8af327b8d0f043dc4d
>
> This patch improves the handling of the requests by the vhost thread, but 
> could perhaps be delayed to a 
> later time , and not submitted as one of the first Elvis patches.
> I'd love to hear some comments about whether this patch needs to be part 
> of the first submission.
>
> Any other feedback on this plan will be appreciated,
> Thank you,
> Razya