This file documents how to use memory mapped I/O with netlink. Author: Patrick McHardy Overview -------- Memory mapped netlink I/O can be used to increase throughput and decrease overhead of unicast receive and transmit operations. Some netlink subsystems require high throughput, these are mainly the netfilter subsystems nfnetlink_queue and nfnetlink_log, but it can also help speed up large dump operations of f.i. the routing database. Memory mapped netlink I/O used two circular ring buffers for RX and TX which are mapped into the processes address space. The RX ring is used by the kernel to directly construct netlink messages into user-space memory without copying them as done with regular socket I/O, additionally as long as the ring contains messages no recvmsg() or poll() syscalls have to be issued by user-space to get more message. The TX ring is used to process messages directly from user-space memory, the kernel processes all messages contained in the ring using a single sendmsg() call. Usage overview -------------- In order to use memory mapped netlink I/O, user-space needs three main changes: - ring setup - conversion of the RX path to get messages from the ring instead of recvmsg() - conversion of the TX path to construct messages into the ring Ring setup is done using setsockopt() to provide the ring parameters to the kernel, then a call to mmap() to map the ring into the processes address space: - setsockopt(fd, SOL_NETLINK, NETLINK_RX_RING, ¶ms, sizeof(params)); - setsockopt(fd, SOL_NETLINK, NETLINK_TX_RING, ¶ms, sizeof(params)); - ring = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0) Usage of either ring is optional, but even if only the RX ring is used the mapping still needs to be writable in order to update the frame status after processing. Conversion of the reception path involves calling poll() on the file descriptor, once the socket is readable the frames from the ring are processsed in order until no more messages are available, as indicated by a status word in the frame header. On kernel side, in order to make use of memory mapped I/O on receive, the originating netlink subsystem needs to support memory mapped I/O, otherwise it will use an allocated socket buffer as usual and the contents will be copied to the ring on transmission, nullifying most of the performance gains. Dumps of kernel databases automatically support memory mapped I/O. Conversion of the transmit path involves changing message contruction to use memory from the TX ring instead of (usually) a buffer declared on the stack and setting up the frame header approriately. Optionally poll() can be used to wait for free frames in the TX ring. Structured and definitions for using memory mapped I/O are contained in . RX and TX rings ---------------- Each ring contains a number of continous memory blocks, containing frames of fixed size dependant on the parameters used for ring setup. Ring: [ block 0 ] [ frame 0 ] [ frame 1 ] [ block 1 ] [ frame 2 ] [ frame 3 ] ... [ block n ] [ frame 2 * n ] [ frame 2 * n + 1 ] The blocks are only visible to the kernel, from the point of view of user-space the ring just contains the frames in a continous memory zone. The ring parameters used for setting up the ring are defined as follows: struct nl_mmap_req { unsigned int nm_block_size; unsigned int nm_block_nr; unsigned int nm_frame_size; unsigned int nm_frame_nr; }; Frames are grouped into blocks, where each block is a continous region of memory and holds nm_block_size / nm_frame_size frames. The total number of frames in the ring is nm_frame_nr. The following invariants hold: - frames_per_block = nm_block_size / nm_frame_size - nm_frame_nr = frames_per_block * nm_block_nr Some parameters are constrained, specifically: - nm_block_size must be a multiple of the architectures memory page size. The getpagesize() function can be used to get the page size. - nm_frame_size must be equal or larger to NL_MMAP_HDRLEN, IOW a frame must be able to hold at least the frame header - nm_frame_size must be smaller or equal to nm_block_size - nm_frame_size must be a multiple of NL_MMAP_MSG_ALIGNMENT - nm_frame_nr must equal the actual number of frames as specified above. When the kernel can't allocate phsyically continous memory for a ring block, it will fall back to use physically discontinous memory. This might affect performance negatively, in order to avoid this the nm_frame_size parameter should be chosen to be as small as possible for the required frame size and the number of blocks should be increased instead. Ring frames ------------ Each frames contain a frame header, consisting of a synchronization word and some meta-data, and the message itself. Frame: [ header message ] The frame header is defined as follows: struct nl_mmap_hdr { unsigned int nm_status; unsigned int nm_len; __u32 nm_group; /* credentials */ __u32 nm_pid; __u32 nm_uid; __u32 nm_gid; }; - nm_status is used for synchronizing processing between the kernel and user- space and specifies ownership of the frame as well as the operation to perform - nm_len contains the length of the message contained in the data area - nm_group specified the destination multicast group of message - nm_pid, nm_uid and nm_gid contain the netlink pid, UID and GID of the