Information Technology Reference
In-Depth Information
you have a pretty good idea of where your system will be under range of
loads
Know your arrival pattern mathematically convenient to assume "expo-
nential distribution" but...
7.5.2
Overload management
load limiting v. scheduling when load is low { nothing matters when load is
medium { scheduling may matter when overload { load limiting matters
many servers do fifo request scheduling + overload management highly vari-
able load {> more sophisticated scheduling not that important { provision the
system so load is usually low {> during normal operation, scheduling doesn't
matter much { overload management to avoid falling over under unexpected
load
overload management { need to do less work when overloaded e.g., reject
some requests (e.g., movie stream server rejects requests to start new movies
so that it can continue to provide good streaming to streams that have already
started. Alternative to giving a few customers good service and others bad (no)
service is to give all customers bad service.
e.g., do less work (e.g., movie stream server reduces video quality/bit rate for
everyone; e.g., ebay updates auction listings every 10 minutes instead of every
minute)
dynamics can also work the other way as well: if amount of work per task
increases as the load increases, then response times can soar even faster, and
throughput decreases as we add load. This makes overload management even
more important.
examples: highway trac, with onramp limiters time-slicing in the presence
of caches (more time slices, fewer cache hits) network trac, with packet losses
as network becomes loaded { keep from sending data into the network another
example: virtual memory
overload management gets less attention than scheduling because messier.
Scheduling is clean policy choice. We build thread systems that work no matter
what scheduler you choose.
You can change scheduler and not affect rest of
system
in contrast, overload management is visible to users; may affect other parts
of system design
Ebay keynote at laddis { under overload, you need to turn something o,
the only question is, do you choose, or let it choose you ebay's scaling odyssey
(cs.cornell.edu)
CNN going to a static page on 9/11
