netdev - Re: Cause of Large Latency Difference Between 1179-byte and 1180-byte UDP Frames?

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Date:	Thu, 21 May 2015 11:33:49 -0700
From:	Todd Bezenek <bezenek@...il.com>
To:	Alexander Duyck <alexander.duyck@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: Cause of Large Latency Difference Between 1179-byte and 1180-byte
 UDP Frames?

(solution below)

The time to send that extra byte over the wire is 8 nsec.  It seems
unlikely that would produce such a clean-cut difference, but I've been
surprised before.  :)

The "idle=poll" parameter sounds like a good idea for any application
which tries to maintain real-time behavior.  I'm putting that into
place.

Result:  About the same.

I believe I have C-states disabled, as the directories
/sys/devices/system/cpu/cpuX/cpufreq/ are not in the filesystem.  Just
to be sure, I removed "idle=poll" and added "processor.max_cstate=1".

Result:  About the same as "idle=poll".

I set the interrupt moderation parameter to 10 on both interfaces
involved in the bridge with "ethtool -C <dev> rx-usecs 10"".

Result:  HUGE difference.

This looks much better.  I'm going to explore the adjustments
available and I think this will solve the problem.

Thank you, Alex,

-Todd

On Thu, May 21, 2015 at 7:55 AM, Alexander Duyck
<alexander.duyck@...il.com> wrote:
> On 05/20/2015 09:50 PM, Todd Bezenek wrote:
>>
>> I'm pushing 10,000 frames per second of UDP traffic through a Linux
>> system with a bridge configured between two 1GbE ports.
>>
>> Iptables is installed and running, but the default rule is ACCEPT with
>> no other rules.
>>
>> When I make the packets 1179 bytes in size (total size includes
>> Ethernet header, etc.), I see the latency of most packets between 60
>> and 200 usec.  When I change the size to 1180 bytes, I start seeing
>> about 1% of the packets with latencies larger than 300 usec, and some
>> as high as 500+ usec.
>>
>> Any ideas what buffer size, cache line count, TLB reach, etc. might be
>> causing this dramatic change?
>>
>> Cut-and-paste to my terminal is not working, so limited info here:
>>
>> Kernel:  3.12.20 x86-64
>> Processor:  Intel E3845 (Atom, 4 cores)
>> Network interface driver:  Intel igb
>>
>> Thank you for any helpful pointers.
>>
>> -Todd
>
>
> The igb driver defaults to an adaptive interrupt moderation scheme. It is
> possible that what you are seeing could be due to that, or it could possibly
> be that the time between packets is becoming large enough that your CPU is
> starting to go into deeper sleep states.
>
> My first suggestion would be to try disabling sleeps states beyond C1.  To
> do that you could boot your kernel with the parameter
> "processor.max_cstate=1" or "idle=poll".  Either one should prevent the CPU
> from going into a sleep state but it will consume more power.
>
> You also might try modifying the interrupt moderation to a fixed value of 10
> or more via "ethtool -C <dev> rx-usecs <x>" to see if the behavior goes away
> after that.  If the device is the cause it might point to a possible glitch
> in the interrupt moderation scheme or possibly lost interrupts for the
> device or driver.
>
> - Alex
>
>

-- 
Todd Bezenek
bezenek@...il.com

A people hire A people, B people hire C people.
  --Jim Gray
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