Subject: Re: PATCH: kmalloc packet slab

From: Patrick McHardy <[email-masked]>
Date: Mon, 27 Dec 2004 18:17:32 +0100

Alan Cox wrote:
> The networking world runs in 1514 byte packets pretty much all the time.
> This adds a 1620 byte slab for such objects and is one of the internally
> generated Red Hat patches we use on things like Fedora Core 3. Original:
> Arjan van de Ven.
> 
> Signed-off-by: Alan Cox <[email-masked]>

Why 1620 bytes ? Most drivers allocate packet_size + 2 bytes.
dev_alloc_skb adds another 16 bytes, finally alloc_skb adds
sizeof(struct skb_shared_info). So we get:

(32bit): 1514b + 2b + 16b + 160b = 1692b
(64bit): 1514b + 2b + 16b + 312b = 1844b

On paths using alloc_skb instead of dev_alloc_skb it's 16 bytes
less, but 1620 bytes is still too small for full-sized packets.

Regards
Patrick