Oracle Hugepages Linux bash script

HugePages is a feature integrated into the Linux kernel with release 2.6. This feature basically provides the alternative to the 4K page size (16K for IA64) providing bigger pages.

Regarding the HugePages, there are some other similar terms that are being used like, hugetlb, hugetlbfs. Before proceeding into the details of HugePages, see the definitions below:

• Page Table: A page table is the data structure of a virtual memory system in an operating system to store the mapping between virtual addresses and physical addresses. This means that on a virtual memory system, the memory is accessed by first accessing a page table and then accessing the actual memory location implicitly.
• TLB: A Translation Lookaside Buffer (TLB) is a buffer (or cache) in a CPU that contains parts of the page table. This is a fixed size buffer being used to do virtual address translation faster.
• hugetlb: This is an entry in the TLB that points to a HugePage (a large/big page larger than regular 4K and predefined in size). HugePages are implemented via hugetlb entries, i.e. we can say that a HugePage is handled by a “hugetlb page entry”. The ‘hugetlb” term is also (and mostly) used synonymously with a HugePage
• hugetlbfs: This is a new in-memory filesystem like tmpfs and is presented by 2.6 kernel. Pages allocated on hugetlbfs type filesystem are allocated in HugePages.
• HugePages can be allocated on-the-fly but they must be reserved during system startup. Otherwise the allocation might fail as the memory is already paged in 4K mostly.
• HugePage sizes vary from 2MB to 256MB based on kernel version and HW architecture
• HugePages are not subject to reservation / release after the system startup unless there is system administrator intervention, basically changing the hugepages configuration (i.e. number of pages available or pool size)
• The actual size of the HugePage on a specific system can be checked by:        $ grep Hugepagesize /proc/meminfo

Below script in bash calculates required Hugepages size in kernel version 2.4 and 2.6. make sure you give chmod 0755 scriptname.sh

#!/bin/bash

# The shared memory segments can be listed by command:

# ipcs -m

# Check for the kernel version

KERN=`uname -r | awk -F. '{ printf("%d.%d\n",$1,$2); }'`

# Find out the HugePage size

HPG_SZ=`grep Hugepagesize /proc/meminfo | awk '{print $2}'`

if [ -z "$HPG_SZ" ];then

echo "The hugepages may not be supported in the system where the script is being executed."

exit 1

fi

# Initialize the counter

NUM_PG=0

# Cumulative number of pages required to handle the running shared memory segments

for SEG_BYTES in `ipcs -m | cut -c44-300 | awk '{print $1}' | grep "[0-9][0-9]*"`

do

MIN_PG=`echo "$SEG_BYTES/($HPG_SZ*1024)" | bc -q`

if [ $MIN_PG -gt 0 ]; then

NUM_PG=`echo "$NUM_PG+$MIN_PG+1" | bc -q`

fi

done

RES_BYTES=`echo "$NUM_PG * $HPG_SZ * 1024" | bc -q`

if [ $RES_BYTES -lt 100000000 ]; then

echo "***********"

echo "** ERROR **"

echo "***********"

echo "Sorry! There are not enough total of shared memory segments allocated for HugePages configuration. HugePages can only be used for shared memory segments that you can list by command:ipcs –m"

exit 1

fi

case $KERN in

'2.4') HUGETLB_POOL=`echo "$NUM_PG*$HPG_SZ/1024" | bc -q`;

echo "Recommended setting: vm.hugetlb_pool = $HUGETLB_POOL" ;;

'2.6') echo "Recommended setting: vm.nr_hugepages = $NUM_PG" ;;

*) echo "Unrecognized kernel version $KERN. Exiting." ;;

esac

Script Output:

$ ./hugepages.sh

Recommended setting: vm.nr_hugepages = 67

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