Date: Fri, 05 Jul 2002 07:34:14 -0700 From: Terry Lambert <tlambert2@mindspring.com> To: Daniel Lang <dl@leo.org> Cc: freebsd-hackers@freebsd.org Subject: Re: Swap configuration for 2GB phys RAM Message-ID: <3D25AE66.2B808DB7@mindspring.com> References: <20020705113532.GA11273@atrbg11.informatik.tu-muenchen.de>
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Daniel Lang wrote: > We've just bought a Dell PowerEdge 2650 with Dual Xeons > and 2 GB of RAM. I thought I would just install it the usual way, > which would include reserving 4 GB swap. Now I'm not sure if > I would run into problems, as this would match the entire > address space. There has been a recent thread about someone > who ran into possible problems because of that. The problem in that case had to do with a page fault in a particular cirumstance when running a UFS in a vn on a UFS. It's a problem with the vn device. My suggested workaround was to modify the daily cron script, or to move the vn devices into subdirectories. He was also hitting near the limit of the KVA space, but was nbot going over because he never needed to swapm with the load he had. Probably should have posted the results back to the list in more detail... > I wonder what I could do to prevent any problems. > Is is advisable to increase the kernel address space > (KVA is this?) in my custom kernel? Not for 2G (IMO), unless you tune stuff up (mbufs, mbuf clusters, etc.)... *way* up. > I've found in the handbook (could also be the FAQ) a paragraph > that stated, if the hardware (MMU) would support it, FreeBSD > could address 8 TB of (I guess virtual) memory. The poweredge > can even be equipped with 6 GB RAM, which is already past > the 4GB limit of a standard 32bit address space. So I wonder > if it would be possible to just use the memory as usual? No. The 8TB limit is elsewhere. The MMU on the 32 bit machine limits you to 4G total KVA + UVA. The 6G in the Dell is bank selected (PAE based). Peter Wemm was talking about hacking it in, but at that point, with what you end up paying, you might as well buy an IA64 instead, and not eat the PAE overhead (IMO). > I don't know of the Xeon's have a different MMU which enables > to address more than 4 GB, but from my guts I would doubt it. > It seems that some people have machines like this in production. > So any hints or advice would be appreciated. See the output of "dmesg | more"; if the CPU line has "PAE" as one of the attributes of the CPU, the CPU itself supports the idea of bank selecting memory above 4G. If you actually modified the VM system to use this (or convinced Peter to), then your limit would be KVA + UVA + PAE_window = 4G, so you would actually lose some overall address space from somewhere to pay for the ability to access the extra RAM, and you could only DMA into it if you passed your hardware 64 bit addresses, and the hardware supported it. See The very long "PAE" discussions that have occurred on these lists (e.g. "Linux supports it, it makes them slower, but we should support it too, because Linux does", etc.). -- Terry To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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