Friday, March 2, 2012

Old rules of thumb always need to be reconsidered

After being in the computer industry for a while you begin to appreciate just how much machine capabilities change and the need to change designs along with them.  For example just 10 years about developers would spend hours trying to find ways to save a few bytes of memory.  Now most of the code the world runs is via interpreters (JiT, PY, PhP, Jscript, ...) and a few bytes is less interesting.  I'm not saying to waste them but I personally would not put it as my first priority.  

Let me give you an example of how changes in machine capabilities caused the rethinking of an OS.  In Windows XP Microsoft Engineers designed the memory manager to aggressively push data from main memory (RAM) to disk.  This was done because RAM was costly and very small (~128MB) at the time and so if more memory could be freed up new applications could start faster.  If you waited until an application started to free memory users would wait 30 seconds to minutes before the application was usable because of paging RAM to disk.  Between the time Windows XP and Vista shipped RAM prices dropped dramatically (from $40 for 128MB to just $2 dollars). 

With the dramatic change in memory prices and the fact disks did not really get any faster Vista fundamentally broke from the past rule of thumb of free up as much RAM as possible and push it to disk to just the reverse.  RAM was cheap and relatively plentiful so a feature called SuperFetch was created to aggressively page in data from disk to RAM.  Based on the decision to not force RAM to disk overall UI performance seemed to be more snappy in Vista.  No more shaking the mouse after lunch with XP and waiting a minute+ before logging in.

Well it looks like with the improved performance of CPU's and networks old rules of thumb around UI responsiveness are starting to be reconsidered.  Some early UI research in 1968 by Miller and 1991 by Card  lead to rules of thumb for UI regularly cited in "Response Times: The 3 important limits" and extended for the World Wide Wait, I mean Web.

Here is a recap of those rules and a few more that have been adopted from experience and very likely paper I've read long ago and forgotten:

  • Users consider 100ms response times fast
  • At around 1 seconds users will notice a delay but are tolerant
  • At 5 seconds users are starting to get impatient and may take action
  • At 10 seconds they lose focus
  • At 15 seconds they are likely to hit “refresh”
  • At 30 seconds they generally navigate away and don’t come back if there is an acceptable alternative.
Well it looks like even hard earned rules of thumb for UI and Web are now falling as seen in a recent NYTIMES article "For Impatient Web Users, an Eye Blink Is Just Too Long to Wait". Based on this article it looks like 250millisec is the new goal for web responsiveness rather than 1 second as we had all used.

The overall morale of the story is don't hold on too dear to those rules of thumb and perhaps you should rethink them often. 

  -- Anthony F. Voellm

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