netdev - Re: [PATCH net-next v3 1/2] net: implement dma cache skb allocator

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Date:	Wed, 11 Mar 2015 14:27:56 +0530 (IST)
From:	Govindarajulu Varadarajan <_govind@....com>
To:	Alexander Duyck <alexander.h.duyck@...hat.com>
cc:	Govindarajulu Varadarajan <_govind@....com>, davem@...emloft.net,
	netdev@...r.kernel.org, ssujith@...co.com, benve@...co.com
Subject: Re: [PATCH net-next v3 1/2] net: implement dma cache skb allocator

On Tue, 10 Mar 2015, Alexander Duyck wrote:

>
> On 03/10/2015 10:43 AM, Govindarajulu Varadarajan wrote:
>> This patch implements dma cache skb allocator. This is based on
>> __alloc_page_frag & __page_frag_refill implementation in net/core/skbuff.c
>> 
>> In addition to frag allocation from order(3) page in __alloc_page_frag,
>> we also maintain dma address of the page. While allocating a frag for skb
>> we use va + offset for virtual address of the frag, and pa + offset for
>> dma address of the frag. This reduces the number of calls to dma_map() by 
>> 1/3
>> for 9000 bytes and by 1/20 for 1500 bytes.
>> 
>> __alloc_page_frag is limited to max buffer size of PAGE_SIZE, i.e 4096 in 
>> most
>> of the cases. So 9k buffer allocation goes through kmalloc which return
>> page of order 2, 16k. We waste 7k bytes for every 9k buffer.
>
> The question I would have is do you actually need to have the 9k buffer? 
> Does the hardware support any sort of scatter-gather receive?  If so that 
> would be preferable as the 9k allocation per skb will have significant 
> overhead when you start receiving small packets.
>

enic hw has limited desc per rq (4096), and we can have only one dma block per
desc. Having sg/header-split will reduce the effective rq ring size for
large packets.

> A classic example is a TCP flow where you are only receiving a few hundred 
> bytes per frame.  You will take a huge truesize penalty for allocating a 9k 
> skb for a frame of only a few hundred bytes, though it sounds like you are 
> taking that hit already.
>

For this we have rx copybreak for pkts < 256 bytes.
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