Date: Tue, 01 May 2001 01:18:05 -0700 From: Terry Lambert <tlambert2@mindspring.com> To: Kirk McKusick <mckusick@mckusick.com> Cc: Robert Watson <rwatson@FreeBSD.ORG>, Julian Elischer <julian@elischer.org>, Rik van Riel <riel@conectiva.com.br>, freebsd-hackers@FreeBSD.ORG, Matt Dillon <dillon@earth.backplane.com>, David Xu <bsddiy@21cn.com> Subject: Re: vm balance Message-ID: <3AEE713D.C0A35C82@mindspring.com> References: <200104180540.WAA58303@beastie.mckusick.com>
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[ ... merging vnode and vm_object_t ... ] Kirk McKusick wrote: > Every vnode in the system has an associated object. Every object > backed by a file (e.g., everything but anonymous objects) has an > associated vnode. So, the performance of one is pretty tied to the > performance of the other. Matt is right that the VM does locking > on a page level, but then has to get a lock on the associated > vnode to do a read or a write, so really is pretty tied to the > vnode lock performance. Merging the two data structures is not > likely to change the performance characteristics of the system for > either better or worse. But it will save a lot of headaches having > to do with lock ordering that we have to deal with at the moment. I really, really dislike the idea of a merge of these objects, still, and not just because it will be nearly impossible to macke object coherency work in a stack of two or more VFS layers if this change ever goes through. When John Dyson originally wrote the FreeBSD unified VM and buffer cache code under contract for Oracle for use in their Oracle 8i and FreeBSD based NC server platform, he did so in such a way to allow anonymous objects, which did not have backing store associated with them. This was the memory pulled off of /dev/zero, and the memory in SYSVSHM. The main benefit of doing this is that it saves an incredible amount of write-through, which would otherwise be necessary to maintain coherency with the backing object (vnode). I think we need to remember that we do not always have a backing object, nor is a backing object always desirable. The performance of an mmap'ed file, or swap-backed anonymous region is _significantly_ below that of unbacked objects. -- Terry To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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