[Libguestfs] [PATCH 0/3] Cleanups for ovirt 4.4

[Nir Soffer]

On Fri, Jan 22, 2021 at 5:42 PM Richard W.M. Jones <rjones at redhat.com>
wrote:

> On Fri, Jan 22, 2021 at 05:11:46PM +0200, Nir Soffer wrote:
> > Based on the tests I did with the imageio plugin this should
> > give significant performance improvement.
> > https://github.com/libguestfs/nbdkit/commit/
> > 78bbe95f754dd404fcbabac57940b1c0e4e49c4d
> >
> > I think the changes are basically:
> >
> > api version: 2
> > thread model: parallel
> >
> > open:
> > - Create a connection pool instead of a single connection.
> > - Based on experience wit imageio client, 4 connections are a good
> default
> >
> > pwrite/zero/pread:
> > - Get connection from the pool, blocking for available instance
> > - Return the connection to the pool when done or after failure
> >
> > flush:
> > - send a flush request for all connections in the pool
> >    (each http connection have a separate nbd connection inside imageio)
> >
> > close:
> > - close all connections in pool
> >
> > What do you think?
>
> Yes.
>
> The tricky thing will be getting locking right.  With the PARALLEL
> model you can have multiple parallel calls into the plugin even with
> the same handle.  The Python GIL is still active, so Python code
> fragments won't be running at the same time, but it's still possible
> for one thread to block in the middle of a plugin method (eg.
> HTTPConnection blocking on a read), releasing the GIL, allowing
> another plugin method call to start with the same handle.
>

The handle is a dict, which is thread safe, and we never modify
the handle during a transfer.

The only change we do is set h["failed"] to True, which is thread
safe and idempotent.

To get a connection from the handle, we will use a queue which is
thread safe. Connection are removed from the queue during use,
so only one thread can access a connection. Returning connection
to the queue is also thread safe.

So I think no extra locking is needed.


>
> Rich.
>
> --
> Richard Jones, Virtualization Group, Red Hat
> http://people.redhat.com/~rjones
> Read my programming and virtualization blog: http://rwmj.wordpress.com
> libguestfs lets you edit virtual machines.  Supports shell scripting,
> bindings from many languages.  http://libguestfs.org
>
>
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