On Mon, 1 Oct 2018 12:56:58 +0300 Ilias Apalodimas <ilias.apalodi...@linaro.org> wrote:
> > > #2: You have allocations on the XDP fast-path. > > > > > > The REAL secret behind the XDP performance is to avoid allocations on > > > the fast-path. While I just told you to use the page-allocator and > > > order-0 pages, this will actually kill performance. Thus, to make this > > > fast, you need a driver local recycle scheme that avoids going through > > > the page allocator, which makes XDP_DROP and XDP_TX extremely fast. > > > For the XDP_REDIRECT action (which you seems to be interested in, as > > > this is needed for AF_XDP), there is a xdp_return_frame() API that can > > > make this fast. > > > > I had an initial implementation that did exactly that (that's why you the > > dma_sync_single_for_cpu() -> dma_unmap_single_attrs() is there). In the > > case > > of AF_XDP isn't that introducing a 'bottleneck' though? I mean you'll feed > > fresh > > buffers back to the hardware only when your packets have been processed from > > your userspace application > > Just a clarification here. This is the case if ZC is implemented. In my case > the buffers will be 'ok' to be passed back to the hardware once the use > userspace payload has been copied by xdp_do_redirect() Thanks for clarifying. But no, this is not introducing a 'bottleneck' for AF_XDP. For (1) the copy-mode-AF_XDP the frame (as you noticed) is "freed" or "returned" very quickly after it is copied. The code is a bit hard to follow, but in __xsk_rcv() it calls xdp_return_buff() after the memcpy. Thus, the frame can be kept DMA mapped and reused in RX-ring quickly. For (2) the zero-copy-AF_XDP, then you need to implement a new allocator of type MEM_TYPE_ZERO_COPY. The performance trick here is that all DMA-map/unmap and allocations go away, given everything is preallocated by userspace. Through the 4 rings (SPSC) are used for recycling the ZC-umem frames (read Documentation/networking/af_xdp.rst). -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat LinkedIn: http://www.linkedin.com/in/brouer
