[lwip-devel] [task #7040] Work on tcp_enqueue

[Jakob Stoklund Olesen]

Follow-up Comment #15, task #7040 (project lwip):

Attached patch tcp-oversize applies on top of tcp-enqueue-split. Please
disregard the earlier tcp-enqueue-concat.

Implement the TCP_OVERSIZE setting
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When relevant, tcp_enqueue_data will allocate pbufs that are larger than
strictly necessary. Data from following writes is copied into the oversized
pbuf before new pbufs are allocated. The TCP_OVERSIZE setting controls the
maximum overallocation. It ranges from 0 to TCP_MSS.

Oversized allocation has the following advantages:

- Pbuf lengths are aligned to help picky DMA engines.
- Pbuf chain length and snd_queuelen can be controlled when using many small
writes.
- Pbuf vs data overhead can be controlled.
- Unfragmented TCP packets can be created for interfaces that can DMA but no
scatter-gather.

The disadvantage is a slightly higher RAM usage:

- struct tcp_pcb has an extra 16-bit field (unsent_oversize). This overhead
disappears due to alignment on 32-bit platforms.
- Unused overallocation in transmitted pbufs is wasted. This typically
happens in the last segment produced by a burst of writes.

To control the extra memory usage, tcp_enqueue_data will not allocate an
oversized pbuf if it thinks the segment will be transmitted immediately. The
heuristic is not perfect, but it works very well for slow stop-and-go
protocols like telnet.

Setting the TF_NODELAY flag on a pcb will disable oversizing unless
TCP_WRITE_FLAG_MORE is passed to tcp_write.



(file #17402)
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Additional Item Attachment:

File name: tcp-oversize                   Size:15 KB


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