Re: [dpdk-dev] Random numbers at line-rate

[Neil Horman <nhorman@tuxdriver.com>]

On Mon, Jul 21, 2014 at 01:43:50PM -0700, Stephen Hemminger wrote:
> On Mon, 21 Jul 2014 15:54:15 -0400
> Neil Horman <nhorman@tuxdriver.com> wrote:
> 
> > On Mon, Jul 21, 2014 at 09:24:36PM +0200, Chris Pappas wrote:
> > > Hi,
> > > 
> > > I need to generate a random number per packet and I used the rte_fast_rand
> > > function to do so. When I run the code for one port-core I get almost
> > > line-rate performance. However, running simultaneously on multiple cores
> > > degrades performance significantly. (in all cases I uses minimum-sized
> > > packets).
> > > 
> > > Shouldn't the implementation scale for multicore and not degrade
> > > performance or am I missing anything? Also, is there another recommendation
> > > for generating randomness at line-rate? (the cpu does not support rdrand).
> > > 
> > > Best regards,
> > > Chris
> > > 
> > 
> > thats an odd random number generator.  I think, without locking, its likely on a
> > multicore system to produce identical values on multiple cores operating in
> > parallel (since multiple cores can read rte_red_rand_seed at the same time).
> > That may well lead to multiple packets having the same nonce, which might cause
> > odd behavior.
> > 
> > If your cpu supports it, I'd suggest writing some inline assembly to use the
> > rdrand instruction instead.  I'm not sure about its performance relative to the
> > current implementation, but IIRC the instruction is handled internal to the
> > core, so it should scale with any number of cpus.
> > 
> > neil
> > 
> 
> Or  just do per-core seed value (and use RTE_PER_LCORE)
rte_fast_rand doesn't seem like it can take advantage of per lcore variables.
Generally speaking its not multi-threaded at all from what I can see.
neil

>