Date: Fri, 8 Mar 2013 16:54:58 +0900 From: YongHyeon PYUN <pyunyh@gmail.com> To: Garrett Wollman <wollman@freebsd.org> Cc: jfv@freebsd.org, freebsd-net@freebsd.org Subject: Re: Limits on jumbo mbuf cluster allocation Message-ID: <20130308075458.GA1442@michelle.cdnetworks.com> In-Reply-To: <20793.36593.774795.720959@hergotha.csail.mit.edu> References: <20793.36593.774795.720959@hergotha.csail.mit.edu>
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On Fri, Mar 08, 2013 at 02:10:41AM -0500, Garrett Wollman wrote: > I have a machine (actually six of them) with an Intel dual-10G NIC on > the motherboard. Two of them (so far) are connected to a network > using jumbo frames, with an MTU a little under 9k, so the ixgbe driver > allocates 32,000 9k clusters for its receive rings. I have noticed, > on the machine that is an active NFS server, that it can get into a > state where allocating more 9k clusters fails (as reflected in the > mbuf failure counters) at a utilization far lower than the configured > limits -- in fact, quite close to the number allocated by the driver > for its rx ring. Eventually, network traffic grinds completely to a > halt, and if one of the interfaces is administratively downed, it > cannot be brought back up again. There's generally plenty of physical > memory free (at least two or three GB). > > There are no console messages generated to indicate what is going on, > and overall UMA usage doesn't look extreme. I'm guessing that this is > a result of kernel memory fragmentation, although I'm a little bit > unclear as to how this actually comes about. I am assuming that this > hardware has only limited scatter-gather capability and can't receive > a single packet into multiple buffers of a smaller size, which would > reduce the requirement for two-and-a-quarter consecutive pages of KVA > for each packet. In actual usage, most of our clients aren't on a > jumbo network, so most of the time, all the packets will fit into a > normal 2k cluster, and we've never observed this issue when the > *server* is on a non-jumbo network. > AFAIK all Intel controllers generate jumbo frame by concatenating multiple mbufs on RX side so there is no physically contiguous 9KB allocation. I vaguely guess there could be mbuf leakage when jumbo frame is enabled. I would check how driver handles mbuf shortage or frame errors while mbuf concatenation for jumbo frame is in progress. > Does anyone have suggestions for dealing with this issue? Will > increasing the amount of KVA (to, say, twice physical memory) help > things? It seems to me like a bug that these large packets don't have > their own submap to ensure that allocation is always possible when > sufficient physical pages are available. > > -GAWollman
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