Libguestfs - Aug 2020 - Re: More parallelism in VDDK driver (was: Re: CFME-5.11.7.3 Perf. Tests)

[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@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.)

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
Fedora Windows cross-compiler. Compile Windows programs, test, and
build Windows installers. Over 100 libraries supported.
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On Wed, Aug 5, 2020 at 2:15 PM Richard W.M. Jones <rjones@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@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
> Read my programming and virtualization blog: http://rwmj.wordpress.com
> Fedora Windows cross-compiler. Compile Windows programs, test, and
> build Windows installers. Over 100 libraries supported.
> http://fedoraproject.org/wiki/MinGW
>

On Wed, Aug 05, 2020 at 02:39:44PM +0300, Nir Soffer
wrote:
> 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?
Yes, but this isn't implemented in the plugins, it's implemented in
the server.  The server always uses a thread pool, but plugins can opt
for more or less concurrency by adjusting the thread model:

  http://libguestfs.org/nbdkit-plugin.3.html#Threads

The file plugin uses PARALLEL:

  $ nbdkit file --dump-plugin | grep thread
  max_thread_model=parallel
  thread_model=parallel

The VDDK plugin currently uses SERIALIZE_ALL_REQUESTS:

  $ nbdkit vddk --dump-plugin | grep thread
  max_thread_model=serialize_all_requests
  thread_model=serialize_all_requests

The proposal is to use SERIALIZE_REQUESTS, with an extra mutex added
by the plugin around VixDiskLib_Open and _Close calls.  PARALLEL is
not possible.
> 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?
The whole server side seems very heavyweight, judging by how long it
takes to answer single requests.  It might just be poor implementation
rather than throttling though.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
Fedora Windows cross-compiler. Compile Windows programs, test, and
build Windows installers. Over 100 libraries supported.
http://fedoraproject.org/wiki/MinGW

Nir, BTW what are you using for performance testing?

As far as I can tell it's not possible to make qemu-img convert use
multi-conn when connecting to the source (which is going to be a
problem if we want to use this stuff in virt-v2v).

Instead I've hacked up a copy of this program from libnbd:

https://github.com/libguestfs/libnbd/blob/master/examples/threaded-reads-and-writes.c

so that it only does reads and aligns requests to 512 bytes.

At least this is testing multi-conn, but there should be an easier way ...

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-top is 'top' for virtual machines.  Tiny program with many
powerful monitoring features, net stats, disk stats, logging, etc.
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