[Openstreetmap-dev] Re: Ideas to lower server resources

[Lars Aronsson]

Immanuel Scholz wrote:

> - How much resources does a tile-server request take?

In the last two weeks, I have made 149 fetches of the URL for a 
specific tile at zoom=7.  For this level, streets are no longer 
drawn, but that was not the case when I started my measurements.
I shall have to make a similar study for one of the deeper zoom 
levels, but for now my stats only report how fast the background 
satellite image can be returned.

The average response time was 0.51 seconds, with a minimum of 0.09 
(including 0.03 seconds ping roundtrip Linköping-London!) and a 
maximum of 30.45 seconds.  The 148 samples fall into the following 
categories:

   13%  took less than .1 seconds
   73%  took .1 - .2 seconds
   10%  took .2 - .5 seconds
    1%  took .5 - 1 seconds
    0%  took 1  - 2 seconds
    1%  took 2  - 5 seconds
    1%  took 5 - 10 seconds
    1%  took 10 - 20 seconds
    1%  took 20 seconds or more

Since I fetch the URL every 2 hours, which equals 24 times in 
every 48 hour interval, I assume that 1/24 = 4% of all calls are 
new computations after the Squid cache expired, and 23/24 = 96% 
are cache hits.  This corresponds very well to the 4% of calls 
that take 2 seconds or more.  In absolute numbers, five samples 
took longer than 1.0 seconds:

  Date         Time     Response time

  20060205 Sun 00:47:00 30.45
  20060207 Tue 04:47:00  2.87
  20060209 Thu 20:47:00  6.24
  20060211 Sat 20:47:00  3.10
  20060213 Mon 20:47:00 12.62

I think you can say that these samples are roughly 48 hours apart.

If these five samples are removed from the statistics, the 
remaining 144 samples have response times ranging between 0.09 and 
0.83, with an average of 0.14 seconds!

In the plain old "dotcom era" also known as "Web 1.0", all website 
response times below 5 seconds were "OK".  You can probably find 
such recommendations from Jacob Nielsen and other "web design" 
gurus (e.g. http://www.useit.com/alertbox/9703a.html).  But an 
average response time of 0.14 seconds is a clear sign that we are 
now building the "Web 2.0".

My conclusions: 1) The Squid is doing a great job!  2) The tile 
server is quite slow, even at zoom=7 where no streets need to be 
drawn.  In two samples out of five, tile generation took longer 
than 10 seconds.

We could immediately reduce (tile and database) server load by 
increasing the Squid cache expiration time to more than 48 hours.
This is especially true for zoom < 10 where no streets are drawn.

Hope this information helps.


-- 
  Lars Aronsson (lars at aronsson.se)
  Aronsson Datateknik - http://aronsson.se