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
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