On Tue, 3 May 2022, rapier wrote: > Hey all, > > Hopefully this is an easy question. In channels.c what is c->remote_window > being used for? Does it correspond to c->local_window on the remote? Does it > indicate free space in a buffer? If so which one? I'm trying to nail down a > stalling situation. AFAIK it is used to record the most recent peer window value (i.e. sent via SSH2_MSG_CHANNEL_WINDOW_ADJUST) minus whatever data has been sent since then. It is used to avoid sending more than the peer's advertised window between receipt of SSH2_MSG_CHANNEL_WINDOW_ADJUST messages It therefore roughly corresponds to c->local_window on the peer, but only after you take into account data that is in-flight or sitting un-acked in the peer's receive buffer. I guess you'd call remote_window "the estimate of the remaining bytes available in their last-advertised window". > On the client I've put in a debug line into channel_input_window_adjust to > report on the size of the c->remote_window. I've noticed that when the > performance drops and seems to stall this value doesn't move much at all. So > I'm wondering if this represents a buffer on the remote reaching a maximum > size and waiting to drain. If you can similarly instrument the peer, then it might shed some light on it. Some possibilities: 1) heaps of data in flight (e.g. bufferbloat on intervening routers) 2) a peer with a full output buffer for the channel, e.g. if it was writing to a disk that is slower than the network. This will cause backpressure via c->local_consumed (see end of channel_handle_wfd()) 3) some stupid bug where we miss a case to update c->channel_handle_wfd or do the math wrong in channel_check_window() hope this helps! -d _______________________________________________ openssh-unix-dev mailing list openssh-unix-dev@xxxxxxxxxxx https://lists.mindrot.org/mailman/listinfo/openssh-unix-dev
