Hello team, Recently I've been using NuttX on a SAMv7 based board with Ethernet. I've tried to use the FTP server for remote access to the SD card. During implementation I faced https://github.com/apache/incubator-nuttx/issues/5973 and did some further analysis.
When CONFIG_NET_TCP_WRITE_BUFFERS is enabled the TCP/IP stack tries to allocate buffers from the IOB pool. There is a throttle mechanism applied to send, but not recv part of TCP/IP. The situation that happens is the following: The host is sending files via TCP stream and the SDcard store speed is slower than TCP send rate. At some point TCP zero window is reached and the upload data rate is limited by device TCP recv -> SDcard store rate. At some point of time the system runs out of IOBs (actually I have CONFIG_IOB_THROTTLE=8, so 8 IOB buffers are still available) and the device wants to send the FTP response message. At this point the deadlock happens. The send can't allocate the buffer because all buffers are used for TCP readhead to buffer incoming data that are not read because the FTP state machine is blocked on send and can't reach the recv point (snake eating its own tail). I was able to overcome this situation by limiting the RX side of the socket with configuring CONFIG_NET_RECV_BUFSIZE. Also I have "CONFIG_SYSLOG_BUFFER=y", so in some cases not only the network subsystem hangs, but the rest of the system is affected because . But I want to highlight the essential problems of the current TCP buffered design: 1. The TCP buffered uses the same pool for RX and TX packets so excessive usage of IOBs at RX side is affecting TX (and also is shared with other parts of the system so other IOB users are affected). 2. The system can't recover even if the TCP connection is closed because the IOB wait allocation process does not know anything about the socket state and does not have a reliable way of waiter interruption to check if connection is still valid or not. I would like to get some thoughts about how to redesign the TCP buffered mode to get more reliable operation. Best regards, Petro
