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From: cmcmanis@sun.uucp (Chuck McManis)
Newsgroups: comp.sys.amiga
Subject: Re: Disk timings....
Message-ID: <17956@sun.uucp>
Date: Mon, 4-May-87 13:38:46 EDT
Article-I.D.: sun.17956
Posted: Mon May  4 13:38:46 1987
Date-Received: Tue, 5-May-87 04:47:51 EDT
References: <3406@jade.BERKELEY.EDU>
Organization: Sun Microsystems, Inc. - Mtn View, CA
Lines: 36
Summary: Hashing is a big win here

In article <3406@jade.BERKELEY.EDU>, (Mike (My watch has windows) Meyer) writes:
> Those who doubt that AmigaDOS does good things for finding files vs.
> the Unix file system might check out the file creations/deletions #'s
> vs.  raw throughput. Now, if someone could explain why scanning a
> directory is so mach faster relative to the raw disk speed on
> AmigaDOS, I'd appreciate it. Numbers of opens/sec would be
> interesting. I may hack the diskperf program to do that.

I think you will find that ADOS is incredibly fast at opening *named*
files. Because of the hashing scheme and the fairly large hash tables
(76 entries in 512 byte blocks). Consequently opening a file usually
takes only 2 reads in most cases (the directory block and the file
header) with digressions to 3 and sometimes 4 reads when the directory
is *really* loaded. DiskPerf's code for checking the 'scan' rate looks
like :
        /*
        ** Time open scan of directory.
        */
        timer(0);
        numRead = 1;
        for(i = 0; i < SCAN_ITER; i++)
                for(j = 0; j < OPEN_TEST_FILES; j++)
                        if(OpenStat(tempname[i]) != 0)
                                numRead++;
        timer(&sDt);

Where OpenStat just gets a Lock on a filename and does an Examine on 
that lock. This wins because of the hashing. There is very little
scanning, and a lot of jump right to the file you want. Unfortunately
optimizing for speed of the 'Dir' command actually hurts lookup time. 

-- 
--Chuck McManis
uucp: {anywhere}!sun!cmcmanis   BIX: cmcmanis  ARPAnet: cmcmanis@sun.com
These views are my own and no one elses. They could be yours too, just
call MrgCop() and then ReThinkonrge Robbr