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From "Ryan Murray" <ryan.mur...@elementn.com>
Subject RE: [users@httpd] lbmethods in mod_proxy_balancer
Date Fri, 30 May 2008 13:25:23 GMT
> -----Original Message-----
> From: Jim Jagielski [mailto:jim@jaguNET.com]
> Sent: Friday, May 30, 2008 3:51 PM
> To: users@httpd.apache.org
> Subject: Re: [users@httpd] lbmethods in mod_proxy_balancer
> 
> 
> On May 29, 2008, at 5:38 PM, andrzej wrote:
> 
> > Hi,
> >
> > module mod_proxy_balancer can make decision basing on volume of
> > traffic or number of requests. How, for example, this volume is
> > counted? Is it counted from the time server starts (hope not) or
> > maybe it is counted in 10 minutes period (after 10 minutes values
> > read and transfered are reset  and counted from zero)? Where/how can
> > I change this time?
> >
> 
> It is counted after each request... that each, the counters are
> updated with info after each request.
> 
> They are not "reset" or "aged" for a number of reasons:
> 
>    1. The overhead associated with it. Esp something like what
>       is done in mod_jk...
>    2. Validity: LBs try to maintain an overall average. If
>       you occasionally "reset" then you lose knowledge of
>       the past and so you are only averaging over smaller
>       time-periods... It's like the difference in tracking
>       stocks short-term rather than long-term. LB is a long
>       term action.
> 
> The only reason I can see for some kind of "reset" is to
> avoid overflow, but even then you don't want to "reset" to
> 0 but rather do a uniform normalization.
> 
> Why is the current behavior bad and why do you want to change
> it?
> 

In fact for many (if not most) monitoring and management situations, moving
averages are more useful than infinitely historical data.  If load patterns
may change over time, it is often reasonable (or necessary) that recent
traffic has significantly more weight than past traffic.  Normalization is
an easy way to achieve this and I doubt even normalizing the counters used
for the LB decisions on a 1 minute basis could have any appreciable impact
on performance of the overall server while making the LB behavior more
predictable based on "current" traffic patterns.

IMHO LB is not at all a long-term action - if my LB worked like current
world stock market I'd return it to the vendor!  Knowing that _over time_ my
"n" clustered servers did the same amount of work is not nearly as useful as
knowing that _right now_ my 2 servers are doing the same amount of work.
I'm not worries about making my servers "wear out" at the same time ;)


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