netdev - Re: [RFC net-next 0/4] gianfar: Use separate NAPI for Tx confirmation processing

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Date:	Wed, 8 Aug 2012 16:06:33 -0700
From:	Tomas Hruby <thruby@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Claudiu Manoil <claudiu.manoil@...escale.com>,
	netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>
Subject: Re: [RFC net-next 0/4] gianfar: Use separate NAPI for Tx confirmation processing

On Wed, Aug 8, 2012 at 9:44 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Wed, 2012-08-08 at 12:24 -0400, Paul Gortmaker wrote:
>> [[RFC net-next 0/4] gianfar: Use separate NAPI for Tx confirmation processing] On 08/08/2012 (Wed 15:26) Claudiu Manoil wrote:
>>
>> > Hi all,
>> > This set of patches basically splits the existing napi poll routine into
>> > two separate napi functions, one for Rx processing (triggered by frame
>> > receive interrupts only) and one for the Tx confirmation path processing
>> > (triggerred by Tx confirmation interrupts only). The polling algorithm
>> > behind remains much the same.
>> >
>> > Important throughput improvements have been noted on low power boards with
>> > this set of changes.
>> > For instance, for the following netperf test:
>> > netperf -l 20 -cC -H 192.168.10.1 -t TCP_STREAM -- -m 1500
>> > yields a throughput gain from oscilating ~500-~700 Mbps to steady ~940 Mbps,
>> > (if the Rx/Tx paths are processed on different cores), w/ no increase in CPU%,
>> > on a p1020rdb - 2 core machine featuring etsec2.0 (Multi-Queue Multi-Group
>> > driver mode).
>>
>> It would be interesting to know more about what was causing that large
>> an oscillation -- presumably you will have it reappear once one core
>> becomes 100% utilized.  Also, any thoughts on how the change will change
>> performance on an older low power single core gianfar system (e.g.  83xx)?
>
> I also was wondering if this low performance could be caused by BQL
>
> Since TCP stack is driven by incoming ACKS, a NAPI run could have to
> handle 10 TCP acks in a row, and resulting xmits could hit BQL and
> transit on qdisc (Because NAPI handler wont handle TX completions in the
> middle of RX handler)

Does disabling BQL help? Is the BQL limit stable? To what value is it
set? I would be very much interested in more data if the issue is BQL
related.
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