Difference: FileSummaryRecord (7 vs. 8)

Revision 82014-04-03 - RobLambert

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META TOPICPARENT name="LHCbComputing"

LHCb File Summary Record

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  • the contained object must have non-zero number of members to interface correctly with what ROOT expects from objects within its trees, some of that information can be stored as metadata of the branch, and some is stored in the leaves, once per data member,
  • The EventCountFSR is a very simple FSR which stores two long long ints.
  • If 8 of them are stored to a file as a tree with only 2 bifurcations at each branch, instead of storing 8*2 packed ints we are now storing
Changed:
<
<
    • Size on file: 1+2+4+8 * (string + pointer + container + root-specific-rubbish ) + 8*2 packed ints
    • Size in memory: 1+2+4+8 * (string + pointer + container + root-specific-rubbish + bucket + cache) + 8*2 packed ints
>
>
    • Size on file: (1+2+4+8) * (string + pointer + container + root-specific-rubbish ) + 8*2 packed ints
    • Size in memory: (1+2+4+8) * (string + pointer + container + root-specific-rubbish + bucket + cache) + 8*2 packed ints
    • Mathematically: [Sum( depth=1 to deepest) branches^depth]*overhead + branches^deepest *2 packed ints
 
    • Note that the root-specific-rubbish + bucket + cache is likely to be of order 10kB and so is vastly bigger than the size of the two packed ints.
Changed:
<
<
  • The deeper the tree and the broader the tree, the exponentially more memory is required to store these objects.
>
>
  • The deeper the tree the exponentially more memory is required to store these objects. The broader the tree, the polynomially more memory is required.
 

It is silly to think about it, but adding one low-level FSR right now is more expensive on memory usage and file size than adding the same information to every event, where it will be compressed away. So FSRs should only be used for the MINIMAL possible use cases which are not addressable in any other way.

 
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