On Thu, Feb 05, 2009 at 11:52:58PM -0800, David Miller wrote:

quoted text
> From: Jarek Poplawski

> Date: Tue, 3 Feb 2009 09:41:08 +0000

>

> > Yes, this looks reasonable. On the other hand, I think it would be

> > nice to get some opinions of slab folks (incl. Evgeniy) on the expected

> > efficiency of such a solution. (It seems releasing with put_page() will

> > always have some cost with delayed reusing and/or waste of space.)

>

> I think we can't avoid using carved up pages for skb->data in the end.

> The whole kernel wants to speak in pages and be able to grab and

> release them in one way and one way only (get_page() and put_page()).

>

> What do you think is more likely?  Us teaching the whole entire kernel

> how to hold onto SKB linear data buffers, or the networking fixing

> itself to operate on pages for it's header metadata? :-)

This idea looks very reasonable, except I wander why nobody else

didn't need this kind of mm interface. Another question is it seems

many mechanisms like fast searching, defragmentation etc. could be

reused.
> What we'll end up with is likely a hybrid scheme.  High speed devices

quoted text
> will receive into pages.  And also the skb->data area will be page

> backed and held using get_page()/put_page() references.

>

> It is not even worth optimizing for skb->data holding the entire

> packet, that's not the case that matters.

>

> These skb->data areas will thus be 128 bytes plus the skb_shinfo

> structure blob.  They also will be recycled often, rather than held

> onto for long periods of time.

Looks fine, except: you mentioned dumb NICs, which would need this

page space on receive, anyway. BTW, don't they need this on transmit

again?
> In fact we can optimize that even further in many ways, for example by

quoted text
> dropping the skb->data backed memory once the skb is queued to the

> socket receive buffer.  That will make skb->data buffer lifetimes

> miniscule even under heavy receive load.

>

> In that kind of situation, doing even the most stupidest page slicing

> algorithm, similar to what we do now with sk->sk_sndmsg_page, is

> more than adequate and things like NTA (purely to solve this problem)

> is overengineering.

Hmm... I don't get it. It seems these slabs do a lot of advanced work,

and still some people like Evgeniy or Nick thought it's not enough,

and even found it worth of their time to rework this.
There is also a question of memory accounting: do you think admins

don't care if we give away say 25% additionally?
Jarek P.

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