Distributed Systems: Naming Continued Synchronization

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Distributed SystemsNaming (Continued)Synchroniz
ation

• CS 654Lecture 13October 30, 2006

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Refresh Distributed Garbage Collection

• The problem. How do you discover when no one
  needs a server object?
• One solution Develop distributed versions of
  garbage collection algorithms.

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Reference Listing

• Distributed reference counting is tricky because
  of failures.
• If you send an increment, how do you know it
  arrived?
• What if the ack is lost?
• If you can design it so that it doesnt matter
  how many times you send a message, then it is
  simpler.
• This is called idempotency. An idempotent
  operation can be done many times without negative
  effect.
• Are these idempotent?
• Withdrawing 50 from your bank account.
• Cancelling a credit card account.
• Registering for a course.

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Idempotent Reference Counting

• How can we make reference counting idempotent?
• What turns non-idempotent registration into
  idempotent registration?
• Keep track of which proxies have been created in
  the skeleton.

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Reference Listing
P1
Reference ListP1P2
P2

• Keep a list of proxies in the skeleton.
• Failures can be handled with heartbeats, etc.
• Main issue is scaling, if millions of proxies.

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Tracing in Groups

• Garbage collect first within a group of
  distributed processes.
• An object is not collected if
• It has a reference from outside the group
• It has a reference from the root set
• This is conservative.
• It is possible that a reference from outside the
  group is not reachable.

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The Model

• Proxies (stubs), skeletons, and objects.
• Only one proxy per object per process.
• A root set of references. The root set is not
  proxies.

Proxy
Skeleton
Object
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Basic Steps

• Find all skeletons in a group that are reachable
  from outside or from root set.
• Mark them hard, others are soft.
• Within a process, proxies that are reachable from
  hard are marked hard, reachable from soft are
  marked soft. Some are marked none.
• Repeat the above until stable (no change).

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• Skeletons
• Hard
• Reachable from proxy outside of group
• Reachable from root object inside of group
• Soft
• Reachable only from proxies inside of group
• Proxies
• Hard
• Reachable from root set
• Soft
• Reachable from skeleton marked soft
• None

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Initial Marking of Skeletons

• All proxies in group report to skeleton.
• If ref count greater than number of proxies, then
  must be external refs.

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Local Propagation

• Propagate hard/soft marks locally.

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Iterate Till Convergence

• Final marking.
• Anything not hard can be collected.

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Synchronization
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Clock Synchronization

• Why is it important?
• Embedded systems (Agilent)
• Stimulus and response
• Ordering debugging messages
• Financial transactions
• Collaborative sensing GPS

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Clock Synchronization

• Figure 6-1. When each machine has its own clock,
  an event that occurred after another event may
  nevertheless be assigned an earlier time.

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Physical Clocks

• A high-frequency oscillator, counter, and holding
  register
• Counter decrements on each oscillation, generates
  a tick and is reset from the holding register
  when it goes to 0.
• On a network, clocks will differ skew.
• Two problems
• How do we synchronize multiple clocks to the
  real time?
• How do we synchronize with each other?

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Measuring Time

• How long is a second?
• How long is a day?
• One solar day is 86,400 solar seconds.
• Some variation in the rotational speed of the
  earth mean solar second.

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A Solar Day

• Computation of the mean solar day.

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Changes

• Big problem the earths rotation is slowing.
• Seconds are getting longer.
• Is this acceptable for science and engineering?
• Use physical clocks not tied to the Earth TAI.
• Hmbut then there are more seconds in a day!
• What are leap years for?
• Leap seconds are periodically added.

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Leap Seconds

• TAI seconds are of constant length, unlike solar
  seconds. Leap seconds are introduced when
  necessary to keep in phase with the sun.
• UTC is TAI with leap seconds.
• Suppose we want to know how much time elapsed
  between two events. Which do we use?

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GPS

• 29 satellites, with atomic clocks, at 20,000 km.
• Each satellite continuously broadcasts its
  position and time.

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Triangulation
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Complications

• Real world facts that complicate GPS
• It takes a while before data on a satellites
  position reaches the receiver.
• The receivers clock is generally not in synch
  with that of a satellite.
• Using four satellites, end up with a system of
  four equations in four unknowns.

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With Four Satellites

• Measured delay includes clock differences.
  Delta-r is the clock difference

• Real distance

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Notation and Terms

• Assume a timer that causes an interrupt H times a
  second.
• When it fires, adds 1 to a software clock, C.
• Let Cp(t) be the value of the clock on machine p
  when the UTC time is t.
• Cp(t) is the frequency, dC/dt.
• Cp(t) 1 is the skew. Offset is Cp(t) t.
• Clocks will drift. If we can bound the drift,
  then we have the maximum drift rate.
• 1 rho

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Clock Drift

• The relation between clock time and UTC when
  clocks tick at different rates.

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Network Time Protocol

• Contact a time server to ask the time.
• What is the major issue here?
• Trick is to get a good estimate of the message
  delays.

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Network Time Protocol

• Getting the current time from a time server.

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NTP Time Corrections

• NTP is about making clocks correspond to the real
  time.
• Clock servers are divided into stratums, starting
  with stratum-0 for the reference physical clock.
• If a stratum-2 clock is running faster than a
  stratum-1 clock, the stratum-2 clock will adjust.
• How should it adjust if it is running fast?
• Time should never go backwards, but should just
  slow down.