tcp: Recover from iob shortage with TCP_WRITE_BUFFERS #11406
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When CONFIG_NET_TCP_WRITE_BUFFERS is enabled, iobs are used for both queuing data from application, and for assembling packets for sending. If there is a system-wide shortage of iobs, it could happen that there is not enough free space to form any packets to send. The buffers allocated for TCP data also can't be released until the packet is sent. Normally this should be avoided by setting suitable values for CONFIG_IOB_NBUFFERS and CONFIG_IOB_THROTTLE. The default values are ok for light usage, but can run out when using multiple simultaneous TCP streams. Before this commit, iob shortage would cause TCP connections to get stuck and eventually timeout. With this change, TCP stack sends smaller packets, eventually freeing some buffers from the write queue.
xiaoxiang781216
approved these changes
Dec 16, 2023
xiaoxiang781216
approved these changes
Dec 16, 2023
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Summary
When
CONFIG_NET_TCP_WRITE_BUFFERSis enabled, iobs are used for both queuing data from application, and for assembling packets for sending. If there is a system-wide shortage of iobs, it could happen that there is not enough free space to form any packets to send. The buffers allocated for TCP data also can't be released until the packet is sent.Normally this should be avoided by setting suitable values for
CONFIG_IOB_NBUFFERSandCONFIG_IOB_THROTTLE. The default values are ok for light usage, but can run out when using multiple simultaneous TCP streams.Before this commit, iob shortage would cause TCP connections to get stuck and eventually timeout. With this change, TCP stack sends smaller packets, eventually freeing some buffers from the write queue.
Impact
In normal usage no effect. If system is running out of io buffers, TCP packet sizes will be smaller than maximum but will be sent sooner. If network warnings are enabled, notice is printed about this.
Testing
Problem first noticed on custom STM32F417 board when network performance testing with the default configuration:
Originally the TCP connection stopped transferring after a few tens of megabytes. Closing and reopening a new connection still worked. Debugger showed that there were 7 free buffers (1372 bytes), while
psock_send_eventhandler()was trying to send the MSS of 1440 bytes.devif_iob_send()then returnedENOMEMand same thing repeated next time around.With this change the connection recovered from buffer shortage and continued transferring.