Re: TCP transmit performance regression

Eric Dumazet
2012-07-10T14:02:16Z

I am kind of annoyed you sent on netdev a copy of a _private_ mail.

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

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.