Re: [Qemu-devel] Anyone seeing huge slowdown launching qemu with Linux 2.6.35?

[Avi Kivity <avi@redhat.com>]

  On 08/03/2010 05:05 PM, Richard W.M. Jones wrote:
> On Tue, Aug 03, 2010 at 04:19:39PM +0300, Avi Kivity wrote:
>>   On 08/03/2010 03:48 PM, Richard W.M. Jones wrote:
>>> Thanks for the explanation.  I'll repost my "DMA"-like fw-cfg patch
>>> once I've rebased it and done some more testing.  This huge regression
>>> for a common operation (implementing -initrd) needs to be solved
>>> without using inb/rep ins.
>> Adding more interfaces is easy but a problem in the long term.
>> We'll optimize it as much as we can.  Meanwhile, why are you loading
>> huge initrds?  Use a cdrom instead (it will also be faster since the
>> guest doesn't need to unpack it).
> Because it involves rewriting the entire appliance building process,
> and we don't necessarily know if it'll be faster after we've done
> that.
>
> Look: currently we create the initrd on the fly in 700ms.  We've no
> reason to believe that creating a CD-ROM on the fly wouldn't take
> around the same time.  After all, both processes involve reading all
> the host files from disk and writing a temporary file.

The time will only continue to grow as you add features and as the 
distro bloats naturally.

Much better to create it once and only update it if some dependent file 
changes (basically the current on-the-fly code + save a list of file 
timestamps).

Alternatively, pass through the host filesystem.

> You have to create these things on the fly, because we don't actually
> ship an appliance to end users, just a tiny (<  1 MB) skeleton.  You
> can't ship a massive statically linked appliance to end users because
> it's just unmanageable (think: security; updates; bandwidth).

Shipping it is indeed out of the question.  But on-the-fly creation is 
not the only alternative.

> Loading the initrd currently takes 115ms (or could do, if a sensible
> 50 line patch was permitted).
>
> So the only possible saving would be the 115ms load time of the
> initrd.  In theory the CD-ROM device could be detected in 0 time.
>
> Total saving: 115ms.

815 ms by my arithmetic.  You also save 3*N-2*P memory where N is the 
size of your initrd and P is the actual amount used by the guest.

> But will it be any faster, since after spending 115ms, everything runs
> from memory, versus being loaded from the CD?
>
> Let's face the fact that qemu has suffered from an enormous
> regression.  From some hundreds of milliseconds up to over a minute,
> in the space of 6 months of development.

It wasn't qemu, but kvm.  And it didn't take six months, just a few 
commits.  Those aren't going back, they're a lot more important than 
some libguestfs problem which shouldn't have been coded differently in 
the first place.

> For a very simple operation:
> loading a file into memory.

Loading a file into memory is plenty fast if you use the standard 
interfaces.  -kernel -initrd is a specialized interface.

-- 
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