Date: Sun, 20 May 2012 12:34:26 -0500 From: Alan Cox <alc@rice.edu> To: Marko Zec <zec@fer.hr> Cc: alc@freebsd.org, freebsd-hackers@freebsd.org, freebsd-amd64@freebsd.org Subject: Re: superpages and kmem on amd64 Message-ID: <4FB92B22.5020304@rice.edu> In-Reply-To: <201205201643.01194.zec@fer.hr> References: <201205200901.32613.zec@fer.hr> <CAJUyCcOWRKw5=NH_WrkKVcOMqGy2f3HroBX=pGbcbS3UbUZkxg@mail.gmail.com> <201205201643.01194.zec@fer.hr>
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On 05/20/2012 09:43, Marko Zec wrote: > On Sunday 20 May 2012 09:25:59 Alan Cox wrote: >> On Sun, May 20, 2012 at 2:01 AM, Marko Zec<zec@fer.hr> wrote: >>> Hi all, >>> >>> I'm playing with an algorithm which makes use of large contiguous blocks >>> of kernel memory (ranging from 1M to 1G in size), so it would be nice if >>> those could be somehow forcibly mapped to superpages. I was hoping that >>> the VM system would automagically map (merge) contiguous 4k pages to >>> superpages, but >>> apparently it doesn't: >>> >>> vm.pmap.pdpe.demotions: 2 >>> vm.pmap.pde.promotions: 543 >>> vm.pmap.pde.p_failures: 266253 >>> vm.pmap.pde.mappings: 0 >>> vm.pmap.pde.demotions: 31 >> No, your conclusion is incorrect. These counts show that 543 superpage >> mappings were created by promotion. > OK, that sounds promising. Does "created by promotion" count reflect > historic / cumulative stats, or is vm.pmap.pde.promotions the actual number > of superpages active? Or should we subtract vm.pmap.pde.demotions from it to > get the current value? The count is cumulative. There is no instantaneous count. Subtracting demotions from promotions plus mappings is not a reliable way to get the instantaneous total because a superpage mapping can be destroyed without first being demoted. > In any case, I wish to be certain that a particular kmem virtual address range > is mapped to superpages - how can I enforce that at malloc time, and / or > find out later if I really got my kmem mapped to superpages? Perhaps > vm_map_lookup() could provide more info, but I'm wondering if someone already > wrote a wrapper function for that, which takes only the base virtual address > as a single argument? Try using pmap_mincore() to verify that the mappings are superpages. > BTW, apparently malloc(size, M_TEMP, M_NOWAIT) requests fail for size> 1G, > even at boot time. Any ideas how to circumvent that (8.3-STABLE, amd64, 4G > physical RAM)? I suspect that you need to increase the size of your kmem map. Alan
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