Date: Wed, 22 Apr 2009 10:20:43 -0400 From: Adam McDougall <mcdouga9@egr.msu.edu> Cc: freebsd-fs@freebsd.org Subject: Re: FreeBSD 7.2-RC1 - ZFS related kernel panic "kmem_map too small" Message-ID: <49EF27BB.1060100@egr.msu.edu> In-Reply-To: <20090422123020.42b756c1@ernst.jennejohn.org> References: <49EE49D8.7000902@free.de> <20090422123020.42b756c1@ernst.jennejohn.org>
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Gary Jennejohn wrote: > On Wed, 22 Apr 2009 00:34:00 +0200 > Kai Gallasch <gallasch@free.de> wrote: > > [snip a lot of stuff] > >> In /usr/src/UPDATING I read: >> >> [..] >> >> 20090207: >> ZFS users on amd64 machines with 4GB or more of RAM should >> reevaluate their need for setting vm.kmem_size_max and >> vm.kmem_size manually. In fact, after recent changes to the >> kernel, the default value of vm.kmem_size is larger than the >> suggested manual setting in most ZFS/FreeBSD tuning guides. >> >> So I understood this as "vm.kmem_size is set unnecessary large by >> default. You should think about decreasing it to save some RAM" >> >> On my amd64 server the default values of kmem_size are >> >> vm.kmem_size_scale: 3 >> vm.kmem_size_max: 3865468109 >> vm.kmem_size_min: 0 >> vm.kmem_size: 1201446912 >> >> Can someone give me a hint how to debug this problem further, or how to >> find some reasonable values for setting vm.kmem_size_max and >> vm.kmem_size with 16G of RAM? >> >> > > Hmm, I wonder whether this applies to 7.2-RC1. I don't know whether > the kernel changes have been committed to 7.2 or whether they were > already present when we started work on 7.2 because I haven't been > paying much attention. > > On my 8-current amd64 machine with only 4GB of RAM I see larger values > than you see with 16GB: > > sysctl vm.kmem_size_max > vm.kmem_size_max: 4509713203 > sysctl vm.kmem_size > vm.kmem_size: 1335824384 > > --- > Gary Jennejohn > _______________________________________________ > freebsd-fs@freebsd.org mailing list > http://lists.freebsd.org/mailman/listinfo/freebsd-fs > To unsubscribe, send any mail to "freebsd-fs-unsubscribe@freebsd.org" > > It has been my experience that after the kmem maximums were raised to allow more than approx 1.6G kmem (a number of months ago), on some systems I still had to specifically raise the vm.kmem_size above the default otherwise I still got out of kmem panics far below the max. I suspect there was pressure for kmem and it was unable to "raise" the limit fast enough, or maybe a fragmentation problem? Additionally, depending on which host, I've found different limits to how high I can set the kmem settings on "recent" builds of 7 and 8 amd64, for example I have one 7.2 system with 4G ram and the kernel would panic if I booted with kmem=2G (1G works fine), but I have a 8.0 system with 2G ram and kmem=2G works fine. Another 8.0 system has 6G ram but we could only boot successfully with kmem=3G, not 4G.
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