netdev - Re: TCP transmit performance regression

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Date:	Tue, 10 Jul 2012 22:22:45 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Network Development <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>
Subject: Re: TCP transmit performance regression

On Tue, Jul 10, 2012 at 10:02 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> I am kind of annoyed you sent on netdev a copy of a _private_ mail.

I am sure that your reply which includes below is not from a private mail:

       Only because skbs were fat (8KB allocated/truesize, for a single
       1500 bytes frame)

>
> Next time, make sure you dont do that without my consent.

OK

> On Tue, 2012-07-10 at 21:37 +0800, Ming Lei wrote:
>
>> Could you explain why the truesize of SKB is 8KB for single
>> 1500bytes frame?
>>
>
> Because the driver uses skb_alloc(4096) for example ?
>
> I don't know, you don't tell us the driver.
>
>
> Goal is to have skb->head points to a 2048 bytes area, so truesize
> should be 2048 + sizeof(sk_buff)  (including struct shared_info)
>
>> I observed it is 2560bytes for RX SKBs inside asix_rx_fixup with
>> rx_urb_size of 2048 on beagle-xm.
>>
>
> Thats because using 2048 bytes for the urb buffer (excluding
> shared_info) means you need :
>
> 2048 + sizeof(struct shared_info) + sizeof(sk_buff) = 2560
>
> In fact 2048 + sizeof(struct shared_info) means a full 4096 area is
> used.
>
> You have 2560 on recent kernels because the way netdev_alloc_frag()
> works.
>
> Thats why copybreak can actually saves ram. Since it is adding a copy,
> we try to use it only on slow devices.

Looks single page allocation won't put too much pressure on MM, that is
why I suggested to avoid copy if the skb buffer size is less or equal one
page. Anyway, unnecessary copy will increase computation and consume power.


Thanks,
-- 
Ming Lei
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