Re: [Qemu-devel] [PATCH 07/15] qapi: use mmap for QmpInputVisitor

[Daniel P. Berrange]

On Tue, Jun 28, 2016 at 11:39:03AM +0200, Peter Lieven wrote:
> Am 28.06.2016 um 11:29 schrieb Dr. David Alan Gilbert:
> > * Peter Lieven (address@hidden) wrote:
> > > this struct is approx 75kB
> > I wonder why it's so large.
> > 
> > The stack size in QmpInputVisitor; it's got a 1024 element stack
> > (QIV_STACK_SIZE) and I bet we never use anywhere near that.
> > 
> > But even then that's 1024 * a 3 pointer stack object, 24 bytes -
> > I don't see where the rest of that 75kB comes from.
> 
> Sorry, I had a wrong size in mind. Its 24736 bytes. But thats
> still larger than expecetd, right?
> 
> > I'm a little wary about turning all these malloc's into mmap's
> > because we do seem to use things like input visitors for small
> > things; don't the cost of doing the mmap's add up in time
> > instead of space?
> 
> Sure, we should discuss this. The series should act as a base for
> discussion.
> 
> None of the things I changed into mmap are continously called
> during VM runtime. So it most likely only happens at setup of the
> vServer.

QmpInputVisitor is used to parse all QMP monitor commands, so will
be used continuously throughout life of QEMU, often very frequently.
eg When migration is running many monitor commands per second are
expected

Regards,
Daniel
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