[dpdk-dev] Defaults for rte_hash

[De Lara Guarch, Pablo]

> -----Original Message-----
> From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Richardson, Bruce
> Sent: Tuesday, September 09, 2014 11:45 AM
> To: Matthew Hall; dev at dpdk.org
> Subject: Re: [dpdk-dev] Defaults for rte_hash
> 
> > -----Original Message-----
> > From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Matthew Hall
> > Sent: Tuesday, September 09, 2014 11:32 AM
> > To: dev at dpdk.org
> > Subject: [dpdk-dev] Defaults for rte_hash
> >
> > Hello,
> >
> > I was looking at the code which inits rte_hash objects in examples/l3fwd.
> It's
> > using approx. 1M to 4M hash 'entries' depending on 32-bit vs 64-bit, but it's
> > setting the 'bucket_entries' to just 4.
> >
> > Normally I'm used to using somewhat deeper hash buckets than that... it
> seems
> > like having a zillion little tiny hash buckets would cause more TLB pressure
> > and memory overhead... or does 4 get shifted / exponentiated into 2**4 ?
> >

That 4 is not shifted, so it is actually 4 entries/bucket. Actually, the maximum number of entries you can use is 16, as bucket will be as big as a cache line.
However, regardless the number of entries, memory size will remain the same, but using 4 entries/bucket, with 16-byte key, all keys stored for a bucket will fit in a cache line, 
so performance looks to be better in this case (although a non-optimal hash function could lead not to be able to store all keys, as chances to fill a bucket are higher).
Anyway, for this example, 4 entries/bucket looks a good number to me.

> > The documentation in
> > http://dpdk.org/doc/api/structrte__hash__parameters.html
> > and http://dpdk.org/doc/api/rte__hash_8h.html isn't that clear... is there a
> > better place to look for this?
> >
> > In my case I'm looking to create a table of 4M or 8M entries, containing
> > tables of security threat IPs / domains, to be detected in the traffic. So it
> > would be good to have some understanding how not to waste a ton of
> memory
> > on a
> > table this huge without making it run super slow either.
> >
> > Did anybody have some experience with how to get this right?
> 
> It might be worth looking too at the hash table structures in the librte_table
> directory for packet framework. These should give better scalability across
> millions of flows than the existing rte_hash implementation. [We're looking
> here to provide in the future a similar, more scalable, hash table
> implementation with an API like that of rte_hash, but that is still under
> development here at the moment.]
> 
> >
> > Another thing... the LPM table uses 16-bit Hop IDs. But I would probably
> have
> > more than 64K CIDR blocks of badness on the Internet available to me for
> > analysis. How would I cope with this, besides just letting some attackers
> > escape unnoticed? ;)
> 
> Actually, I think the next hop field in the lpm implementation is only 8-bits,
> not 16 :-). Each lpm entry is only 16-bits in total.
> 
> >
> > Have we got some kind of structure which allows a greater number of
> CIDRs
> > even
> > if it's not quite as fast?
> >
> > Thanks,
> > Matthew.