drawback of transparent_hugepage=always?

[Reindl Harald]

Google around brings no clear statement

is there any drawback of boot with "transparent_hugepage=always" which 
seems not to be the default currently - as far as i understand it should 
recude the overhead of memory management (especially in virtual guests) 
and seems to have been the default for some time

http://oracle-base.com/articles/linux/configuring-huge-pages-for-oracle-on-l... 
at least talks about "You can check the current setting using the 
following command, which is displaying the default value of 
"enabled=[always]"

https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/6/htm...
____________________________________________________

[root@testserver:~]$ cat /sys/kernel/mm/transparent_hugepage/enabled
always [madvise] never

[root@testserver:~]$ grep Huge /proc/meminfo
AnonHugePages:         0 kB
HugePages_Total:       0
HugePages_Free:        0
HugePages_Rsvd:        0
HugePages_Surp:        0
Hugepagesize:       2048 kB
____________________________________________________

[root@testserver:~]$ cat /sys/kernel/mm/transparent_hugepage/enabled
[always] madvise never

[root@testserver:~]$ grep Huge /proc/meminfo
AnonHugePages:    194560 kB
HugePages_Total:       0
HugePages_Free:        0
HugePages_Rsvd:        0
HugePages_Surp:        0
Hugepagesize:       2048 kB
____________________________________________________

[Josh Boyer]

On Thu, Dec 11, 2014 at 01:51:41AM +0100, Reindl Harald wrote:
> Google around brings no clear statement
>
> is there any drawback of boot with "transparent_hugepage=always" which seems
> not to be the default currently - as far as i understand it should recude
> the overhead of memory management (especially in virtual guests) and seems
> to have been the default for some time
>
> http://oracle-base.com/articles/linux/configuring-huge-pages-for-oracle-on-l...
> at least talks about "You can check the current setting using the following
> command, which is displaying the default value of "enabled=[always]"
>
> https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/6/htm...

We had this set to always for quite a while.  However, it isn't a win on
all workloads.  You can run into overhead in cases where you have a
smaller amount of RAM that is often allocated and freed frequently,
which fragments it quite a bit and causes a lot of hugepages to be
broken into regular pages, etc.  We discussed this a bit with the
upstream developers and they didn't expect big wins in a typical desktop
or laptop machine.  We switched to madvise by default so that
applications that can benefit still do, while the overhead for
everything else is reduced.

For static workloads like databases, or virt hosts, or machines with
larger amounts of RAM it might make sense to have it as always.

josh

[Reindl Harald]

Am 11.12.2014 um 15:04 schrieb Josh Boyer:
> On Thu, Dec 11, 2014 at 01:51:41AM +0100, Reindl Harald wrote:
> > Google around brings no clear statement
> >
> > is there any drawback of boot with "transparent_hugepage=always" which seems
> > not to be the default currently - as far as i understand it should recude
> > the overhead of memory management (especially in virtual guests) and seems
> > to have been the default for some time
> >
> > http://oracle-base.com/articles/linux/configuring-huge-pages-for-oracle-on-l...
> > at least talks about "You can check the current setting using the following
> > command, which is displaying the default value of "enabled=[always]"
> >
> > https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/6/htm...
>
> We had this set to always for quite a while.  However, it isn't a win on
> all workloads.  You can run into overhead in cases where you have a
> smaller amount of RAM that is often allocated and freed frequently,
> which fragments it quite a bit and causes a lot of hugepages to be
> broken into regular pages, etc.  We discussed this a bit with the
> upstream developers and they didn't expect big wins in a typical desktop
> or laptop machine.  We switched to madvise by default so that
> applications that can benefit still do, while the overhead for
> everything else is reduced.
>
> For static workloads like databases, or virt hosts, or machines with
> larger amounts of RAM it might make sense to have it as always

thank you for feedback confirming my considerations i liked to be backed 
by people knowing more of the internals!

i am thinking about our big virtual servers with 8-16 GB RAM per VM and 
a constant workload (dbmail with large innodb and such)

maybe the following links are helpful for others too

http://www.vmware.com/files/pdf/large_pg_performance.pdf

http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd...