netdev - RE: [PATCH net-next v2 1/2] r8152: add RTL8152_EARLY_AGG_TIMEOUT

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Date:	Thu, 13 Mar 2014 09:28:12 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Hayes Wang' <hayeswang@...ltek.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:	"nic_swsd@...ltek.com" <nic_swsd@...ltek.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>
Subject: RE: [PATCH net-next v2 1/2] r8152: add
 RTL8152_EARLY_AGG_TIMEOUT_SUPER

From: Hayes Wang
> For slow CPU, the frequent bulk transfer would cause poor throughput.
> One solution is to increase the timeout of the aggregation. It let
> the hw could complete the bulk transfer later and fill more packets
> into the buffer. Besides, it could reduce the frequency of the bulk
> transfer efficiently and improve the performance.
> 
> However, the optimization value of the timeout depends on the
> capability of the hardware, especially the CPU. For example, according
> to the experiment, the timeout 164 us is better than the default
> value for the chromebook with the ARM CPU.

The best value probably depends on the workload as well as the
cpu speed.
I wonder if a sane algorithm for dynamically setting this value exists.
It really needs a CBU (crystal ball unit), but working ones are
difficult to come by.

Passing the buck to the 'user' is rather a cop-out (but we all do it).

> Now add RTL8152_EARLY_AGG_TIMEOUT_SUPER to let someone could choose
> desired timeout value if he wants to get the best performance.
...
>  /* USB_RX_EARLY_AGG */
> -#define EARLY_AGG_SUPPER	0x0e832981
> +#define EARLY_AGG_SUPER	((((rx_buf_sz - 1522) / 4) << 16) | \
> +	(u32)(CONFIG_RTL8152_EARLY_AGG_TIMEOUT_SUPER <= 0 ? 0x2981 : \
> +	((CONFIG_RTL8152_EARLY_AGG_TIMEOUT_SUPER * 125) < 0xffff ? \
> +	CONFIG_RTL8152_EARLY_AGG_TIMEOUT_SUPER * 125 : 0xffff)))

The 0x2981 would be better written as (85 * 125).
But maybe replace with something like:
	min((CONFIG_RTL8152_EARLY_AGG_TIMEOUT_SUPER <= 0 ? 85 :
		CONFIG_RTL8152_EARLY_AGG_TIMEOUT_SUPER) * 125u, 0xffffu)

	David



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