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Date: Wed, 09 Jan 2002 18:49:53 -0800
From: Brian Pane <brian.pane@cnet.com>
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To: dev@httpd.apache.org
Subject: Re: cvs commit: httpd-2.0/docs/manual/mod mod_log_config.html
References: <ELEDJONBOPPAEGANDEEIAENDCAAA.joshua@slive.ca>
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Joshua Slive wrote:

>>-----Original Message-----
>>From: Ian Holsman [mailto:ianh@apache.org]
>>
>
>>>> new option LogExcludeByType
>>>> main use is so that you can ignore logging images
>>>>
>>>Hmmm... I'm pretty close to a -1 on this one.  We were just discussing
>>>simplifying the logging directives, while this will compilicate
>>>
>>them for no
>>
>>>good reason.  Can you justify why this is an improvement over
>>>http://httpd.apache.org/docs/logs.html#conditional and
>>>http://httpd.apache.org/docs/env.html#examples ?
>>>
>>I guess the answer is mainly a speed one.
>>
>>setenvif is a regex call while this isn't.
>>
>
>Can you really benchmark speed differences between the two?
>

Yes.  And the difference is quite large.  The cost of a single regex
comparison isn't too bad compared to the cost of a single hash table
lookup (about 300 cycles for the regex match vs ~100 for the hash table
lookup on a Sparc, based on our most recent round of profiling data).
The problem is that mod_setenvif does a *lot* of regex operations per
request.  Specifically, it does O(N) regex comparisons for N SetEnvIf
rules, compared to O(1) hash table lookups per request for LogExcludeByType.
At least in its present form, mod_setenvif simply isn't a scalable
solution.

I'm mildly alarmed that we're actually advocating it as a general-purpose
solution in the documentation URLs listed above; we really should warn
readers that the processing cost scales linearly with the number of rules.

>Wouldn't we be better off designing a fast-path for mod_setenvif that could
>take something like
>SetEnvIf REQUEST_URI \.gif$
>and test it without a regex?  (I admit that I don't know how much work that
>would be.)
>

That would work if you could also reduce the O(N) comparisons per request
to O(1)...without that additional improvement, though, it still won't 
scalewell as the number of SetEnvIf rules increases.

--Brian