CMSSW I/O out-of-the-box

This page discusses CMSSW I/O under the following conditions:
  • CMSSW_3_5_4, unpatched
  • Running PAT tuple creation.
  • cacheSize set to 0.
  • 300 events run

Is a graphs showing the I/O activity:

  • I/O activity on analyzed file over the whole run of the application:
    There are four main bands of activity - first, when ROOT opens the file. Second, when ROOT starts opening and processing the TTree itself (figuring out what it needs to deserialize, event metadata). Third is the read of the first event. After this, there's a long pause in I/O when the conditions data is loaded. Finally, there is the run through the file itself.
  • Notice there is a lot of blocking and a very low level of parallelism.

In general, the I/O behavior is bad because:

  1. Completely random I/O (this is true, but not visible from blot)
  2. Too many small reads (this is true, but not visible from blot)
  3. Stalling on basically every read - we do no read hints.
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Topic revision: r2 - 2010-05-13 - BrianBockelman
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