netdev - RE: [PATCH v1 4/6] net: fec: Increase buffer descriptor entry number

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Date:	Fri, 30 May 2014 15:52:01 +0000
From:	"fugang.duan@...escale.com" <fugang.duan@...escale.com>
To:	David Laight <David.Laight@...LAB.COM>,
	Eric Dumazet <eric.dumazet@...il.com>,
	"ezequiel.garcia@...e-electrons.com" 
	<ezequiel.garcia@...e-electrons.com>
CC:	"Frank.Li@...escale.com" <Frank.Li@...escale.com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"shawn.guo@...aro.org" <shawn.guo@...aro.org>,
	"bhutchings@...arflare.com" <bhutchings@...arflare.com>,
	"stephen@...workplumber.org" <stephen@...workplumber.org>
Subject: RE: [PATCH v1 4/6] net: fec: Increase buffer descriptor entry number

Hi, David

From: David Laight <David.Laight@...LAB.COM> Data: Friday, May 30, 2014 11:35 PM
>To: Duan Fugang-B38611; Eric Dumazet; ezequiel.garcia@...e-electrons.com
>Cc: Li Frank-B20596; davem@...emloft.net; netdev@...r.kernel.org;
>shawn.guo@...aro.org; bhutchings@...arflare.com;
>stephen@...workplumber.org
>Subject: RE: [PATCH v1 4/6] net: fec: Increase buffer descriptor entry
>number
>
>From: fugang.duan@...escale.com
>> >64KB TSO packet, with MSS=1460 -> 44 segments  (44*1460 = 64240) with
>> >MSS=1448 (TCP timestamps) -> 45 segments  (45*1448 = 65160)
>> >
>> >This software TSO emulation uses at least 2 descriptors per MSS
>> >
>> >one descriptor to hold the headers (ethernet + ip + tcp) one
>> >descriptor (or two) to hold the payload for this MSS
>> >
>> Thanks for Eric's detail explain.
>>
>> If frag page data is not match the alignment for ethernet DMA
>> controller, there need three descriptor for one MSS:
>> One descriptor for headers, one for the first non-align bytes copied
>> from frag page, one for the rest of frag page.
>>
>> So one frame may cost descriptor number is: 3 x 45
>
>No - that is 45 frames, typically needing 3 ring entries each.
>
>> And I will add interrupt coalescing support for tx and rx, which also
>cost some more descriptors.
>>
>> So the descriptors slots set to 512 is not big, just is reasonable. Do
>you think ?
>
>Software TSO generates lots of separate ethernet frames, there is no
>absolute requirement to be able to put all of them into the tx ring at
>once.
>
>The required size for the tx ring is much more likely to be related to any
>interrupt mitigation that delays the refilling of ring entries.
>512 sounds like a lot of tx ring entries.
>
>The receive ring doesn't need to allow for fragments - your driver is in
>control of allocating the buffers.
>I didn't understand why you've based the number of rx ring entries on
>PAGE_SIZE - IIRC that might be 64k, or even larger.
>I'd have expected 128 or 256 rx ring entries to be typical, but how many
>are needed is a separate issue.
>If the system can keep up with the maximum ethernet data rate I'd expect
>that a smaller number would be fine.
>If it can't keep up you'll lose packets anyway.
>Aggressive power saving (with wake on LAN) might need more.
>
>	David

Thanks for your information. I will do more test to weigh the smaller number for maximum data.
Summary you and Eric's thoughts, maybe rx set to 256, tx set to 512 are fine.

Thanks,
Andy