Description of problem:
currently net-snmp overestimates RHEL7 memory usage which result lot of wrong memory alert

Description	
net-snmp overestimates the memory usage than that of free output

Reason : free command includes slab allocation in cache, which is not included by HOST-RESOURCES-MIB i.e. net-snmp. 


# snmpwalk -Os -v2c -cpublic localhost hrStorage | grep '\.[167] ='
hrStorageIndex.1 = INTEGER: 1
hrStorageIndex.6 = INTEGER: 6
hrStorageIndex.7 = INTEGER: 7
hrStorageType.1 = OID: hrStorageRam
hrStorageType.6 = OID: hrStorageOther
hrStorageType.7 = OID: hrStorageOther
hrStorageDescr.1 = STRING: Physical memory
hrStorageDescr.6 = STRING: Memory buffers
hrStorageDescr.7 = STRING: Cached memory
hrStorageAllocationUnits.1 = INTEGER: 1024 Bytes
hrStorageAllocationUnits.6 = INTEGER: 1024 Bytes
hrStorageAllocationUnits.7 = INTEGER: 1024 Bytes
hrStorageSize.1 = INTEGER: 19799404
hrStorageSize.6 = INTEGER: 19799404
hrStorageSize.7 = INTEGER: 2428420
hrStorageUsed.1 = INTEGER: 6758148
hrStorageUsed.6 = INTEGER: 279280
hrStorageUsed.7 = INTEGER: 2428420

hrStorageUsed includes buffers and cache, so you can get an estimate of total free RAM like this:
totFree = total - used + buffers + cache

As per above outputs
totFree = hrStorageSize.1 - hrStorageUsed.1 + hrStorageUsed.6 + hrStorageUsed.7
        = 19799404        - 6792488         + 280040          + 2447248
        = 15734204 KB

As per free output total free memory would be as follows :
# free
              total        used        free      shared  buff/cache   available
Mem:       19799404     3709992    13008312      846124     3081100    14787728
Swap:      10027004           0    10027004

totFree = free   + buff/cache
        = 13008312 + 3081100
        = 16089412 KB

Here is the discrepancy in memory usage between net-snmp and free. Though both free and HOST-RESOURCES-MIB get their data from /proc/meminfo, but use slightly different metrics. From man free:

buffers : Memory used by kernel buffers (Buffers in /proc/meminfo)
cache : Memory used by the page cache and slabs (Cached and Slab in /proc/meminfo)

So free includes slab allocation in cache, while HOST-RESOURCES-MIB doesn't. If we get the slab allocation:

# grep Slab /proc/meminfo
Slab:             353696 kB

and add it to what we got from SNMP, we have:

totFree = 15734204 + 353696
        = 16087900 KB

Is it possible to include slab allocation in net-snmp memroy usage ?

Version-Release number of selected component (if applicable):
net-snmp-5.7.2-24.el7_3.2.x86_64


How reproducible:
1. Install net-snmp
2. Start the service
3. Collect memory usage using net-snmp

Steps to Reproduce:
1. Check memory usage using net-snmp
2. Check memory usage using free command 
3. Compare the memory usage

Actual results:
net-snmp over estimates memory usage on the system

Expected results:
net-snmp and free should show similar memory usage

Additional info:

slab allocation is not considered by net-snmp memory usage