How to Read a Varnish Histogram?

How is a varnishhist histogram/graph read? I understand that the x-axis is a log scale.

Specifically:

• I see 9 "|"s or cache hits occur around `y=1e-4`, what does each "|" refer to? Page? File?

• I see fewer hits to the left of these 9 "|"s and some to the right. What are those? Did they take longer (right) and shorter (left) time ... ?

• In the top, left what are `1:2`? and `n = 134`

• Also, I've noticed that the cache hits are always to the left of the cache misses, i.e., they are on the `1e-x` side of things, while cache misses are under + exponent. What is the significance of that? Is that time, if so, time of what ... ?

Many Thanks!

x-axis

``````1:2, n = 134                                                              hostname

#
#
#
#
#
|        #
|        #
|        #
|        #
|       ###
|       ###
||       ###                              ##
|||      ###         ## #   #             ##
||||     ####      #### ## ##  #          ###
+---------+---------+---------+---------+---------+---------+---------+---------+---------
|1e-6     |1e-5     |1e-4     |1e-3     |1e-2     |1e-1     |1e0      |1e1  |1e2
``````
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So the x-axis is the time it takes for the request to come into varnish and get sent back to the client. The |'s are the cache hits and the #'s are the misses. So you should see all the |'s on the left since that is a faster time.

So the left most numbers are faster.. the right most are slower..

now the 1:2 n= number.. Its numbers for the vertical scale and sample size. I wouldn't worry much about those.

But from that output.. you have a pretty fast cache going on.

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Thanks @Mike. There isn't much going on this box yet (-: So I take it, the x-axis is in seconds ... ? – KM. Aug 23 '11 at 18:40
yes.. 1e0 = 1 second – Mike Aug 23 '11 at 18:53
Actually, I am more intrigued by `1:2, n=_num_` (_: Any ideas what that represents? – KM. Aug 25 '11 at 0:34
it's just numbers how the graph is scaled.. has nothing to do with how varnish is performing – Mike Aug 25 '11 at 3:33
• '|' is cache HIT
• '#' is cache MISS
• 'n:m' numbers in left top corner is vertical scale
• 'n = 123' is number of requests that are being displayed
• X-axis is logarithmic time between request request from kernel to Varnish and response from Varnish to kernel.

The X-axis works like this:

• 1e1 = 10 sec
• 1e0 = 1 sec
• 1e-1 = 0.1 secs or 100 ms (milliseconds)
• 1e-2 = 0.01 secs or 10 ms
• 1e-3 = 0.001 secs or 1 ms or 1000 µs (microseconds)
• 1e-4 = 0.0001 secs or 0.1 ms or 100 µs
• 1e-5 = 0.00001 secs or 0.01 ms or 10 µs
• 1e-6 = 0.000001 secs or 0.001 ms or 1 µs or 1000 ns (nanoseconds)

Whole article explaining 'varnishhist' very nicely can be found here: Varnishhist – What Does it Tell Us, and official doc here: Varnish request histogram.

Another useful command is 'varnishstat' - Varnish Cache statistics.

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