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FV Decipher Unterstützung

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Decipher Limits and Capacity

1:  Decipher Limits

What is the maximum amount of <something> you can have in v2?

1.1:  Data Files

The maximum total amount of real or virtual text in a survey cannot exceed 2 gigabytes. 

Partial data (data/state.db files) cannot exceed 1024 GB and must be cleared out before it does so. This is usually not a problem unless you have > 10,000,000 completes or extreme amounts of persistent data (Limit was changed from 64 to 1024 GB in 2015 and applies to all surveys).

1.2:  Timeouts

There is a 30 minute limit on runtime of a report (beyond that your browser/proxy server times out).

1.3:  Surveys

Surveys have the following limits:

  • 8192 of each type of questions (virtual/real) [Increased to 8192 as of September 2013]
  • 1 million data cells (e.g. rows times columns) per question
  • 20,000 quota cells and markers (beyond that executing the quota is going to take rather long time and you have to greatly limit the amount of respondents in a survey)

2:  Decipher Capacity

This section provides information about the limits of the respondent process capacity of Decipher and offers guidelines to respondent infusion limits.

2.1:  Technical Introduction

Surveys execute on a number of "backend processes." A backend process can only do one survey at a time. There are 24 processes and 24 computing units (CPUs).

A single survey can be loaded on up to 4 backend processes by default; this can increase up to 5 through the scaling='5' parameter. Beyond 5, payoff is limited as processes have to wait for each other to synchronize data access.

A single process can generally show 30 survey pages per second however this can vary greatly depending on survey.

You can find our current average traffic load at: http://v2.decipherinc.com/scoreboard

  Requires Decipher Cloud

The "howfast" script can be used to determine likely performance. It requires a complete survey and an SST run. 

2.2:  Calculating Simultaneous Respondents

  Requires Decipher Cloud

The "howfast" script performs this calculation: if a SST run reports SST can show a page in 0.012 seconds then dividing 5000 by this number gives the rough number of page views that our system can display. Divide this number of page views again by number of pages in the survey (there are always 2: start and exit page) to get the number of completes per hour.

This assumes the following: the completes are evenly divided and that the shared processes are not overwhelmed with other surveys. To be sure, keep a safety margin.

Clients that expect high capacity can be moved on the secondary survey system dedicated to high-traffic clients. That allows better throughput without disturbing the main clients.

2.3:  Hosting Static Files

As for hosting static client files (images, javascript files, etc.), we have a caching server that can offset the load of our main server. That server is used automatically if [versioned ...] or [image] is used in the survey. The serving can take some load off the main server however at this point the bandwidth capacity we have is beyond that of Rackspace in 2014 - around 30 megabytes per second.

2.4:  What's Our Capacity?

How many respondents we can handle depends on complexity of the survey and number of pages in the survey. We can handle about 800 survey page views per second all in all. A specific survey can be served at a rate of about 100 page views per second. If a respondent needs to view just 2 pages (the minimum to finish a survey), we could handle 50 new respondents per second. In practice we have not seen more than 150 pages per second served by our system total over the last year.