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Linux HugePages and virtual memory (VM) tuning

 

Preamble

With huge hardware prices decrease and all consolidation/virtualization projects going around your Oracle database servers are most probably quite powerful and running multiple instances. The ones I’m using are made of two X7560 x86 processors wit 8 cores each (so 16 cores and you even see 32 CPUs if Hyper Threading is activated) and 64 GB of RAM. My servers are running Red Hat Enterprise Linux Server release 5.5 (Tikanga) (Red Hat 6 not certified at the time we installed servers) and Oracle 11gR2 and 10gR2 (test database of this post is Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 – 64bit Production).

The bigger number of databases per server increases complexity and obviously requires more expertise to understand if a database performance issue is linked to Oracle or server or another instance runing on same server and eating all resources. The performance problem we have experienced was linked to server but reported by one application only: database queries of this application very slow (AWR reports automatically canceled) and/or databases not even reachable. This, even if allocated Oracle memory for all instances was far below maximum. CPU usage on server when experiencing slow response time was also low.

What happens in the background is very well explained is My Oracle Support (MOS) note 361670.1 and in Pythian post called Performance tuning: HugePages in Linux (see references section).

Obviously I have experienced it on my Linux box:

[root@server1 ~]# vmstat -s |grep paged
 159809376876 pages paged in
  12513585830 pages paged out
[root@server1 ~]# vmstat -s |grep paged
 159814312328 pages paged in
  12513708731 pages paged out

You also see it using the files generated by crontab jobs coming with sysstat package. When performance are bad you should see high usage of kswapd process and huge changes in memory free percentage and swap usage:

[root@server1 ~]# export LANG=C
[root@server1 ~]# sar -r -f /var/log/sa/sa09
Linux 2.6.18-274.3.1.el5 (server1)      07/09/12
 
00:00:01    kbmemfree kbmemused  %memused kbbuffers  kbcached kbswpfree kbswpused  %swpused  kbswpcad
.
.
12:50:01     54859152  11002672     16.71     46424   7517192  43599680   5912760     11.94    687912
13:00:02     57649756   8212068     12.47     10492   4952724  40570716   8941724     18.06    751720
.
.
21:10:01     52864804  12997020     19.73    278332   8989132  41647328   7865112     15.89    892780
21:20:01     35866680  29995144     45.54    280840  25883808  41654116   7858324     15.87    896004
.
.
22:00:01     35516316  30345508     46.07    289664  26246756  41698240   7814200     15.78    910264
22:10:01     32650008  33211816     50.43    291904  29011612  43184728   6327712     12.78    929100
.
.

And lots of swap in/out pages:

[root@server1 ~]# export LANG=C
[root@server1 ~]# sar -W -f /var/log/sa/sa09
Linux 2.6.18-274.3.1.el5 (server1)      07/09/12
 
00:00:01     pswpin/s pswpout/s
.
.
12:50:01         1.60      0.00
13:00:02        19.54   1204.79
.
.
21:10:01         6.83      0.00
21:20:01         4.17      0.00
.
.
22:00:01         6.70      0.00
22:10:01       627.21      0.00
.
.

So then digging around you will come to Linux HugePages which have the following benefit (please refer to document in references section):

  • Bigger page size (2MB on Linux x86_64) so reduced Page Table (512 times smaller: 2MB/4KB), reduced Translation Lookaside Buffer (TLB) and then less contention/CPU usage.
  • Not swappable.

The only cons I could see is its non-flexibility means you may need to reboot your server to allocate HugePages which can happen if you add a new database to your server and/or want to change current memory allocated for an already running database.

Huge pages size, available, reserved and free can be obtained with:

[root@server1 ~]# cat /proc/meminfo | grep Huge
HugePages_Total:     0
HugePages_Free:      0
HugePages_Rsvd:      0
Hugepagesize:     2048 kB

HugePages_Rsvd
is short for “reserved,” and is the number of huge pages for which a commitment to allocate from the pool has been made, but no allocation has yet been made. Reserved huge pages guarantee that an application will be able to allocate a huge page from the pool of huge pages at fault time.

Page table size can be obtained with:

[root@server1 ~]# cat /proc/meminfo | grep PageTables
PageTables:    2137628 kB

More information on HugePages can be found by installing kernel-uek-doc-2.6.39-200.29.1.el6uek.noarch on Oracle Linux Server release 6.3:

[root@server1 ~]# cat /usr/share/doc/kernel-doc-2.6.39/Documentation/vm/hugetlbpage.txt

HugePages is NOT compatible with new 11g Automatic Memory Management (AMM) i.e. memory_max_target and memory_target so you have to use Automatic Shared Memory Management (ASMM) i.e. sga_max_size and sga_target.

Clearly a regression on this, this may change with further kernel releases, so it deserves a quick bench of your system to understand if performance gain is worth flexibility lost.

