Messages in this thread |  | | Date | Tue, 9 Jan 2001 15:27:02 +0000 | From | "Stephen C. Tweedie" <> | Subject | Re: [PLEASE-TESTME] Zerocopy networking patch, 2.4.0-1 |
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Hi,
On Tue, Jan 09, 2001 at 04:00:34PM +0100, Ingo Molnar wrote: > > On Tue, 9 Jan 2001, Stephen C. Tweedie wrote: > > we do have SLAB [which essentially caches structures, on a per-CPU basis] > which i did take into account, but still, initializing a 600+ byte kiovec > is probably more work than the rest of sending a packet! I mean i'd love > to eliminate the 200+ bytes skb initialization as well, it shows up.
Reusing a kiobuf for a request involves setting up the length, offset and maybe errno fields, and writing the struct page *'s into the maplist[]. Nothing more.
> > Bad bad bad. We already have SCSI devices optimised for bandwidth > > which don't approach decent performance until you are passing them 1MB > > IOs, [...] > > The fact that we're using single-page interfaces doesnt preclude us from > having nicely clustered requests, this is what IO-plugging is about!
We've already got measurements showing how insane this is. Raw IO requests, plus internal pagebuf contiguous requests from XFS, have to get broken down into page-sized chunks by the current ll_rw_block() API, only to get reassembled by the make_request code. It's *enormous* overhead, and the kiobuf-based disk IO code demonstrates this clearly.
We have already shown that the IO-plugging API sucks, I'm afraid.
> > and even in networking the 1.5K packet limit kills us in some cases > > and we need an interface capable of generating jumbograms. > > which cases?
Gig Ethernet, HIPPI... It's not so bad with an intelligent controller, admittedly.
> > but if you're doing udp jumbograms (or STP or VIA), you do need an > > interface which can give the networking stack more than one page at > > once. > > nothing prevents the introduction of specialized interfaces - if they feel > like they can get enough traction.
So you mean we'll introduce two separate APIs for general zero-copy, just to get around the problems in the single-page-based on?
> I was talking about the normal Linux IO > APIs, read()/write()/sendfile(), which are byte granularity and invoke an > almost mandatory buffering/clustering mechanizm in every kernel subsystem > they deal with.
Only tcp and ll_rw_block. ll_rw_block has already been fixed in the SGI patches, and gets _much_ better performance as a result. udp doesn't do any such clustering. That leaves tcp.
The presence of terrible performance in the old ll_rw_block code is NOT a good excuse for perpetuating that model.
Cheers, Stephen - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org Please read the FAQ at http://www.tux.org/lkml/
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