[libvirt] [PATCH] nwfilter: increase pcap buffer size to be compatible with TPACKET_V3

[Daniel P. Berrangé]

On Thu, Apr 26, 2018 at 11:55:55AM -0400, Laine Stump wrote:
> >> That will help the next one stumbling over this code.
> > FWIW, having just checked libpcap git history, the current 2 MB default
> > size has been there since 2008 !   I'm guessing we don't use that as we
> > don't want to consume lots of RAM when many guests are running.
> >
> 
> Yeah, I'm guessing this is why Stefan chose to set the buffer size.

> For the case of dhcp snooping, though, the handle remains open for the
> entire lifetime of the domain (I guess in case the address is released
> and re-requested and a different address is handed out the next time).
> So if we're using the default 2MB buffer, there would be 2MB for each
> client. On one hand, for a system with 100 clients, that's 200MB of
> memory. On the other hand, any system that's miniscule in relation to
> the total amount of memory (it might reduce the capacity of a very large
> host by one guest at most).

Hmm, I didn't realize dhcp snooping stayed active forever, but that
totally makes sense given way dhcp works.

2MB is not much per VM, but bear in mind there are 100's of places
where libvirt or qemu has decided it is "not much", so overall we
do end up with alot of fixed overhead per guest, beyond its RAM
allocation.

So I'm still wary of using 2MB per guest for something which hangs
around for entire life of the guest.

Lets just use 256kb and put a big comment explaining the mess so
when it breaks again in 5 years time, we don't have to investigate
it starting from zero knowledge aain.


Regards,
Daniel
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