On 11/11/2020 16:49, Victor Stewart wrote: > On Wed, Nov 11, 2020 at 1:00 AM Pavel Begunkov <asml.silence@xxxxxxxxx> wrote: >> On 11/11/2020 00:07, Victor Stewart wrote: >>> On Tue, Nov 10, 2020 at 11:26 PM Pavel Begunkov <asml.silence@xxxxxxxxx> wrote: >>>>> NIC ACKs, instead of finding the socket's error queue and putting the >>>>> completion there like MSG_ZEROCOPY, the kernel would find the io_uring >>>>> instance the socket is registered to and call into an io_uring >>>>> sendmsg_zerocopy_completion function. Then the cqe would get pushed >>>>> onto the completion queue.> >>>>> the "recvmsg zerocopy" is straight forward enough. mimicking >>>>> TCP_ZEROCOPY_RECEIVE, i'll go into specifics next time. >>>> >>>> Receive side is inherently messed up. IIRC, TCP_ZEROCOPY_RECEIVE just >>>> maps skbuffs into userspace, and in general unless there is a better >>>> suited protocol (e.g. infiniband with richier src/dst tagging) or a very >>>> very smart NIC, "true zerocopy" is not possible without breaking >>>> multiplexing. >>>> >>>> For registered buffers you still need to copy skbuff, at least because >>>> of security implications. >>> >>> we can actually just force those buffers to be mmap-ed, and then when >>> packets arrive use vm_insert_pin or remap_pfn_range to change the >>> physical pages backing the virtual memory pages submmited for reading >>> via msg_iov. so it's transparent to userspace but still zerocopy. >>> (might require the user to notify io_uring when reading is >>> completed... but no matter). >> >> Yes, with io_uring zerocopy-recv may be done better than >> TCP_ZEROCOPY_RECEIVE but >> 1) it's still a remap. Yes, zerocopy, but not ideal >> 2) won't work with registered buffers, which is basically a set >> of pinned pages that have a userspace mapping. After such remap >> that mapping wouldn't be in sync and that gets messy. > > well unless we can alleviate all copies, then there isn’t any point > because it isn’t zerocopy. > > so in my server, i have a ceiling on the number of clients, > preallocate them, and mmap anonymous noreserve read + write buffers > for each. > > so say, 150,000 clients x (2MB * 2). which is 585GB. way more than the > physical memory of my machine. (and have 10 instance of it per > machine, so ~6TB lol). but at any one time probably 0.01% of that > memory is in usage. and i just MADV_COLD the pages after consumption. > > this provides a persistent “vmem contiguous” stream buffer per client. > which has a litany of benefits. but if we persistently pin pages, this > ceases to work, because pin pages require persistent physical memory > backing pages. > > But on the send side, if you don’t pin persistently, you’d have to pin > on demand, which costs more than it’s worth for sends less than ~10KB. having it non-contiguous and do round-robin IMHO would be a better shot > And I guess there’s no way to avoid pinning and maintain kernel > integrity. Maybe we could erase those userspace -> physical page > mappings, then recreate them once the operation completes, but 1) that > would require page aligned sends so that you could keep writing and > sending while you waited for completions and 2) beyond being > nonstandard and possibly unsafe, who says that would even cost less > than pinning, definitely costs something. Might cost more because > you’d have to get locks to the page table? > > So essentially on the send side the only way to zerocopy for free is > to persistently pin (and give up my per client stream buffers). > > On the receive side actually the only way to realistically do zerocopy > is to somehow pin a NIC RX queue to a process, and then persistently > map the queue into the process’s memory as read only. That’s a > security absurdly in the general case, but it could be root only > usage. Then you’d recvmsg with a NULL msg_iov[0].iov_base, and have > the packet buffer location and length written in. Might require driver > buy-in, so might be impractical, but unsure. https://blogs.oracle.com/linux/zero-copy-networking-in-uek6 scroll to AF_XDP > > Otherwise the only option is an even worse nightmare how > TCP_ZEROCOPY_RECEIVE works, and ridiculously impractical for general > purpose… Well, that's not so bad, API with io_uring might be much better, but still would require unmap. However, depending on a use case overhead for small packets and/or shared b/w many thread mm can potentially be a deal breaker. > “Mapping of memory into a process's address space is done on a > per-page granularity; there is no way to map a fraction of a page. So > inbound network data must be both page-aligned and page-sized when it > ends up in the receive buffer, or it will not be possible to map it > into user space. Alignment can be a bit tricky because the packets > coming out of the interface start with the protocol headers, not the > data the receiving process is interested in. It is the data that must > be aligned, not the headers. Achieving this alignment is possible, but > it requires cooperation from the network interface should support scatter-gather in other words > > It is also necessary to ensure that the data arrives in chunks that > are a multiple of the system's page size, or partial pages of data > will result. That can be done by setting the maximum transfer unit > (MTU) size properly on the interface. That, in turn, can require > knowledge of exactly what the incoming packets will look like; in a > test program posted with the patch set, Dumazet sets the MTU to > 61,512. That turns out to be space for fifteen 4096-byte pages of > data, plus 40 bytes for the IPv6 header and 32 bytes for the TCP > header.” > > https://lwn.net/Articles/752188/ > > Either receive case also makes my persistent per client stream buffer > zerocopy impossible lol. it depends > > in short, zerocopy sendmsg with persistently pinned buffers is > definitely possible and we should do that. (I'll just make it work on > my end). > > recvmsg i'll have to do more research into the practicality of what I > proposed above. 1. NIC is smart enough and can locate the end (userspace) buffer and DMA there directly. That requires parsing TCP/UDP headers, etc., or having a more versatile API like infiniband. + extra NIC features. 2. map skbufs into the userspace as TCP_ZEROCOPY_RECEIVE does. -- Pavel Begunkov
