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leandromoreira / linux-network-performance-parameters

Learn where some of the network sysctl variables fit into the Linux/Kernel network flow. Translations: 🇷🇺
https://github.com/leandromoreira/linux-network-performance-parameters
BSD 3-Clause "New" or "Revised" License
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kernel linux network performance sysctl tcp

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🇷🇺

TOC

Introduction

Sometimes people are looking for sysctl cargo cult values that bring high throughput and low latency with no trade-off and that works on every occasion. That's not realistic, although we can say that the newer kernel versions are very well tuned by default. In fact, you might hurt performance if you mess with the defaults.

This brief tutorial shows where some of the most used and quoted sysctl/network parameters are located into the Linux network flow, it was heavily inspired by the illustrated guide to Linux networking stack and many of Marek Majkowski's posts.

Feel free to send corrections and suggestions! :)

Linux network queues overview

linux network queues

Fitting the sysctl variables into the Linux network flow

Ingress - they're coming

  1. Packets arrive at the NIC
  2. NIC will verify MAC (if not on promiscuous mode) and FCS and decide to drop or to continue
  3. NIC will DMA packets at RAM, in a region previously prepared (mapped) by the driver
  4. NIC will enqueue references to the packets at receive ring buffer queue rx until rx-usecs timeout or rx-frames
  5. NIC will raise a hard IRQ
  6. CPU will run the IRQ handler that runs the driver's code
  7. Driver will schedule a NAPI, clear the hard IRQ and return
  8. Driver raise a soft IRQ (NET_RX_SOFTIRQ)
  9. NAPI will poll data from the receive ring buffer until netdev_budget_usecs timeout or netdev_budget and dev_weight packets
  10. Linux will also allocate memory to sk_buff
  11. Linux fills the metadata: protocol, interface, setmacheader, removes ethernet
  12. Linux will pass the skb to the kernel stack (netif_receive_skb)
  13. It will set the network header, clone skb to taps (i.e. tcpdump) and pass it to tc ingress
  14. Packets are handled to a qdisc sized netdev_max_backlog with its algorithm defined by default_qdisc
  15. It calls ip_rcv and packets are handed to IP
  16. It calls netfilter (PREROUTING)
  17. It looks at the routing table, if forwarding or local
  18. If it's local it calls netfilter (LOCAL_IN)
  19. It calls the L4 protocol (for instance tcp_v4_rcv)
  20. It finds the right socket
  21. It goes to the tcp finite state machine
  22. Enqueue the packet to the receive buffer and sized as tcp_rmem rules
    1. If tcp_moderate_rcvbuf is enabled kernel will auto-tune the receive buffer
  23. Kernel will signalize that there is data available to apps (epoll or any polling system)
  24. Application wakes up and reads the data

Egress - they're leaving

  1. Application sends message (sendmsg or other)
  2. TCP send message allocates skb_buff
  3. It enqueues skb to the socket write buffer of tcp_wmem size
  4. Builds the TCP header (src and dst port, checksum)
  5. Calls L3 handler (in this case ipv4 on tcp_write_xmit and tcp_transmit_skb)
  6. L3 (ip_queue_xmit) does its work: build ip header and call netfilter (LOCAL_OUT)
  7. Calls output route action
  8. Calls netfilter (POST_ROUTING)
  9. Fragment the packet (ip_output)
  10. Calls L2 send function (dev_queue_xmit)
  11. Feeds the output (QDisc) queue of txqueuelen length with its algorithm default_qdisc
  12. The driver code enqueue the packets at the ring buffer tx
  13. The driver will do a soft IRQ (NET_TX_SOFTIRQ) after tx-usecs timeout or tx-frames
  14. Re-enable hard IRQ to NIC
  15. Driver will map all the packets (to be sent) to some DMA'ed region
  16. NIC fetches the packets (via DMA) from RAM to transmit
  17. After the transmission NIC will raise a hard IRQ to signal its completion
  18. The driver will handle this IRQ (turn it off)
  19. And schedule (soft IRQ) the NAPI poll system
  20. NAPI will handle the receive packets signaling and free the RAM

How to check - perf

If you want to see the network tracing within Linux you can use perf.

docker run -it --rm --cap-add SYS_ADMIN --entrypoint bash ljishen/perf
apt-get update
apt-get install iputils-ping

# this is going to trace all events (not syscalls) to the subsystem net:* while performing the ping
perf trace --no-syscalls --event 'net:*' ping globo.com -c1 > /dev/null

perf trace network

What, Why and How - network and sysctl parameters

Ring Buffer - rx,tx

Interrupt Coalescence (IC) - rx-usecs, tx-usecs, rx-frames, tx-frames (hardware IRQ)

Interrupt Coalescing (soft IRQ) and Ingress QDisc

Egress QDisc - txqueuelen and default_qdisc

TCP Read and Write Buffers/Queues

The policy that defines what is memory pressure is specified at tcp_mem and tcp_moderate_rcvbuf.

Honorable mentions - TCP FSM and congestion algorithm

Accept and SYN Queues are governed by net.core.somaxconn and net.ipv4.tcp_max_syn_backlog. Nowadays net.core.somaxconn caps both queue sizes.

How to monitor:

tcp finite state machine Source: https://commons.wikimedia.org/wiki/File:Tcp_state_diagram_fixed_new.svg

Network tools for testing and monitoring

References