Date: Mon, 8 Oct 2007 11:27:42 -0500 From: Craig Boston <cb@severious.net> To: Bakul Shah <bakul@bitblocks.com> Cc: freebsd-fs@FreeBSD.org, freebsd-current@FreeBSD.org, Pawel Jakub Dawidek <pjd@FreeBSD.org> Subject: Re: ZFS kmem_map too small. Message-ID: <20071008162730.GA98555@nowhere> In-Reply-To: <20071006174614.E1D575B52@mail.bitblocks.com> References: <20071005180119.GE98210@garage.freebsd.pl> <20071006174614.E1D575B52@mail.bitblocks.com>
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On Sat, Oct 06, 2007 at 10:46:14AM -0700, Bakul Shah wrote: > There two differences: R1.11 arc_reclaim_needed() returns 1 when 80% > of kmem is used, while R1.10 does so at 50% of kmem. I'll bet it's this change that provokes the problem; as even when manually tuning the ARC size to 1/2 kmem_size or lower I still sometimes get panics. Probably what's happening is kmem usage gets high from other things, and when there's a sudden spike zfs can't react fast enough and shrink the ARC. At 50% it acts more conservatively so there's more memory available for burst usage. I noticed that some of the time, on my mostly-stable system, the panic happens when the nvidia driver is trying to allocate a 128K chunk. vmstat -m only shows nvidia at ~12MB total though, so I think it just gets hit because it malloc/frees large blocks more than most subsystems. The 512MB memory one doesn't run X at all, but it's by far the least stable of the bunch. Unfortunately it doesn't seem to want to create crash dumps for some reason. > Still, fiddling with limits to make the panic go away seems > to somehow miss the point as I always worry it will show up > under other conditions. May be there a way to ensure that > kmem_map is never too small or may be zfs can reserve a few > resources for its own use so that it can get out of a tight > spot? I don't think having ZFS reserve resources would help, as at least for me the kmem_map panic doesn't always happen within ZFS code. It's just that the increased kernel memory demands from ZFS are causing it to run out at times. I still think the best course is to have ZFS's cache use VM objects like the buffer cache does, but I know this is a very nontrivial thing to do. Craig
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