linux-kernel - Re: questions on NAPI processing latency and dropped network packets

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Message-ID: <479529DF.5030707@nortel.com>
Date:	Mon, 21 Jan 2008 17:25:19 -0600
From:	"Chris Friesen" <cfriesen@...tel.com>
To:	Eric Dumazet <dada1@...mosbay.com>
CC:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: questions on NAPI processing latency and dropped network packets

Eric Dumazet wrote:
> Chris Friesen a écrit :
> 
>> I've done some further digging, and it appears that one of the 
>> problems we may be facing is very high instantaneous traffic rates.
>>
>> Instrumentation showed up to 222K packets/sec for short periods (at 
>> least 1.1 ms, possibly longer), although the long-term average is down 
>> around 14-16K packets/sec.
> 
> 
> Instrumentation done where exactly ?

I added some code to e1000_clean_rx_irq() to track rx_fifo drops, total 
packets received, and an accurate timestamp.

If rx_fifo errors changed, it would dump the information.

>> Is there anything else we can do to minimize the latency of network 
>> packet processing and avoid having to crank the rx ring size up so high?

> You have some tasks that disable softirqs too long. Sometimes, bumping 
> RX ring size is OK (but you will still have delays), sometimes it is not 
> an option, since 4096 is the limit on current hardware.

I added some instrumentation to take timestamps in __do_softirq() as 
well.  Based on these timestamps, I can see the following code sequence:

2374604616 usec, start processing softirqs in __do_softirq()
2374610337 usec, log values in e1000_clean_rx_irq()
2374611411 usec, log values in e1000_clean_rx_irq()

In between the successive calls to e1000_clean_rx_irq() the rx_fifo 
counts went up.

Does anyone have any patchsets to track down what softirqs are taking a 
long time, and/or who's disabling softirqs?

Chris
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