Date: Thu, 23 Mar 2000 15:21:04 -0800 From: Greg Lehey <grog@lemis.com> To: Matthew Dillon <dillon@apollo.backplane.com> Cc: Poul-Henning Kamp <phk@critter.freebsd.dk>, current@FreeBSD.ORG Subject: Re: patches for test / review Message-ID: <20000323152104.B9318@mojave.worldwide.lemis.com> In-Reply-To: <200003202204.OAA72087@apollo.backplane.com>; from dillon@apollo.backplane.com on Mon, Mar 20, 2000 at 02:04:48PM -0800 References: <20074.953579833@critter.freebsd.dk> <200003202204.OAA72087@apollo.backplane.com>
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On Monday, 20 March 2000 at 14:04:48 -0800, Matthew Dillon wrote: > > If a particular subsystem needs b_data, then that subsystem is obviously > willing to take the virtual mapping / unmapping hit. If you look at > Greg's current code this is, in fact, what is occuring.... the critical > path through the buffer cache in a heavily loaded system tends to require > a KVA mapping *AND* a KVA unmapping on every buffer access (just that the > unmappings tend to be for unrelated buffers). The reason this occurs > is because even with the larger amount of KVA we made available to the > buffer cache in 4.x, there still isn't enough to leave mappings intact > for long periods of time. A 'systat -vm 1' will show you precisely > what I mean (also sysctl -a | fgrep bufspace). > > So we will at least not be any worse off then we are now, and probably > better off since many of the buffers in the new system will not have > to be mapped. For example, when vinum's RAID5 breaks up a request > and issues a driveio() it passes a buffer which is assigned to b_data > which must be translated (through page table lookups) to physical > addresses anyway, so the fact that that vinum does not populate > b_pages[] does *NOT* help it in the least. It actually makes the job > harder. I think you may be confusing two things, though it doesn't seem to make much difference. driveio() is used only for accesses to the configuration information; normal Vinum I/O goes via launch_requests() (in vinumrequest.c). And it's not just RAID-5 that breaks up a request, it's any access that goes over more than one subdisk (even concatenated plexes in exceptional cases). Greg -- Finger grog@lemis.com for PGP public key See complete headers for address and phone numbers To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-current" in the body of the message
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