netdev - Re: [RFC 0/2] pm,net: Introduce QoS requests per CPU

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Date:	Wed, 26 Mar 2014 17:42:33 +0200
From:	Amir Vadai <amirv@...lanox.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	"David S. Miller" <davem@...emloft.net>,
	<linux-pm@...r.kernel.org>, <netdev@...r.kernel.org>,
	Pavel Machek <pavel@....cz>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Len Brown <len.brown@...el.com>, <yuvali@...lanox.com>,
	Or Gerlitz <ogerlitz@...lanox.com>,
	Yevgeny Petrilin <yevgenyp@...lanox.com>, <idos@...lanox.com>
Subject: Re: [RFC 0/2] pm,net: Introduce QoS requests per CPU

[This mail might be double posted due to problems I have with the mail 
server]

On 25/03/14 08:14 -0700, Eric Dumazet wrote:
> On Tue, 2014-03-25 at 15:18 +0200, Amir Vadai wrote:
>
> > The current pm_qos implementation has a problem. During a short pause in a high
> > bandwidth traffic, the kernel can lower the c-state to preserve energy.
> > When the pause ends, and the traffic resumes, the NIC hardware buffers may be
> > overflowed before the CPU starts to process the traffic due to the CPU wake-up
> > latency.
>
> This is the point I never understood with mlx4
>
> RX ring buffers should allow NIC to buffer quite a large amount of
> incoming frames. But apparently we miss frames, even in a single TCP
> flow. I really cant understand why, as sender in my case do not have
> more than 90 packets in flight (cwnd is limited to 90)

Hi,

We would like to nail down the errors you experience.

>
> # ethtool -S eth0 | grep error
>      rx_errors: 268
This is an indication for a bad cable

>      tx_errors: 0
>      rx_length_errors: 0
>      rx_over_errors: 40
>      rx_crc_errors: 0
>      rx_frame_errors: 0
>      rx_fifo_errors: 40
>      rx_missed_errors: 40

did you experience the rx_over_errors, rx_fifo_errors and
rx_missed_errors on a setup where rx_errors is 0?

The above 3 counters are actually the same HW counter, which indicates
that the HW buffer is full - probably as Ben indicated, because the
DMA wasn't fast enough.

>      tx_aborted_errors: 0
>      tx_carrier_errors: 0
>      tx_fifo_errors: 0
>      tx_heartbeat_errors: 0
>      tx_window_errors: 0
> # ethtool -g eth0
> Ring parameters for eth0:
> Pre-set maximums:
> RX:		8192
> RX Mini:	0
> RX Jumbo:	0
> TX:		8192
> Current hardware settings:
> RX:		4096
> RX Mini:	0
> RX Jumbo:	0
> TX:		4096
This is relevant to the buffers on the host memory, the error
statistics above indicates that problem is in the HW buffers on the
NIC memory.

Assuming that there are no cable issues here, please give us
instructions how to reproduce the issue.

Just to make sure, are you running with flow control disabled?

When flow control is enabled, we didn't see any errors - single or
multi stream traffic.
When flow control is disabled, we didn't see any errors on a single
stream of 27Gbe. Only with a multi stream traffic (full line rate) we
did see drops - but it is expected.
In any case we didn't get rx_errors.

Amir
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