Date: Mon, 28 Jan 2013 13:20:37 -0600 From: Alan Cox <alc@rice.edu> To: Ian Lepore <ian@FreeBSD.org> Cc: alc@FreeBSD.org, freebsd-current@FreeBSD.org, Andre Oppermann <andre@FreeBSD.org> Subject: Re: Trouble with recent auto-tuning changes Message-ID: <5106CF85.8060004@rice.edu> In-Reply-To: <1359382968.93359.66.camel@revolution.hippie.lan> References: <1359310302.93359.48.camel@revolution.hippie.lan> <CAJUyCcOGE-86Y_fMkFyDwORd%2Bve0mpwrePv5uSey0L-mfD-9bA@mail.gmail.com> <1359382968.93359.66.camel@revolution.hippie.lan>
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On 01/28/2013 08:22, Ian Lepore wrote: > On Mon, 2013-01-28 at 00:09 -0600, Alan Cox wrote: >> On Sun, Jan 27, 2013 at 12:11 PM, Ian Lepore <ian@freebsd.org> wrote: >> >>> I ran into a panic while attempting to un-tar a large file on a >>> DreamPlug (arm-based system) running -current. The source and dest of >>> the un-tar is the root filesystem on sdcard, and I get this: >>> >>> panic: kmem_malloc(4096): kmem_map too small: 12582912 total allocated >>> >>> Just before the panic I see the tar process get hung in a "nokva" wait. >>> 12582912 is the value of VM_KMEM_SIZE from arm/include/vmparam.h. >>> >>> In r245575 the init order for mbuf limits was changed from >>> SI_SUB_TUNABLES to SI_SUB_KMEM so that mbuf limits could be based on the >>> results of sizing kernel memory. Unfortunately, the process of sizing >>> kernel memory relies on the mbuf limits; in kmeminit(): >>> >>> vm_kmem_size = VM_KMEM_SIZE + nmbclusters * PAGE_SIZE; >>> >>> Since r245575, nmbclusters is zero when this line of code runs. If I >>> manually plugin "32768" (the number tunable_mbinit() comes up with for >>> this platform) in that line, the panic stops happening. >>> >>> So we've got two problems here... one is the circular dependency in >>> calculating the mbuf limits. The other is the fact that some >>> non-trivial amount of kernel memory we're allowing for mbufs is actually >>> being used for other things. That is, if my system was actually using >>> all the mbufs that tunable_mbinit() allowed for, then this panic while >>> untarring a huge file would still have happened. >>> >>> >> All of this is factually correct. However, it's a red herring. The real >> problem is that arm, unlike every other architecture in the tree, does not >> enable auto-sizing of the kmem map based on the physical memory size. >> Specifically, you'll find VM_KMEM_SIZE_SCALE defined in >> "arch"/include/vmparam.h on every other architecture, just not on arm. >> This auto-sizing overrides the value of VM_KMEM_SIZE. >> > Aha. I'll investigate what other architectures do with that and try to > get the same thing going for arm. > i386 or (32-bit) MIPS would be the most similar. Also, I would encourage you to look for other definitions that those architectures have that arm doesn't. As physical memory sizes continue to grow on arm-based systems, they may require other changes in vmparam.h and the machine-dependent param.h that were made on those other architectures year ago. If you have any questions about any of the definitions, feel free to e-mail me. Alan P.S. When I get a little more free time, I intend to get in touch with Andre to address the apparent circular dependency. For now just know that unless that circular dependency is combined with a lack of kmem map auto-sizing, like arm, it's basically harmless.
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