Date: Wed, 29 Aug 2012 19:49:04 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Patrick Mahan <PMahan@adaranet.com> Cc: "freebsd-hackers@freebsd.org" <freebsd-hackers@freebsd.org> Subject: Re: Need some explanation on a field in struct vmtotal Message-ID: <20120829164904.GD33100@deviant.kiev.zoral.com.ua> In-Reply-To: <DA0D02C15AF8CF408F8A03D8FE5AD16C1777FDDA@SJ-EXCH-1.adaranet.com> References: <DA0D02C15AF8CF408F8A03D8FE5AD16C1777FDDA@SJ-EXCH-1.adaranet.com>
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On Wed, Aug 29, 2012 at 09:09:17AM -0700, Patrick Mahan wrote:
> All,
>
> I'm trying to determine if this is a bug or for real. We have a customer that pointed his
> NMS at our appliance (running FBSD 9.0). These are 64-bit intel platforms (8 core Xeons)
> with 8 Gbytes of RAM and a 16 Gbyte swap.
>
> The customer claims that the NMS shows him that these boxes have a tetrabyte of VM, which
> surprised him somewhat. These platforms are using net-snmp 5.7.1 out of the ports tree.
>
> I did an snmpget query on one of our boxes here in our lab and saw the following:
>
> comet1# snmpget -v2c -c public localhost hrMemorySize.0
> HOST-RESOURCES-MIB::hrMemorySize.0 = INTEGER: 8351368 KBytes
>
>
> SNMP table: HOST-RESOURCES-MIB::hrStorageTable
>
> hrStorageIndex hrStorageType hrStorageDescr hrStorageAllocationUnits hrStorageSize hrStorageUsed hrStorageAllocationFailures
>
> ...
>
> 3 HOST-RESOURCES-TYPES::hrStorageVirtualMemory Virtual memory 4096 Bytes 269040967 268460681 0
>
> My understanding is that the 'hrStorageSize' is in 'hrStorageAllocationUnits', so the total
> byte size should be
>
> 4096 x 269040967 = 1085607800832
>
> Now, I have looked at the net-snmp code for retrieving this value and it seems to be
> via a sysctl to vm.vmtotal. This value is the field t_vm in the vmtotal structure
> (defined in sys/sys/vmmeter.h).
>
> Looking at the code in sys/vm/vm_meter.c I see the following:
>
> TAILQ_FOREACH(object, &vm_object_list, object_list) {
> /*
> * Perform unsynchronized reads on the object to avoid
> * a lock-order reversal. In this case, the lack of
> * synchronization should not impair the accuracy of
> * the reported statistics.
> */
> if (object->type == OBJT_DEVICE || object->type == OBJT_SG) {
> /*
> * Devices, like /dev/mem, will badly skew our totals.
> */
> continue;
> }
> if (object->ref_count == 0) {
> /*
> * Also skip unreferenced objects, including
> * vnodes representing mounted file systems.
> */
> continue;
> }
> total.t_vm += object->size;
>
> But I cannot find any description of what object->size is defined. The
> vm_object structure is defined in vm/vm_object.h as type vm_pindex_t.
>
> Which is an architecturally defined in <machine>/_types.h (for amd64 this
> is defined as __uint64_t.
>
> My FreeBSD internals (McKusick's book) for 5.5 doesn't seem to reference it.
>
> So my question is - Is the object->size in bytes? If this is so, then
> total.t_vm is in bytes and net-snmp is using that value unmodified. Instead
> it should divide that value by 4096 and report that number instead?
>
> Or is my understanding screwed up here?
struct vm_object size is in pages.
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