Subject: Re: [PATCH iproute2] Re: HTB accuracy for high speed

From: Patrick McHardy <[email-masked]>
Date: Wed, 03 Jun 2009 09:06:37 +0200

Jarek Poplawski wrote:
> Jarek Poplawski wrote, On 06/02/2009 11:37 PM:
> ...
> 
>> I described the reasoning here:
>> http://permalink.gmane.org/gmane.linux.network/128189
> 
> The link is stuck now, so here is a quote:

Thanks.

> Jarek Poplawski wrote, On 05/17/2009 10:15 PM:
> 
>> Here is some additional explanation. It looks like these rates above
>> 500Mbit hit the design limits of packet scheduling. Currently used
>> internal resolution PSCHED_TICKS_PER_SEC is 1,000,000. 550Mbit rate
>> with 800byte packets means 550M/8/800 = 85938 packets/s, so on average
>> 1000000/85938 = 11.6 ticks per packet. Accounting only 11 ticks means
>> we leave 0.6*85938 = 51563 ticks per second, letting for additional
>> sending of 51563/11 = 4687 packets/s or 4687*800*8 = 30Mbit. Of course
>> it could be worse (0.9 tick/packet lost) depending on packet sizes vs.
>> rates, and the effect rises for higher rates.

I see. Unfortunately changing the scaling factors is pushing the lower
end towards overflowing. For example Denys Fedoryshchenko reported some
breakage a few years ago when I changed the iproute-internal factors
triggered by this command:

.. tbf buffer 1024kb latency 500ms rate 128kbit peakrate 256kbit 
minburst 16384

The burst size calculated by TBF with the current parameters is
64000000. Increasing it by a factor of 16 as in your patch results
in 1024000000. Which means we're getting dangerously close to
overflowing, a buffer size increase or a rate decrease of slightly
bigger than factor 4 will already overflow.

Mid-term we really need to move to 64 bit values and ns resolution,
otherwise this problem is just going to reappear as soon as someone
tries 10gbit. Not sure what the best short term fix is, I feel a bit
uneasy about changing the current factors given how close this brings
us towards overflowing.