jm + packet-loss   4

Comcast
Nice wrapper for 'tc' and 'netem', for network latency/packet loss emulation
networking  testing  linux  tc  netem  latency  packet-loss  iptables 
january 2015 by jm
Netflix packets being dropped every day because Verizon wants more money | Ars Technica
With Cogent and Verizon fighting, [peering capacity] upgrades are happening at a glacial pace, according to Schaeffer.

"Once a port hits about 85 percent throughput, you're going to begin to start to drop packets," he said. "Clearly when a port is at 120 or 130 percent [as the Cogent/Verizon ones are] the packet loss is material."

The congestion isn't only happening at peak times, he said. "These ports are so over-congested that they're running in this packet dropping state 22, 24 hours a day. Maybe at four in the morning on Tuesday or something there might be a little bit of headroom," he said.
packet-loss  networking  internet  cogent  netflix  verizon  peering 
february 2014 by jm
_Measuring Mobile Web Performance_ [slides]
Notable slide is #13, displaying a graph of HSDPA packet RTTs measured from a train. Max RTT gets up to 20,266ms. ouch
rtt  packets  latency  hsdpa  mobile  internet  trains  packet-loss 
june 2013 by jm
Building a Modern Website for Scale (QCon NY 2013) [slides]
some great scalability ideas from LinkedIn. Particularly interesting are the best practices suggested for scaling web services:

1. store client-call timeouts and SLAs in Zookeeper for each REST endpoint;
2. isolate backend calls using async/threadpools;
3. cancel work on failures;
4. avoid sending requests to GC'ing hosts;
5. rate limits on the server.

#4 is particularly cool. They do this using a "GC scout" request before every "real" request; a cheap TCP request to a dedicated "scout" Netty port, which replies near-instantly. If it comes back with a 1-packet response within 1 millisecond, send the real request, else fail over immediately to the next host in the failover set.

There's still a potential race condition where the "GC scout" can be achieved quickly, then a GC starts just before the "real" request is issued. But the incidence of GC-blocking-request is probably massively reduced.

It also helps against packet loss on the rack or server host, since packet loss will cause the drop of one of the TCP packets, and the TCP retransmit timeout will certainly be higher than 1ms, causing the deadline to be missed. (UDP would probably work just as well, for this reason.) However, in the case of packet loss in the client's network vicinity, it will be vital to still attempt to send the request to the final host in the failover set regardless of a GC-scout failure, otherwise all requests may be skipped.

The GC-scout system also helps balance request load off heavily-loaded hosts, or hosts with poor performance for other reasons; they'll fail to achieve their 1 msec deadline and the request will be shunted off elsewhere.

For service APIs with real low-latency requirements, this is a great idea.
gc-scout  gc  java  scaling  scalability  linkedin  qcon  async  threadpools  rest  slas  timeouts  networking  distcomp  netty  tcp  udp  failover  fault-tolerance  packet-loss 
june 2013 by jm

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