Re: [Xen-devel] libxl - avoid calling block script

[Ian Jackson <ian.jackson@xxxxxxxxxx>]

Marek Marczykowski-Górecki writes ("Re: [Xen-devel] libxl - avoid calling block 
script"):
> On Fri, Feb 09, 2018 at 11:03:55AM +0000, Roger Pau Monné wrote:
> > Really adding Ian and Wei.
> > 
> > On Fri, Feb 09, 2018 at 10:55:24AM +0000, Roger Pau Monné wrote:
> > > So the problem is creation time for domains that have quite a lot of
> > > disks attached. Adding Ian and Wei who know more about the async
> > > dispatch system, but I think (at least from a technical PoV) it
> > > should be possible to parallelize device attachment and thus hotplug
> > > script execution. Devices are independent from each other.
> 
> In theory yes, but in practice block script (at least on Linux) takes a
> lock and serialize execution...

Indeed.

> > > Also the Linux hotplug scripts in general seem extremely convoluted,
> > > I'm not sure whether we could gain some speed there just by
> > > simplification.
> 
> Well, we're comparing a bunch of fork+exec(), including starting bash
> (default /bin/sh on most systems), with just a single stat() call...
> Handling scripts in libxl itself also takes some time (in my case libxl
> live in libvirt, which may or may not have an impact). For a domU with
> 4 disks, getting rid of hotplug scripts saved about 2s of startup time.

The scripts themselves are terribly terribly slow.  They are as Roger
says incredibly convoluted.  I'm sure your 2s is right, but almost all
of that will be actual script execution.

I am not opposed to moving the functionality for very simplest case
into libxl.

But I think from your pov it would be worth trying a simple shell
script which doesn't take a lock, but just provides the physical
device information.

Ian.

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