Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!cbmvax!jesup
From: jesup@cbmvax.commodore.com (Randell Jesup)
Newsgroups: comp.sys.amiga.hardware
Subject: Re: Filesystem Speeds (was Re:  GVP Trade-in)
Keywords: SCSI, GVP
Message-ID: <14260@cbmvax.commodore.com>
Date: 6 Sep 90 20:47:44 GMT
References: <552@DIALix.UUCP> <14069@cbmvax.commodore.com> <558@DIALix.UUCP> <6499@sugar.hackercorp.com> <1159@mpirbn.mpifr-bonn.mpg.de>
Reply-To: jesup@cbmvax (Randell Jesup)
Organization: Commodore, West Chester, PA
Lines: 42

In article <1159@mpirbn.mpifr-bonn.mpg.de> p554mve@mpirbn.UUCP (Michael van Elst) writes:
>In article <6499@sugar.hackercorp.com> peter@sugar.hackercorp.com (Peter da Silva) writes:
>>Last time I saw any benchmarks, the Amiga *with equivalent hardware* was way
>>faster than SunOS.
>
>Yes and no. The Amiga "with equivalent hardware" is faster with disk i/o.
>It is faster in that sense that it can move data faster into a processes
>address space. This is because the Amiga does not use a buffer cache
>and therefore does not need the extra copy operation involved in using
>a cache. This advantage becomes less when looking at small reads or writes,
>the filesystem overhead of the filesystem will make the cache overhead
>nearly invisible.

	True, but the filesystem overhead is lower also, due to faster
context switches, etc.

>On the other side, the Amiga (the filesystem) is very slow when dealing
>with concurrent tasks. The filesystem deals with requests in
>first-come-first-served order and concurrent accesses lead to heavy
>thrashing of the disk. This is because there is no buffer cache, no
>read-ahead and no "elevator"-algorithm. But it is not necessary most
>of the time since the Amiga is a single user system and only few users
>will put stress on the disk by loading several programs or files at once.
>And UNIX, it has to deal with multiple users and a lot of system tasks
>requiring disk access therefore it needs these mechanisms to improve
>_its_ average demands.

	Elevator only helps in the case of 3 or more tasks making
accesses at the same time.  Read-ahead does help in multiple-access 
situations, but more and more SCSI drives have built-in read-ahead.  Your
points on single-user are on-target.  Remember, though, that a lot of
Unix machines are effectively single-user and rarely have multiple-accesses
occurring (though more often than AmigaDos).

	Multiple track buffers in trackdisk can reduce thrash a lot (for
floppies).  LoadSeg uses larger internal buffers under 2.0 which will
reduce thrashing (more data xfered per seek).

-- 
Randell Jesup, Keeper of AmigaDos, Commodore Engineering.
{uunet|rutgers}!cbmvax!jesup, jesup@cbmvax.cbm.commodore.com  BIX: rjesup  
Common phrase heard at Amiga Devcon '89: "It's in there!"