Re: [RFC PATCH 0/5] net: low latency Ethernet device polling

From: Eliezer Tamir
Date: Wed Feb 27 2013 - 15:40:37 EST

On 27/02/2013 21:58, Rick Jones wrote:
On 02/27/2013 09:55 AM, Eliezer Tamir wrote:

Performance numbers:
Kernel Config C3/6 rx-usecs TCP UDP
3.8rc6 typical off adaptive 37k 40k
3.8rc6 typical off 0* 50k 56k
3.8rc6 optimized off 0* 61k 67k
3.8rc6 optimized on adaptive 26k 29k
patched typical off adaptive 70k 78k
patched optimized off adaptive 79k 88k
patched optimized off 100 84k 92k
patched optimized on adaptive 83k 91k
*rx-usecs=0 is usually not useful in a production environment.

I would think that latency-sensitive folks would be using rx-usecs=0 in
production - at least if the NIC in use didn't have low enough latency
with its default interrupt coalescing/avoidance heuristics.

It will only work well if you have no bulk traffic on the same port as the low latency traffic at all.

If I take the first "pure" A/B comparison it seems that the change as
benchmarked takes latency for TCP from ~27 usec (37k) to ~14 usec (70k).
At what request/response size does the benefit taper-off? 13 usec
seems to be about 16250 bytes at 10 GbE.

It's pretty easy to get a result of 80K+ with a little tweaking, an rx-usecs value of 100 with C3/6 enabled will get you that.

When I last looked at netperf TCP_RR performance where something similar
could happen I think it was IPoIB where it was possible to set things up
such that polling happened rather than wakeups (perhaps it was with a
shim library that converted netperf's socket calls to "native" IB). My
recollection is that it "did a number" on the netperf service demands
thanks to the spinning. It would be a good thing to include those
figures in any subsequent rounds of benchmarking.

I will get service demand numbers, but we are busy polling so I can tell you right now that one core will be at 100%.

Am I correct in assuming this is a mechanism which would not be used in
a high aggregate PPS situation?

The current design has in mind situations where you want to react very fast to a trigger but that reaction could involve more than short messages. so we are willing to burn CPU cycles when there is nothing better to do, but we also want to work well when there is bulk traffic.
Ideally I would want the system to be smart about this and to know when not to allow busy polling.

happy benchmarking,
we love netperf.

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