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List:       freebsd-hackers
Subject:    Re: Need some explanation on a field in struct vmtotal
From:       Konstantin Belousov <kostikbel () gmail ! com>
Date:       2012-08-29 16:49:04
Message-ID: 20120829164904.GD33100 () deviant ! kiev ! zoral ! com ! ua
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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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