sending process. These values correspond to the data available using SOCK_PASSCRED in the SCM_CREDENTIALS cmsg. The possible values in the status word are: - NL_MMAP_STATUS_UNUSED: RX ring: frame belongs to the kernel and contains no message for user-space. Approriate action is to invoke poll() to wait for new messages. TX ring: frame belongs to user-space and can be used for message construction. - NL_MMAP_STATUS_RESERVED: RX ring only: frame is currently used by the kernel for message construction and contains no valid message yet. Appropriate action is to invoke poll() to wait for new messages. - NL_MMAP_STATUS_VALID: RX ring: frame contains a valid message. Approriate action is to process the message and release the frame back to the kernel by setting the status to NL_MMAP_STATUS_UNUSED or queue the frame by setting the status to NL_MMAP_STATUS_SKIP. TX ring: the frame contains a valid message from user-space to be processed by the kernel. After completing processing the kernel will release the frame back to user-space by setting the status to NL_MMAP_STATUS_UNUSED. - NL_MMAP_STATUS_COPY: RX ring only: a message is ready to be processed but could not be stored in the ring, either because it exceeded the frame size or because the originating subsystem does not support memory mapped I/O. Appropriate action is to invoke recvmsg() to receive the message and release the frame back to the kernel by setting the status to NL_MMAP_STATUS_UNUSED. - NL_MMAP_STATUS_SKIP: RX ring only: user-space queued the message for later processing, but processed some messages following it in the ring. The kernel should skip this frame when looking for unused frames. The data area of a frame begins at a offset of NL_MMAP_HDRLEN relative to the frame header. TX limitations -------------- Kernel processing usually involves validation of the message received by user-space, then processing its contents. The kernel must assure that userspace is not able to modify the message contents after they have been validated. In order to do so, the message is copied from the ring frame to an allocated buffer if either of these conditions is false: - only a single mapping of the ring exists - the file descriptor is not shared between processes This means that for threaded programs, the kernel will fall back to copying. Example ------- Ring setup: unsigned int block_size = 16 * getpagesize(); struct nl_mmap_req req = { .nm_block_size = block_size, .nm_block_nr = 64, .nm_frame_size = 16384, .nm_frame_nr = 64 * block_size / 16384, }; unsigned int ring_size; void *rx_ring, *tx_ring; /* Configure ring parameters */ if (setsockopt(fd, NETLINK_RX_RING, &req, sizeof(req)) < 0) exit(1); if (setsockopt(fd, NETLINK_TX_RING, &req, sizeof(req)) < 0) exit(1) /* Calculate size of each invididual ring */ ring_size = req.nm_block_nr * req.nm_block_size; /* Map RX/TX rings. The TX ring is located after the RX ring */ rx_ring = mmap(NULL, 2 * ring_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0); if ((long)rx_ring == -1L) exit(1); tx_ring = rx_ring + ring_size: Message reception: This example assumes some ring parameters of the ring setup are available. unsigned int frame_offset = 0; struct nl_mmap_hdr *hdr; struct nlmsghdr *nlh; unsigned char buf[16384]; ssize_t len; while (1) { struct pollfd pfds[1]; pfds[0].fd = fd; pfds[0].events = POLLIN | POLLERR; pfds[0].revents = 0; if (poll(pfds, 1, -1) < 0 && errno != -EINTR) exit(1); /* Check for errors. Error handling omitted */ if (pfds[0].revents & POLLERR) /* If no new messages, poll again */ if (!(pfds[0].revents & POLLIN)) continue; /* Process all frames */ while (1) { /* Get next frame header */ hdr = rx_ring + frame_offset; if (hdr->nm_status == NL_MMAP_STATUS_VALID) /* Regular memory mapped frame */ nlh = (void *hdr) + NL_MMAP_HDRLEN; len = hdr->nm_len; /* Release empty message immediately. May happen * on error during message construction. */ if (len == 0) goto release; } else if (hdr->nm_status == NL_MMAP_STATUS_COPY) { /* Frame queued to socket receive queue */ len = recv(fd, buf, sizeof(buf), MSG_DONTWAIT); if (len <= 0) break; nlh = buf; } else /* No more messages to process, continue polling */ break; process_msg(nlh); release: /* Release frame back to the kernel */ hdr->nm_status = NL_MMAP_STATUS_UNUSED; /* Advance frame offset to next frame */ frame_offset = (frame_offset + frame_size) % ring_size; } } Message transmission: This example assumes some ring parameters of the ring setup are available. A single message is constructed and transmitted, to send multiple messages at once they would be constructed in consecutive frames before a final call to sendto(). unsigned int frame_offset = 0; struct nl_mmap_hdr *hdr; struct nlmsghdr *nlh; struct sockaddr_nl addr = { .nl_family = AF_NETLINK, }; hdr = tx_ring + frame_offset; if (hdr->nm_status != NL_MMAP_STATUS_UNUSED) /* No frame available. Use poll() to avoid. */ exit(1); nlh = (void *)hdr + NL_MMAP_HDRLEN; /* Build message */ build_message(nlh); /* Fill frame header: length and status need to be set */ hdr->nm_len = nlh->nlmsg_len; hdr->nm_status = NL_MMAP_STATUS_VALID; if (sendto(fd, NULL, 0, 0, &addr, sizeof(addr)) < 0) exit(1); /* Advance frame offset to next frame */ frame_offset = (frame_offset + frame_size) % ring_size;