Re: TCP, and clamping MSS..?
Re: TCP, and clamping MSS..?
- Subject: Re: TCP, and clamping MSS..?
- From: John Heffner <email@hidden>
- Date: Thu, 11 Jan 2007 17:39:59 -0500
Andrew Gallatin wrote:
Vincent Lubet writes:
> Andrew,
>
> This is an optimization that is simple missin in xnu -- thanks for
> bringing it to our attention.
>
> Feel free to file an enhancement request at http://
> bugreporter.apple.com/ to make sure this won't fall through the cracks.
>
Thanks Vince.. I've filed problem id 4919145.
FWIW, it looks like the MSS clamping code was removed sometime between
10.2 and 10.3. I've never really understood why it helps so much.
This isn't just my NIC that it helps for. See the thread starting at
http://lists.freebsd.org/pipermail/freebsd-net/2004-January/002594.html
where I actually proposed removing it, and found that it really did
help!
Do you have any intuition on why keeping things nicely aligned helps
so much? Linux, on the same hardware, does not need to clamp the MTU,
and easily maxes out the 10GbE link with far lower CPU utilization.
Is the BSD stack just that much more sensitive to alignment?
FWIW, Linux does exhibits this effect, too, though you probably won't
see it if it's doing TSO. I observed exactly this phenomenon on a
Pentium 3 back in the days when GigE was fast. I'm not up on the BSD
kernel, but in Linux, when using devices that support scatter/gather
DMA, packets are built as chains of pages. There's a certain amount of
allocation/copy overhead per page, so you get the best performance when
you just fill up 2 pages, and get a bit of a dip once you need 3. (This
might not be the same effect as in BSD, since it's using a constant
that's not page size...)
If it goes back in, maybe it should have a sysctl? It's not clear this
optimization is always the right thing to do. There's a trade-off
between CPU efficiency and network efficiency which is a moving target
depending on the relative speeds of the CPU and NIC. When it's really
mattered for me, I've been satisfied just setting the interface MTU to
get the desired effect.
-John
_______________________________________________
Do not post admin requests to the list. They will be ignored.
Darwin-kernel mailing list (email@hidden)
Help/Unsubscribe/Update your Subscription:
This email sent to email@hidden