On 11.10.2011, at 11:26, Avi Kivity wrote:
On 10/11/2011 11:19 AM, Alexander Graf wrote:
Of this, 1.4 seconds is the time required by LinuxBoot to copy the
kernel+initrd. If I used an uncompressed initrd, which I really want
to, to avoid decompression overhead, this increases to ~1.7 seconds.
So the LinuxBoot ROM is ~60% of total QEMU execution time, or 40%
of total sandbox execution overhead.
One thing we can do is boot a guest and immediately snapshot it, before it
runs any application specific code. Subsequent invocations will MAP_PRIVATE
the memory image and COW their way. This avoids the kernel initialization time
as well.
That doesn't allow modification of -append
Is it really needed?
For our use case for example yes. We pass the cifs user/pass using the kernel
cmdline, so we can reuse existing initrd code and just mount it as root.
and gets you in a pretty bizarre state when doing updates of your host files,
since then you have 2 different paths: full boot and restore. That's yet
another potential source for bugs.
Typically you'd check the timestamps to make sure you're running an up-to-date
version.
Yes. That's why I said you end up with 2 different boot cases. Now imagine you
get a bug once every 10000 bootups and try to trace that down that it only
happens when running in the non-resume case.
For comparison I also did a test building a bootable ISO using ISOLinux.
This required 700 ms for the boot time, which is appoximately 1/2 the
time reqiured for direct kernel/initrd boot. But you have to then add
on time required to build the ISO on every boot, to add custom kernel
command line args. So while ISO is faster than LinuxBoot currently
there is still non-negligable overhead here that I want to avoid.
You can accept parameters from virtio-serial or some other channel. Is there
any reason you need them specifically as *kernel* command line parameters?
That doesn't work for kernel parameters. It also means things would have to be
rewritten needlessly. Some times we can't easily change the way parameters are
passed into the guest either, for example when running a random (read: old,
think of RHEL5) distro installation initrd.
This use case is not installation, it's for app sandboxing.
I thought we were talking about plenty different use cases here? I'm pretty
sure there are even more out there that we haven't even thought about.
And I don't see the point why we would have to shoot yet another hole into the
guest just because we're too unwilling to make an interface that's perfectly
valid horribly slow.
rep/ins is exactly like dma+wait for this use case: provide an address, get a
memory image in return. There's no need to add another interface, we should
just optimize the existing one.
Whatever we do, the interface will never be as fast as DMA. We will always have
to do sanity / permission checks for every IO operation, can batch up only so
many IO requests and in QEMU again have to call our callbacks in a loop.
I don't see where the problem is in admitting that we were wrong back then. The
fw_cfg interface as it is is great for small config variables, but nobody sane
would even consider using IDE without DMA these days for example, because
you're transferring bulk data. And that's exactly what we do in this case. We
transfer bulk data.
However, I'll gladly see myself proven wrong with an awesomely fast rep/ins
implementation that loads 100MB in< 1/10th of a second.
Alex