Structured Partially Caching
Proxies for Mixed Media
Frank T. Johnsen,
Carsten Griwodz,
Pål Halvorsen
Dept. of Informatics,
University of Oslo
Mixed media data
An object may be any
multimedia data type, be it
continuous (video, audio) or
discrete (text, pictures).
How the client should use
the objects is described by
metadata - a presentation
plan.
Objects or parts of them can
be reused.
Proxies for NoD
We want to accommodate the needs of mixed media
news on demand (NoD) data.
Characteristics of mixed media NoD:
–
–
–
–
–
User interaction.
Different client types.
A news item may consist of several different objects.
The relationship between objects is described by a
presentation plan.
Some content is reused.
So far not addressed by a single solution.
Partial caching
Hypothesis:
–
A proxy utilizing knowledge about structure will be more
efficient (exhibit a higher byte hit ratio) than a proxy that
does not.
We employ a means of partial caching to
accommodate the needs of mixed media
applications.
Different types of partial caching:
–
–
Time domain (Rexford et al 1999)
Quality domain (Rejaie et al 2000, Schojer et al 2003)
We introduce structured partial caching.
Caching with structure
There are two kinds of structure:
–
Structure between objects
For example, objects are part of the same presentation plan.
–
Structure within objects
Described by metadata; for example reuse of parts of a video.
Format specific; for example layered video and progressive
jpeg. This should also be described by the metadata.
We have looked into applying quality partial caching
in our scenario utilizing knowledge of structure within
objects.
Structured caching example
The news item A consists of
two media objects. This is
external structure.
Media objects may be
layered as shown. This is
internal structure.
All structure is described by
metadata in the presentation
plan, thus allowing the proxy
to cache data consistently
regardless of format.
Structured caching example
News item B reuses the
media objects from A in a
new presentation. This
adds further internal
structure to the media
objects. The proxy tracks
this and collects access
statistics.
The shaded area indicates
that presentation A has been
cached for low quality, i.e.,
lower layers only.
Structured caching example
Time passes, A is no
longer popular.
B is still being
requested, leading to
caching of only the
parts in use.
If higher quality is
requested later, further
layers will be cached.
Simulations
SERVER
− infinite disk space
Other assumptions:
–
− zero delay
− unlimited bandwidth
PROXY
− finite disk space
− zero delay
− unlimited bandwidth
CLIENT
–
–
The proxy has the
presentation plan.
The presentation plans and
the media objects all have
unique IDs.
Client requests follow the
Multi-selection Zipf
distribution (from Clustered
Multimedia NoD:
Popularity-Based Article
Prefetching and Placement,
Kim et al.).
Simulation results / quality
0.9
0.8
byte hit ratio
0.7
0.6
100
0.5
90-10
0.4
50-50
0.3
0
0.2
0.1
0
full object
structured partial
Simulation results / cache size
0.9
0.8
byte hit ratio
0.7
0.6
small
0.5
medium
0.4
large
0.3
0.2
0.1
0
full object
structured partial
Simulation results / adm. policy
0.9
0.8
byte hit ratio
0.7
0.6
small
0.5
medium
0.4
large
0.3
0.2
0.1
0
full object
full object 3 day
admission
structured partial
structured partial 3
day admission
Is structured partial caching smart?
Conclusions for mixed media NoD scenario:
–
–
–
Gains when utilizing knowledge of structure.
More CPU intensive than a scheme not
considering structure (3.5-5 times more).
CPU is not a bottleneck.
Future work/open issues
Investigate additional gains possible by proxy
cooperation.
Refined interactivity patterns (Kim et al a bit
coarse grained).
Thank you for your attention!
Questions?