Re: ixg(4) performances

[Thor Lancelot Simon <tls%panix.com@localhost>]

On Tue, Aug 26, 2014 at 07:03:06PM -0700, Jonathan Stone wrote:
> Thor,
> 
> The NetBSD  TCP stack can't handle 8K payload by page-flipping the payload 
> and prepending an mbuf for XDR/NFS/TCP/IP headers? Or is the issue the extra 
> page-mapping for the prepended mbuf?

The issue is allocating the extra page for a milligram of data.  It is almost
always a lose.  Better to choose the MTU so that the whole packet fits neatly
in 8192 bytes.

It is helpful to understand where MTU 9000 came from: SGI was trying to
optimise UDP NFS performance, for NFSv2 with 8K maximum RPC size, on
systems that had 16K pages.  You can't fit two of that kind of NFS request
in a 16K page, so you might as well allocate something a little bigger than
8K but that happens to leave your memory allocator some useful-sized chunks
to hand out to other callers.

I am a little hazy on the details, but I believe they ended up at MTU 9024
which is 8K + 768 + 64 (leaving a bunch of handy power-of-2 split sizes
as residuals: 4096 + 2048 + 1024 + 128 + 64) which just made no sense to
anyone else so everyone _else_ picked random sizes around 9000 that happened
to work for their hardware.  But at the end of the day, if you do not have
16K pages or are not optimizing for 8K NFSv2 requests on UDP, an MTU that
fits in 8K is almost always better.

Thor