First off, I would like to thank Per Buer for pointing out that I am off
by a factor of 1000 in the following
statements. I have corrected for that below so that my statements are
more clear. My mistake was in
considering modern processors as 2 megahertz instead of 2 gigahertz.
On 3/7/2011 1:52 PM, David Helkowski wrote:
A modern CPU can run, at most, around 10 million -assembly based-
instructions per second.
Make that 10 billion. The math I am using is 5 x 2 gigahertz.
See http://en.wikipedia.org/wiki/Instructions_per_second
A regular expression compare is likely at least 20 or so assembly
instructions.
That gives around 500,000 regular expression compares if you are using
100% of the
CPU just for that. A reasonable amount of CPU to consume would be 30%
( at most ).
So; you are left with around 150k regular expression checks per second.
The correct numbers here are 500 million. A regular expression compare
more likely takes
40 assembly instructions, so I am going to cut this to 250 million.
LIkewise, at 30%, that
leads to about 80 million.
Lets suppose there are 500 different domains. On average, you will be
doing 250 if/else
checks per call. 150k / 250 = 600.
The new number is 80 million / 250 = 320k
That means that you will get, under fair conditions, a max
of about 600 hits per second.
320,000 hits per second.
Obviously, no server is capable of serving up such a number.
Just using regular expressions in a cascading if/then will work fine in
this case.
My apologies for the confusion in this regard.
What I can see is a server serving around 10,000 hits per second.
That would require about 30x the number of domains. You don't really
want to eat up CPU usage for just if/then though, so probably at around 10x
the number of domains you'd want to switch to a hash table.
So; correcting my conclusion; if you are altering configuration for 5000
domains, then you are going to need a hash table. Otherwise you are going
to be fine just using a cascading if/then, despite it being ugly.
The person asking the question likely has 500 domains running.
That gives a little over 1 hit possible per second per domain. Do you
think that is an acceptable
solution for this person? I think not.
Compare it to a hash lookup. A hash lookup, using a good minimal
perfect hashing algorithms,
will take at most around 10 operations. Using the same math as above,
that gives around 300k
lookups per second. A hash would be roughly 500 times faster than
using if/else...
Note that despite my being off by a factor of 1000, the multiplication
still holds out. If you use
a hash table, even with only 500 domains, a hash table will -still- be
500 times faster. I still think
it would be great to have a hash table solution available for use in VCL.
On 3/7/2011 1:35 PM, Per Buer wrote:
Hi,
On Sun, Mar 6, 2011 at 11:39 PM, AD <[email protected]
<mailto:[email protected]>> wrote:
what is the best way to run an instance of varnish that may need
different vcl configurations for each hostname. This could end
up being 100-500 includes to map to each hostname and then a long
if/then block based on the hostname. Is there a more scalable
way to deal with this?
CPU and memory bandwidth is abundant on modern servers. I'm actually
not sure that having a 500 entries long if/else statement will hamper
performance at all. Remember, there will be no system calls. I would
guess a modern server will execute at least a four million
regex-based if/else per second per CPU core if most of the code and
data will be in the on die cache. So executing 500 matches should
take about 0.5ms.
It might not make sense to optimize this.
--
Per Buer, Varnish Software
Phone: +47 21 98 92 61 / Mobile: +47 958 39 117 / Skype: per.buer
Varnish makes websites fly!
Want to learn more about Varnish?
http://www.varnish-software.com/whitepapers
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