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

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Date:	Tue, 25 Mar 2014 08:14:07 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Amir Vadai <amirv@...lanox.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

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)

# ethtool -S eth0 | grep error
     rx_errors: 268
     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
     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




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