[rhelv6-list] THP: Why are they on by default?

[Bryan J Smith]

Troels Arvin wrote:
> I've just had a situation with an RHEL 7 server where transparent
> hugepages (THP) might have contributed to a problem.

There are many cases of THP causing issues, this is why Red Hat used
to turn it off.  But with the memory sizes of average servers today,
sticking with 4KiB pages in the age of kernel 3+ is just not a
remotely viable default.

I.e., anything "certified" for RHEL7 _should_ have an installer, tuned
profile, etc... that _disables_ them included.  If not, that's on your
ISV, not Red Hat.  ;)

> I have also seen reports about THP ruining the performance on
> workstations.

Er, um, what?  Can you give a scenario where that is the case?

I would tend to think it is the opposite, with large mallocs only
_benefiting_ from THP.  I cannot think of a case where they would not.

I would think there would be general 32-bit malloc issues that have
nothing to do with THP that might come up from time-to-time.  I can
think of a lot of cases of 32-bit issues on x86-64, but so little that
are really impacted by THP.

> So which workloads actually benefit significantly from THP?

Things with large mallocs, which is very typical these days with all
sorts of x86-64 services.  And that's even before we look to
virtualization.  ;)

> Does someone know if Red Hat has considered turning them off
> by default?

Red Hat _used_ to do this, because of select ISVs.  But with RHEL7,
ISVs have long been warned that it was going to change, and they
needed to include installer changes, tools, etc... to address it.

ISVs that did not do such during the long, extensive, high-touch Alpha
and Beta periods really only have themselves to blame, especially if
the application is certified on RHEL7.

> (There doesn't seem to be a mailing list for RHEL 7, so I'm using this
> mailing list instead. Which should be OK, as I believe there isn't much
> difference WRT THP between RHEL 6 and 7.)

Red Hat has tried to concentrate everything into its Portal for far
more searchable reference, especially since Google tends to not always
give a "RHEL preferable answer" -- at least that has long been my
experience.

There are other, independent lists and sites, from reddit to even
CentOS (even if much of CentOS et al. is Red Hat hosted/sponsored
these days).  Otherwise, just create a login at the Portal and try
searching there.

-- bjs

-- 
Bryan J Smith - http://www.linkedin.com/in/bjsmith