[libvirt] [PATCH] rpc: message related sizes enlarged

[Daniel P. Berrange]

On Mon, Apr 22, 2013 at 06:18:04PM +0200, Viktor Mihajlovski wrote:
> On 04/22/2013 04:34 PM, Daniel P. Berrange wrote:
> >On Mon, Apr 22, 2013 at 04:22:26PM +0200, Viktor Mihajlovski wrote:
> >I'm not against this in general, but before we enlarge this so much
> >there needs to be some code work to make RPC message encoding more
> >efficient. Currently virNetMessageEncodeHeader will allocate
> >VIR_NET_MESSAGE_MAX immediately, regardless of whether the message
> >being encoded acutally needs so much. The problem is that we can't
> >tell ahead of time how much space a message needs. We just can't go
> >on doing this if we're going to have such large max message size.
> >We need to start with small allocs & grow if we hit the size limit.
>
> So something along the lines of allocating VIR_NET_MESSAGE_INITIAL
> bytes, maybe well below 1M, and then bumping it in the
> EncodePayloadxxx calls if required. Do you foresee any pitfalls
> one should consider?

We have an ever increasing variability in RPC message size - some of
them will remain absolutely tiny - 24 bytes in size, while others can
be huge. Deciding on the optimal initial allocation size is a tradeoff
between the overheads of frequently re-alloc'ing to enlarge the buffer,
vs the overhead of allocting too much memory and/or calling memset()
on the buffers.

I'd suggest an initial size of just a perhaps as small as a few 10's
of KB ? Then do a loop doubling allocation each time until it hits
the hard limit. In cases where we even up triggering the re-alloc
loop many times, we'll presumably be on large hardware with many
many VMs, so the overhead of the re-allocs will be dwarved by all
the other activity on the machine.

Daniel
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