Date: Wed, 17 Feb 1999 13:44:19 -0500 (EST) From: "John S. Dyson" <dyson@iquest.net> To: tlambert@primenet.com (Terry Lambert) Cc: dyson@iquest.net, tlambert@primenet.com, toasty@home.dragondata.com, mike@smith.net.au, hackers@FreeBSD.ORG Subject: Re: vm_page_zero_fill Message-ID: <199902171844.NAA69882@y.dyson.net> In-Reply-To: <199902171838.LAA20158@usr07.primenet.com> from Terry Lambert at "Feb 17, 99 06:38:39 pm"
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Terry Lambert said: > > > The thing that appalled me was what you said about BSS being zero'ed > > > in the kernel space using zeroed pages instead of as a result of an > > > explicit zeroing by the execution class loader. > > > > That is the way that it works. Explict zeroing is wasteful because > > it cannot easily take advantage of background prezeroing... However, > > recently prezeroed pages make for efficient usage of cache. The zero > > queue (and all others) are designed to take advantage of recent cache > > usage. > > This is robbing Peter to pay Paul; in a way. The base assumption > that you are hiding is that you aren't constrained by memory > bandwidth. This isn't true if you are nearly saturating a PCI > bus with 4 BT848's (to pick the highest memory bandwidth application > I know about). > Prezeroing doesn't take any significant CPU if there are no cycles available. It does increase latency slightly, if zeroing is allowed to happen. > > Maybe we need to go back to first principles, and examine the > assumptions about what constraints are in effect under various > usage models, and make trades like these optional instead of > mandatory. I think that's all he wants, anyway. > The prezeroing isn't adding any cost to him, the ability to support returning non-initialized data from the kernel would be useful. In that case, turning off prezeroing *might* help (but probably won't.) -- John | Never try to teach a pig to sing, dyson@iquest.net | it makes one look stupid jdyson@nc.com | and it irritates the pig. To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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