Date: Sat, 17 Apr 2004 02:19:53 +0400 From: =?koi8-r?Q?=22?=Igor Shmukler=?koi8-r?Q?=22=20?= <shmukler@mail.ru> To: =?koi8-r?Q?=22?=Jim C.Nasby=?koi8-r?Q?=22=20?= <jim@nasby.net> Cc: freebsd-performance@freebsd.org Subject: Re[2]: How does disk caching work? Message-ID: <E1BEbgf-0005hv-00.shmukler-mail-ru@f13.mail.ru> In-Reply-To: <20040416221211.GM87362@nasby.net>
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> > > Is there a document anywhere that describes in detail how FreeBSD > > > handles disk caching? I've read Matt Dillon's description of the VM > > > system, but it deals mostly with programs, other than vague statements > > > such as 'FreeBSD uses all available memory for disk caching'. > > > > Well, the statement is not vague. FreeBSD has a unified buffer cache. This means that ALL AVAILABLE > > MEMORY IS A BUFFER CACHE for all device IO. > > > > > I think I know how caching memory mapped IO works for the most part, > > > since it should be treated just like program data, but what about files > > > that aren't memory mapped? What impact is there as pages move from > > > active to inactive to cache to free? What role do wired and buffer pages > > > play? > > > > If file is not memory mapped it is not in memory, is it? Where do you cache it? Maybe I am missing > > somewhing? Do you maybe want to know about node caching? > > What if the file isn't memory mapped? You can access a file without > mapping it into memory, right? Read/write/execute require mmap as far as I know. What type of access do you want to perform without the mmap? Not that you asked, but I strongly suggest reading AST's book than Design of 44BSD. > > When pages are rotated from active to inactive and then to cache buckets they is still retains vnode > > references. Once it is in free queue, there is no way to put it back to cache. Association is lost. > > > > Wired pages are to pin memory. So that we do not get situation when fault handling code is paged out. > > > > I am not FreeBSD guru so I never heard of BUFFER pages. Is there such a concept? > > I'm reffering to the 'Buf' column at the top of top. I remember reading > something about that being used to cache file descriptors before the > files are mapped into memory, but I'm not very clear on what is actually > happening. Ok, I thought that there was an unknown concept of buffer pages. :)
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