On Sun, Aug 31, 2014 at 06:42:38PM -0700, Andy Lutomirski wrote: > On Sun, Aug 31, 2014 at 5:58 PM, Rusty Russell <rusty@xxxxxxxxxxxxxxx> wrote: > > Andy Lutomirski <luto@xxxxxxxxxxxxxx> writes: > >> The only unusual thing about virtio's use of scatterlists is that > >> two of the APIs accept scatterlists that might not be terminated. > >> Using function pointers to handle this case is overkill; for_each_sg > >> can do it. > >> > >> There's a small subtlely here: for_each_sg assumes that the provided > >> count is correct, but, because of the way that virtio_ring handles > >> multiple scatterlists at once, the count is only an upper bound if > >> there is more than one scatterlist. This is easily solved by > >> checking the sg pointer for NULL on each iteration. > > > > Hi Andy, > > > > (Sorry for the delayed response; I was still catching up from > > my week at KS.) > > No problem. In the grand scheme of maintainer response time, I think > you're near the top. :) > > > > > Unfortunately the reason we dance through so many hoops here is that > > it has a measurable performance impact :( Those indirect calls get inlined. > > gcc inlines that? That must nearly double the size of the object file. :-/ > > > > > There's only one place which actually uses a weirdly-formed sg now, > > and that's virtio_net. It's pretty trivial to fix. This path in virtio net is also unused on modern hypervisors, so we probably don't care how well does it perform: why not apply it anyway? It's the virtio_ring changes that we need to worry about. > > However, vring_bench drops 15% when we do this. There's a larger > > question as to how much difference that makes in Real Life, of course. > > I'll measure that today. > > Weird. sg_next shouldn't be nearly that slow. Weird. I think that's down to the fact that it's out of line, so it prevents inlining of the caller. > > > > Here are my two patches, back-to-back (it cam out of of an earlier > > concern about reducing stack usage, hence the stack measurements). > > > > I like your version better than mine, except that I suspect that your > version will blow up for the same reason that my v2 patches blow up: > you probably need the skb_to_sgvec_nomark fix, too. > > IOW, what happens if you apply patches 1-4 from my v3 series and then > apply your patches on top of that? > > There'll be a hit on some virtio_pci machines due to use of the DMA > API. I would argue that, if this is measurable, the right fix is to > prod the DMA API maintainers, whoever they are, to fix it. The DMA > API really out to be very fast on identity-mapped devices, but I don't > know whether it is in practice. > > --Andy Right but we'd have to fix that before applying the patches to avoid performance regressions. -- MST _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