Remark:
I have ever wonder the added value of SGA_TARGET versus SGA_MAX_SIZE as on all OS where I have tested it even if SGA_TARGET is much lower than SGA_MAX_SIZE the memory is anyway requested and allocated at OS level (ipcs -m) so in clear useless functionality. Apparently this is not true on Solaris and on this OS the behavior is the one you can expect…

Implementation

First get the gid of your Unix dba group:

[root@server1 ~]# grep dba /etc/group
dba:x:501:oracle
[root@server1 ~]# id -g oracle
501

Set this number to vm.hugetlb_shm_group kernel parameter:

[root@server1 ~]# sysctl -w vm.hugetlb_shm_group=501
vm.hugetlb_shm_group = 501

Change “The maximum size that may be locked into memory”, value that can be checked with:

[oradmspoc@server1 ~]$ ulimit -l
32

Modify /etc/security/limits.conf and add something like (it is advise to set value a bit lower than your total system memory, 64GB in my case):

[root@server1 ~]# tail -2 /etc/security/limits.conf
oradmspoc   soft   memlock    60397977
oradmspoc   hard   memlock    60397977

Remark:
Even if it looks more secure to put account name. In a consolidated environment and/or when databases can switch from one server to another it could becomes complex to handle. So huge temptation to replace account name (i.e. oradmspoc) by * character.

As your Oracle Unix accounts are most probably in same Unix group (dba for example) a safer solution could be:

@dba soft memlock unlimited
@dba hard memlock unlimited

Logoff/logon and check it is active:

[oradmspoc@server1 ~]$ ulimit -l
60397977

Finally third parameter to change is vm.nr_hugepages, even if dynamic parameter you may encounter difficulties to change it and you may need to reboot your server (to change its value kernel must find contiguous free space and if you server is running since long memory is most probably quite fragmented). I have partially resolved it by submitting the kernel change multiple time. Please note it’s a number of pages (2MB) and not a size in bytes:

[root@server1 ~]# echo 252 > /proc/sys/vm/nr_hugepages
[root@server1 ~]# sysctl -w vm.nr_hugepages=252
vm.nr_hugepages = 252
[root@server1 ~]# for i in $(seq 1 10); do echo 252 > /proc/sys/vm/nr_hugepages; sleep 10; done

Control it is effective:

[root@server1 ~]# sysctl vm.nr_hugepages
vm.nr_hugepages = 252
[root@server1 ~]# cat /proc/sys/vm/nr_hugepages
252
[root@server1 ~]# cat /proc/meminfo | grep HugePages_Total
HugePages_Total:   252

To which value set vm.nr_hugepages kernel parameter ? If your Oracle database are already running you may use Oracle script (MOS note 401749.1):

[root@server1 ~]# /home/oradmspoc/yannick/hugepages_settings.sh
 
This script is provided by Doc ID 401749.1 from My Oracle Support
(http://support.oracle.com) where it is intended to compute values for
the recommended HugePages/HugeTLB configuration for the current shared
memory segments. Before proceeding with the execution please make sure
that:
 * Oracle Database instance(s) are up and running
 * Oracle Database 11g Automatic Memory Management (AMM) is not setup
   (See Doc ID 749851.1)
 * The shared memory segments can be listed by command:
     # ipcs -m
 
Press Enter to proceed...
 
Recommended setting: vm.nr_hugepages = 252

Or allocate your (SGA size in MB/2)+1 pages (sum for all your Oracle database), take care of rounding on SGA i.e. allocation a SGA of exactly 401MB is not possible. I take the opportunity to write that RedHat formula I have seen in multiple document i.e. (SGA+PGA+(20KB * # of Oracle processes running)) / 2MB is wrong as HugePages is ONLY for SGA and not for PGA. If you really don’t know how much databases will be finally running on your server Red Hat generic recommendation is to set HugePages size to half your server physical memory.

You can control usage with (or using nmon):

[root@server1 ~]# watch -n 10 cat /proc/meminfo
[root@server1 ~]# cat /proc/meminfo | grep Huge

Remark:
The real number of free HugePages is HugePages_Free – HugePages_Rsvd, if you want Oracle to initialize all HugePages use initialization parameter pre_page_sga=true.

Remark:
Starting with 11.2.0.2 there is a new initialization parameter called use_large_pages than can forbid Oracle to start (only value) if no HugePages are available. This avoid mistake and Oracle using normal page size (4KB).

Not swapping PGA

As we have seen above this HugePages story is only true for SGA parameter. But, obviously, we also want NO swapping for PGA memory so a bit of tuning on virtual memory (VM) subsystem is needed.

More information on VM kernel parameters can be found by installing kernel-uek-doc-2.6.39-200.29.1.el6uek.noarch on Oracle Linux Server release 6.3:

[root@server1 ~]# cat /usr/share/doc/kernel-doc-2.6.39/Documentation/sysctl/vm.txt

There are a lot of available RedHat documentation on the subject, they more or less all suggest the same parameter values except for vm.swappiness. Keeping in mind that on an Oracle server I rate swapping really bad I think setting it to 0 is a good idea:

Kernel Parameter Description Default Value Recommended Value
vm.swappiness This control is used to define how aggressive the kernel will swap memory pages. Higher values will increase aggressiveness, lower values decrease the amount of swap. 60 0
vm.dirty_background_ratio Contains, as a percentage of total system memory, the number of pages at which the pdflush background writeback daemon will start writing out dirty data. 10 3
vm.dirty_ratio Contains, as a percentage of total system memory, the number of pages at which a process which is generating disk writes will itself start writing out dirty data. 40 15
vm.dirty_expire_centisecs This tunable is used to define when dirty data is old enough to be eligible for writeout by the pdflush daemons. It is expressed in 100′ths of a second. Data which has been dirty in-memory for longer than this interval will be written out next time a pdflush daemon wakes up. 3000 500
vm.dirty_writeback_centisecs The pdflush writeback daemons will periodically wake up and write `old’ data out to disk. This tunable expresses the interval between those wakeups, in 100′ths of a second. Setting this to zero disables periodic writeback altogether. 500 100

References

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