From:	CRDGW2::CRDGW2::MRGATE::"SMTP::CRVAX.SRI.COM::RELAY-INFO-VAX" 25-JUL-1989 02:36
To:	MRGATE::"ARISIA::EVERHART"
Subj:	Re: Algorithm for LAT Load Balancing?

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Date: 24 Jul 89 14:46:05 GMT
From: uhccux!munnari.oz.au!comp.vuw.ac.nz!dsiramd!windy!gpwd!gpwrmdh@ames.arc.nasa.gov  (Martin D. Hunt)
Organization: Government Printing Office, Wellington, New Zealand
Subject: Re: Algorithm for LAT Load Balancing?
Message-Id: <76@gp.govt.nz>
References: <89Jul18.171439edt.57358@ugw.utcs.utoronto.ca>
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In article <89Jul18.171439edt.57358@ugw.utcs.utoronto.ca>, SYSJOHN@UTOROCI.BITNET writes:
> Does anyone know what algorithm is used in DEC's LAT load balancing?
> The manual says:
> 
>>The service rating for a service is calculated dynamically by each
>>service node that offers it. The node announces the rating value over
>>the LAN as part of the service announcement message. The service
>>rating is based on the OVERALL LEVEL OF ACTIVITY OF THE NODE AND THE
>>PROCESSOR TYPE. A high level of system activity gives a low service
>>rating and hence inhibits new connections. (LCP-3, LATPlus/VMS Service
>>Node Management Guide)
> 
> We have found that the number of INTERACTIVE LOGINS on each node also
> contributes to the service rating. For example, our VAX/11-780 has a
> much higher service rating than our uVAX/II (with the same CPU load).
> When we set interactive logins as follows:
> 
>      SET LOGIN/INTERACTIVE=16   ! VAX/11-780
>      SET LOGIN/INTERACTIVE=640  ! uVAX/II
> 
> the service ratings become the same. Moreover, if we set interactive
> logins beyond the maximum possible value (i.e. SET LOGIN/INTER=35000)
> the service rating of that node drops to zero! (this on VMS 5.1-1)
> 
> Any thoughts on what is going on here?
> 

Yes, this is a case of DEC removing useful information from their manuals. 
When LATPlus was distributed as a product separate from VMS, the formula 
was listed in the LATPlus release notes. Luckily we hang on to such 
information. The following is the formula as it appears in the release 
notes from LATPlus/VMS V1.2 (October 1986) - I don't think it would have
changed much, if at all: 
				       ijoblim - ijobcnt
Rating = (100 * %cpu_idle_time + 155 * ----------------- ) * cpu_type_factor
				            ijoblim

So, yes it does take into account the interactive job limit - the rating 
will drop to zero when the number logged in is equal to this limit.

The release notes then go on list the CPU ratings, as follows:

CPU Type		Weight Factor
--------		-------------

8800, 8650, 8600, 8500	1.0
785, 8300		0.5
780, 8200		0.375
750, MicroVAX II	0.25
730, MicroVAX I		0.125

The remaining question is - Why does DEC omit potentially very useful 
information from their manuals when it has previously been freely 
available.
-- 
--------------------------------------------------------------------------------
Martin D. Hunt
Government Printing Office
Wellington
New Zealand

USEnet address : MARTINH@GP.GOVT.NZ
PSI address    : PSI%0530147000028::MARTINH
--------------------------------------------------------------------------------

