On Wed, May 30, 2012 at 8:30 AM, Andrew Gallatin <galla...@cs.duke.edu>wrote:
> On 05/30/12 10:59, Colin Percival wrote: > >> Hi all, >> >> The Xen virtual network interface has an issue (ok, really the issue is >> with >> the linux back-end, but that's what most people are using) where it can't >> handle scatter-gather writes with lots of pieces, aka. long mbuf chains. >> This currently bites us hard with TSO enabled, since it produces said long >> mbuf chains. >> > > Colin, > > Thanks for pointing me at this. I've been talking about this > with bz@ a little. > > I've never been clear about what the max TSO size supported by FreeBSD > is. The NIC I maintain (mxge) is limited to 64K - epsilon for both > IPv4 *AND* IPv6. Up until now, this has been enforced by the 16-bit > ip length limit of IPv4 and we have not had IPv6 TSO until this week. > With IPv6, I'm worried that FreeBSD may now send packets down larger > than I could handle. In my case, however, the problem is not s/g list > length, but rather it is internal limits in the NIC which limit us to > 64K - epsilon for IPv6 as well. I think there may be other NICs in > the same boat for IPv6 (and maybe even some which cannot handle the > full 64K for IPv4). > > Your approach would not work well for my size limit. For > example, I'd have to set the limit to 4 mbufs to stay under 64KB. > This would be assuming the worst case of 16KB jumbo mbufs, so > that would limit me to ~8KB per TSO if 2KB mbufs were used. > > I think a better approach would be to have a limit on the size of the > pre-segmented TCP payload size sent to the driver. I tend to think > that this would be more generically useful, and it is a better match > for the NDIS APIs, where a driver must specify the max TSO size. I > think the changes to the TCP stack might be simpler (eg, they > would seem to jive better with the existing "maxmtu" approach). > > I think this could work for you as well. You could set the Xen max > tso size to be 32K (derived from 18 pages/skb, multiplied by a typical > 2KB mbuf size, with some slack built in). If the chain was too large, > you could m_defrag it down to size. > Think I favor Drew's idea as well for what that's worth. Jack _______________________________________________ freebsd-net@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-net To unsubscribe, send any mail to "freebsd-net-unsubscr...@freebsd.org"
