Date: Wed, 30 May 2012 15:35:45 -0700 From: Colin Percival <cperciva@freebsd.org> To: Andrew Gallatin <gallatin@cs.duke.edu> Cc: freebsd-net@freebsd.org Subject: Re: [please review] TSO mbuf chain length limiting patch Message-ID: <4FC6A0C1.3010307@freebsd.org> In-Reply-To: <4FC63D27.70807@cs.duke.edu> References: <4FC635CC.5030608@freebsd.org> <4FC63D27.70807@cs.duke.edu>
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On 05/30/12 08:30, Andrew Gallatin wrote: > On 05/30/12 10:59, Colin Percival wrote: >> 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. > > 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. Right, the problem you describe isn't the one I was trying to solve. :-) > 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. Sounds good -- I don't want to m_defrag too often, but I imagine in most cases when TSO is being invoked most of the mbufs will have 2 kB each. This should also make the patch simpler by avoiding the need to modify uipc_mbuf.c; if we just limit the TSO payload size then the TCP stack can figure things out by itself. Are you working on a patch, or should I put one together? -- Colin Percival Security Officer Emeritus, FreeBSD | The power to serve Founder, Tarsnap | www.tarsnap.com | Online backups for the truly paranoid
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