[Libguestfs] More parallelism in VDDK driver (was: Re: CFME-5.11.7.3 Perf. Tests)

[Nir Soffer]

On Wed, Aug 5, 2020 at 2:15 PM Richard W.M. Jones <rjones at redhat.com> wrote:
>
> [NB: Adding PUBLIC mailing list because this is upstream discussion]
>
> On Mon, Aug 03, 2020 at 06:27:04PM +0100, Richard W.M. Jones wrote:
> > On Mon, Aug 03, 2020 at 06:03:23PM +0300, Nir Soffer wrote:
> > > On Mon, Aug 3, 2020 at 5:47 PM Richard W.M. Jones <rjones at redhat.com> wrote:
> > > All this make sense, but when we upload 10 disks we have 10 connections
> > > but still we cannot push data fast enough. Avoiding copies will help,
> > > but I don't
> > > expect huge difference.
> > >
> > > My guess is the issue is on the other side - pulling data from vmware.
> >
> > I can believe this too.  VDDK is really slow, and especially the way
> > we use it is probably not optimal either -- but it has a confusing
> > threading model and I don't know if we can safely use a more parallel
> > thread model:
> >
> > https://github.com/libguestfs/nbdkit/blob/89a36b1fab8302ddc370695d386a28a03a74eae7/plugins/vddk/vddk.c#L505
> >
> > I may have a play around with this tomorrow.
>
> The threading model allowed by VDDK is restrictive.  The rules are here:
>
> https://code.vmware.com/docs/11750/virtual-disk-development-kit-programming-guide/GUID-6BE903E8-DC70-46D9-98E4-E34A2002C2AD.html
>
> I did a bit of testing, and it's possible to do better than what we
> are doing at the moment.  Not sure at present if this will be easy or
> will add a lot of complexity.  Read on ...
>
> I found through experimentation that it is possible to open multiple
> VDDK handles pointing to the same disk.  This would allow us to use
> SERIALIZE_REQUESTS (instead of SERIALIZE_ALL_REQUESTS) and have
> overlapping calls through different handles all pointing back to the
> same server/disk.  We should have to change all open/close calls to
> make the request through a single background thread - see document
> above for why.
>
> Adding a background thread and all the RPC needed to marshall these
> calls is the part which would add the complexity.
>
> However I suspect we might be able to get away with just adding a
> mutex around open/close.  The open/close requests would happen on
> different threads but would not overlap.  This is contrary to the
> rules above, but it could be sufficient.  This is what I'm testing at
> the moment.
>
> It is definitely *not* possible to move to PARALLEL since nbdkit would
> make requests in parallel on the same VDDK handle, which is not
> allowed.  (I did try this to see if the document above was serious,
> and it crashed in all kinds of strange ways, so I guess yes they are
> serious.)

Can we use something like the file plugin? thread pool of workers, each keeping
open vddk handle, and serving requests in parallel from the same nbd socket?

This is kind of ugly but simple, and it works great for the file
plugin - we get better
performance than qemu-nbd.

But since we get low throughput even when we have 10 concurrent handles for 10
different disks, I'm sure this will help, and the issue may be deeper
in vmware. Maybe
they intentionally throttle the clients?

> Rich.
>
> --
> Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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