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From: "John S. Dyson" <toor@dyson.iquest.net>
Message-Id: <199705161758.MAA02020@dyson.iquest.net>
Subject: Re: mmap()
In-Reply-To: <199705161744.KAA17629@phaeton.artisoft.com> from Terry Lambert at "May 16, 97 10:44:41 am"
To: terry@lambert.org (Terry Lambert)
Date: Fri, 16 May 1997 12:58:01 -0500 (EST)
Cc: dyson@FreeBSD.ORG, james@westongold.com, freebsd-hackers@FreeBSD.ORG
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> > Okay!!!  Firstly, the FFS FS dependent VOP_GETPAGES does do read-aheads iff
> > the object is marked with MADV_SEQUENTIAL.  Secondly, it would be fairly
> > easy to detect sequential behavior automatically.  Right now, there are
> > much bigger fish to fry!!!  :-) (The reason that it is in the FS dependent
> > code, is that it is only optional that one uses the cluster read ahead
> > code on a per filesystem basis.)  It is likely that the FFS dependent
> > VOP_GETPAGES code will work with other filesystem types (perhaps with
> > minor mods.)
> 
> It seems to me that the OBJ_SEQUENTIAL blocks the VOP_GETPAGE() caller
> until all pages have been faulted, instead of asynchronously doing the
> pages following the requested page, and returing immediately for the
> requested page.  This would introduce "bursty" behaviour, as sequential
> access would incur a large latence for every read-ahead trigger.
> 
The read-ahead isn't blocked for in that case.  It only waits for the
blocks that have been explicitly requested.

John