On Fri, Jun 3, 2011 at 1:16 PM, Peter Robinson <pbrobin...@gmail.com> wrote: There are some back of the envelope computations that can help with OLPC in a wireless mesh or from a server.
Bandwidth is fixed. So if there are two OLPCs connecting to a server you need to divide the bandwidth by two and target a sub second send buffer allocation configuration. Watching latency can prove important because latency problems indicate that one application near or far could fill the buffers. By keeping the send buffers small a fair share access to the net can effectively be established by the system process scheduler. Receive buffers can be big, but it is better to have the system advertise a modest buffer space. Next in the talk is a mention of critical services. One of which is name services. DNS is in the critical path for almost all operations in the OLPC and XS server. Time outs for DNS lookups are many seconds so a slow lookup can keep a window from opening for many seconds. Most often overlooked is localhost (127.0.0.1). After localhost are the lookups for private networks (192.168.x.x, 172.16.x.x, 10.x.x.x). These are often allocated by DHCP for each OLPC but even these need to be resolved quickly because timeouts are long. The system will continue after the timeout but timeouts are long. The XS must be able to resolve any address it allocates via DHCP. And each OLPC must be able to lookup names for all the IP addresses it connects to. It is possible to setup a name server on the XS that is authorative or with a host res order that places hosts before dns in the /etc/host.conf file. Populating /etc/hosts with all the private name lookups is a valuable trick when establishing a class room that is not known to the world because it is hidden behind a NAT box. Also watch for another type of private networking uses the link-local address range (169.254.1.0 to 169.254.255.254). If link-local or Zero configuration networking is involved these addresses also need to be resolved promptly. It helps but is not sufficient to just use numbers. A secure shell connection (ssh me@192.168.4.24) can take fifteen seconds to connect if the lookups to three name servers fail. If both ends are quickly resolved the connection can take place in the blink of a screen refresh. It also makes sense for the XS server to run a squid proxy server. DHCP can be configured so DHCP clients get the proxy server info. The big value of a squid server is all the rich web content that sites serve up. The proxy server also places a number of critical lookups on the XS where they can be evaluated, measured and managed. IP filters can also firewall many problems..... Some of this is analogous to the issues that HPC clusters like a ROCKS or Beowulf cluster encounter. Like an XS setup there is one larger system that serves as a gateway and central "hub" and behind it are many compute servers. These clusters like a school server can be isolated or fully connected to the internet and have the same living in isolation or full network service connected issues and problems. > On Fri, Jun 3, 2011 at 7:37 PM, Sameer Verma <sve...@sfsu.edu> wrote: >> On Thu, Jun 2, 2011 at 7:32 PM, James Cameron <qu...@laptop.org> wrote: >>> On Thu, Jun 02, 2011 at 09:50:57AM -0700, Sameer Verma wrote: >>>> I don't know if any traffic shaping implications will affect the >>>> school server, but in the hopes that it might, I'm copying that list >>>> as well. -- T o m M i t c h e l l mitch-at-niftyegg-dot-com "My lifetime goal is to be the kind of person my dogs think I am." _______________________________________________ Server-devel mailing list Server-devel@lists.laptop.org http://lists.laptop.org/listinfo/server-devel
