Messages in this thread | | | Date | Fri, 6 Feb 2009 09:10:34 +0000 | From | Jarek Poplawski <> | Subject | Re: [PATCH v2] tcp: splice as many packets as possible at once |
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On Thu, Feb 05, 2009 at 11:52:58PM -0800, David Miller wrote: > From: Jarek Poplawski <jarkao2@gmail.com> > 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 > 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 > 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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