[dpdk-dev] Random numbers at line-rate

[dpdk-dev] Random numbers at line-rate

[Chris Pappas]

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

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

[Neil Horman]

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

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

[Stephen Hemminger]

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)

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

[Neil Horman]

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

>