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Re: pty(4) 1024 bytes buffer limit
On Fri, Sep 09, 2011 at 05:11:17PM -0700, Erik Fair wrote:
> ... unless they're using jumbo frames. Potentially 9Kbytes, depending upon
> NICs and switches.
...which reminds me that someday, I will eventually finish my "9k MTU
demonstrated harmful" informational RFC.
The original research which seemed to show a benefit for 9K MTU was done
with NFS over UDP (so IP layer fragmentation instead of TCP layer
segmentation) on systems with a 16K page size and a page-based memory
coherency algorithm for multiple processors. Guess what the NFS RPC
size limit was set to? Right, 8K. If they'd used 16K there, they would
have concluded they needed a 17K MTU.
In practice, large MTU helps, particularly for receive (modern adapters
do TCP segmentation offload on send so you get the whole efficiency
benefit of a large MTU for your stack, and then some), but 9K is a very
bad choice of size: on most systems it means you allocate 4K three times
and waste the last 3K of it. The FDDI MTU of 4K would have been a much
better choice; for some applications 8K-plus-headers is good too.
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