[Bugs] #1656 UNSP: logging is slow

[Sugar Labs Bugs]

#1656: logging is slow
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    Reporter:  quozl                      |          Owner:  tomeu                      
        Type:  defect                     |         Status:  new                        
    Priority:  Unspecified by Maintainer  |      Milestone:  Unspecified by Release Team
   Component:  sugar                      |        Version:  0.84.x                     
    Severity:  Unspecified                |       Keywords:                             
Distribution:  Unspecified                |   Status_field:  Unconfirmed                
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 Logging to .sugar/default/logs is quite slow; activities that emit more
 log lines are slower.

 The effect is more easily observed on hardware that has a slow path to log
 storage, such as the XO-1 and XO-1.5.  The effect is absent when logs are
 written to a temporary filesystem.

 The effect is most apparent as a variability in the starting time of an
 activity, such as Terminal or Write.  Terminal takes between 2 and 3
 seconds to start on XO-1.5 hardware, but roughly one out of four starts
 takes up to 8 seconds.

 The cause of the slowness is the blocking writes to the log, and these are
 caused by the explicit use of O_SYNC when the log file is opened.

 [http://dev.laptop.org/ticket/9924 dev.laptop.org ticket 9924] is where
 the problem was first reported.  A patch that removes O_SYNC is attached
 to that ticket.  Only that patch is proposed.

 Reasons to continue using O_SYNC are:
  * increase the chances of useful logs being available for post-event
 diagnosis, especially when a child may force a power fail or reset.

 Reasons to avoid using O_SYNC are:
  * activity startup time and responsiveness.

 Monitoring logs with tail(1) or cat(1) while developing or testing is
 unaffected.

 Reducing log output during normal operation of an activity would also
 reduce the effect of this symptom.

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Ticket URL: <http://bugs.sugarlabs.org/ticket/1656>
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