Date: Fri, 16 May 1997 10:44:41 -0700 (MST) From: Terry Lambert <terry@lambert.org> To: dyson@FreeBSD.ORG Cc: james@westongold.com, freebsd-hackers@FreeBSD.ORG Subject: Re: mmap() Message-ID: <199705161744.KAA17629@phaeton.artisoft.com> In-Reply-To: <199705161426.JAA01426@dyson.iquest.net> from "John S. Dyson" at May 16, 97 09:26:24 am
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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. It seems to me that this is not what he is asking for? Else how do you explain the factor of 2 performance degradation he is seeing when using mmap() I/O over standard file I/O (with associated copies)? Regards, Terry Lambert terry@lambert.org --- Any opinions in this posting are my own and not those of my present or previous employers.
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