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Re: [Qemu-devel] Why is SeaBIOS used with -kernel?
From: |
Richard W.M. Jones |
Subject: |
Re: [Qemu-devel] Why is SeaBIOS used with -kernel? |
Date: |
Fri, 1 Apr 2016 21:46:05 +0100 |
User-agent: |
Mutt/1.5.20 (2009-12-10) |
On Fri, Apr 01, 2016 at 04:05:46PM -0400, Kevin O'Connor wrote:
> On Fri, Apr 01, 2016 at 07:41:31PM +0100, Richard W.M. Jones wrote:
> > On Fri, Apr 01, 2016 at 11:35:40AM -0400, Kevin O'Connor wrote:
> > > > +# general stuff
> > > > +CONFIG_QEMU=y
> > > > +CONFIG_ROM_SIZE=128
> > >
> > > Why force a size of 128K - I would think 64K would be fine.
> >
> > Agreed. Setting this to =0 seems the best thing, and it does fit fine
> > inside 64K.
> >
> > > > +# no input, no boot menu
> > > > +CONFIG_MOUSE=n
> > > > +CONFIG_KEYBOARD=n
> > > [...]
> > > > +CONFIG_DRIVES=n
> > >
> > > I would not recommended disabling CONFIG_MOUSE, CONFIG_KEYBOARD,
> > > CONFIG_DRIVES - I only had those in my config so as to avoid having to
> > > specify all the device drivers. Ideally these would remain on and the
> > > individual device drivers would be disabled.
> >
> > We are always use this in a virtual appliance. Interaction with the
> > user is both impossible and undesirable. It either boots or not, and
> > the whole appliance is discarded in seconds. We're always using
> > -kernel with this SeaBIOS build, so probing drives is never needed.
>
> Okay, but if it doesn't change the boot time, then it would be nicer
> to use a standard rom for all boots.
I don't think I know which option (or options) make the boot faster,
but bios-fast.bin is certainly much faster than the qemu standard
bios.bin -- no amount of variability in the test framework would
account for that huge difference of 44ms.
I had convinced myself before that maybe CONFIG_DRIVES=n was the
factor, but that setting alone doesn't actually make much difference:
bios-fast.bin (CONFIG_DRIVES=n):
Result: 1093.3ms ±3.4ms
Result: 1100.6ms ±8.9ms
bios-fast.bin + CONFIG_DRIVES=y:
Result: 1100.3ms ±8.4ms
Result: 1101.7ms ±1.2ms
It could be just one setting or an accumulation of several settings.
At some point I may have the patience to try each one but probably not
late on Friday night :-/
> I looked closer at your setup and it appears the SeaBIOS virtio-scsi
> driver is very slow because it does a full search of all 256 possible
> scsi targets. This full scan takes a lot of time. I put together a
> quick patch (see below) to stop the scan early. Gerd/Paulo, do you
> know if what I've done is valid and/or if there is a better way we can
> limit the virtio-scsi scan?
>
> I also found a way to reduce the overhead of the "shadow ram" code a
> little. I have a patch (see below) for that as well.
I would say the patches on the KevinOConnor/testing branch don't make
any measurable difference.
Stock qemu SeaBIOS:
Result: 1218.6ms ±1.0ms
Result: 1214.1ms ±0.0ms
KevinOConnor/testing branch:
Result: 1221.9ms ±0.5ms
Result: 1216.4ms ±0.2ms
> Another consumer of time is ACPI table deployment. I wonder if you
> could get similar results by running QEMU with "-no-acpi"?
No apparent difference:
Stock qemu SeaBIOS:
Result: 1211.5ms ±4.8ms
Stock qemu SeaBIOS + qemu -no-acpi:
Result: 1213.9ms ±6.5ms
> Beyond that, I think the only other big time consumers of the default
> seabios is debug messages. If so, then I think we can come up with a
> way to limit these debug messages in SeaBIOS.
>
> The SeaBIOS testing patches are at:
>
> https://github.com/KevinOConnor/seabios/tree/testing
Thanks,
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
- Re: [Qemu-devel] Why is SeaBIOS used with -kernel?, (continued)
- Re: [Qemu-devel] Why is SeaBIOS used with -kernel?, Kevin O'Connor, 2016/04/01
- Re: [Qemu-devel] Why is SeaBIOS used with -kernel?, Paolo Bonzini, 2016/04/01
- Re: [Qemu-devel] Why is SeaBIOS used with -kernel?, Richard W.M. Jones, 2016/04/01
- Re: [Qemu-devel] Why is SeaBIOS used with -kernel?, Richard W.M. Jones, 2016/04/01
- Re: [Qemu-devel] Why is SeaBIOS used with -kernel?, Kevin O'Connor, 2016/04/01
- Re: [Qemu-devel] Why is SeaBIOS used with -kernel?, Richard W.M. Jones, 2016/04/01
- Re: [Qemu-devel] Why is SeaBIOS used with -kernel?, Richard W.M. Jones, 2016/04/01
- Re: [Qemu-devel] Why is SeaBIOS used with -kernel?, Kevin O'Connor, 2016/04/01
- Re: [Qemu-devel] Why is SeaBIOS used with -kernel?, Richard W.M. Jones, 2016/04/01
- Re: [Qemu-devel] Why is SeaBIOS used with -kernel?, Kevin O'Connor, 2016/04/01
- Re: [Qemu-devel] Why is SeaBIOS used with -kernel?,
Richard W.M. Jones <=
- Re: [Qemu-devel] Why is SeaBIOS used with -kernel?, Kevin O'Connor, 2016/04/01
- Re: [Qemu-devel] Why is SeaBIOS used with -kernel?, Richard W.M. Jones, 2016/04/02
- Re: [Qemu-devel] Why is SeaBIOS used with -kernel?, Paolo Bonzini, 2016/04/02
- Re: [Qemu-devel] Why is SeaBIOS used with -kernel?, Kevin O'Connor, 2016/04/01
- Re: [Qemu-devel] Why is SeaBIOS used with -kernel?, Kevin O'Connor, 2016/04/01
- Re: [Qemu-devel] Why is SeaBIOS used with -kernel?, Richard W.M. Jones, 2016/04/01
- Re: [Qemu-devel] Why is SeaBIOS used with -kernel?, Kevin O'Connor, 2016/04/01
- Re: [Qemu-devel] Why is SeaBIOS used with -kernel?, Richard W.M. Jones, 2016/04/01