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Re: st: Does Blasnik's Law apply to -use-?


From   "David Elliott" <dcelliott@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Does Blasnik's Law apply to -use-?
Date   Fri, 14 Sep 2007 11:07:05 -0300

Being Stata users, we should approach this in a rigorous scientific fashion:

X-----begin-----X

program define intest
version 9.0

*! version 1.0.0  2007.09.13
*! Simulate using part of file with in #/##
*! by David C. Elliott
*!
*! using name of trial dataset
*! postname specifies filename of postfile
*! numblocks is number of file blocks to create


syntax using/ ,POSTname(string) NUMblocks(int)

local more `c(more)'
set more off

use `using', clear //Load first to eliminate any first pass caching effects
local recblock = round(`c(N)'/`numblocks',1)

tempname post
postfile  `post' double block float timein timeif using `postname',
every(10) replace

timer clear 1
n di _n(2) "{txt}{col 11}{center 10:-- IF --}{center 10:-- IN --}" _n ///
  "{center 10:Block}{center 10:Time}{center 10:Time}" _n ///
  "{hline 30}"
local lastblock = `c(N)' - `recblock'
forvalues i=1(`recblock')`lastblock ' {
	local block = `i'
	foreach I in if in {
		if "`I'" == "in" {
			local ifin in `i'/`=`i'+`recblock''
			}
			else {
				local ifin if inrange(_n, `i', `=`i'+`recblock'')
				}
		timer on 1
		use `using' `ifin', clear
		timer off 1
		qui timer list 1
		local time`I' :display %5.2f round(`r(t1)',.01)
		timer clear 1
		}
	post `post' (`block') (`timein') (`timeif')
	n di "{res}{ralign 10:`block'}{ralign 10:`timeif'}{ralign 10:`timein'}"
	}
postclose `post'
set more `more'
use `postname', clear
lab var block "Record Block"
lab var timein "Load Time using IN"
lab var timeif "Load Time using IF"
tw line timein block || line timeif block
end

X-----end-----X

eg:

. intest using dss_data_06_07.dta , postname(intest.dta) numblocks(100)


           -- IN --  -- IF --
  Block      Time      Time
------------------------------
         1      0.64      0.88
     17278      0.47      0.77
     34555      0.47      0.77
     51832      0.47      0.78
     69109      0.45      0.78
     86386      0.45      0.78
    103663      0.47      0.78
    120940      0.47      0.77
 ...

This adofile will run an -if- versus -in- simulation and graph the
results.  From my findings I can confirm a speed advantage of about
50% using -in- on dataset with obs:1,727,673 vars:28 size:266,061,642

However, things get murkier.  Run a simulation, then max out Stata's
memory setting with as much memory as the system will give you and run
the simulation again.  When you do this, you eliminate the system's
ability to cache the file.  Ordinarily, subject to filesize and
available memory, Stata may be reading the file from cache.  If this
is the case, one will see an advantage to using -in-.  However, if the
caching advantage is eliminated by increasing Stata memory, my
simulations show the speed reduction using -in- is negated.  I also
tested this on large network databases and was unable to demonstrate
any advantage to -in-.

So back to Roger's initial question.  It would appear that for
cacheable filesizes and large numbers of bygroups a strategy using
-in- might be feasible.  There is an overhead penalty of setting up
the bygroups to make them selectable using -in- involving sorts and
the like.  For a small number of bygroups the speed advantages might
be lost, but for many levels and a large number of iterations there
would be an advantage.

DC Elliott
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