Re: Plumbers 2018 - Performance and Scalability Microconference

[Daniel Jordan]

On 9/10/18 1:34 PM, John Hubbard wrote:
> On 9/10/18 10:20 AM, Davidlohr Bueso wrote:
> > On Mon, 10 Sep 2018, Waiman Long wrote:
> > > On 09/08/2018 12:13 AM, John Hubbard wrote:
> [...]
> > > > It's also interesting that there are two main huge page systems (THP and Hugetlbfs), and I sometimes
> > > > wonder the obvious thing to wonder: are these sufficiently different to warrant remaining separate,
> > > > long-term? Yes, I realize they're quite different in some ways, but still, one wonders. :)
> > >
> > > One major difference between hugetlbfs and THP is that the former has to
> > > be explicitly managed by the applications that use it whereas the latter
> > > is done automatically without the applications being aware that THP is
> > > being used at all. Performance wise, THP may or may not increase
> > > application performance depending on the exact memory access pattern,
> > > though the chance is usually higher that an application will benefit
> > > than suffer from it.
> > >
> > > If an application know what it is doing, using hughtblfs can boost
> > > performance more than it can ever achieved by THP. Many large enterprise
> > > applications, like Oracle DB, are using hugetlbfs and explicitly disable
> > > THP. So unless THP can improve its performance to a level that is
> > > comparable to hugetlbfs, I won't see the later going away.
> >
> > Yep, there are a few non-trivial workloads out there that flat out discourage
> > thp, ie: redis to avoid latency issues.
>
> Yes, the need for guaranteed, available-now huge pages in some cases is
> understood. That's not the quite same as saying that there have to be two different
> subsystems, though. Nor does it even necessarily imply that the pool has to be
> reserved in the same way as hugetlbfs does it...exactly.
>
> So I'm wondering if THP behavior can be made to mimic hugetlbfs enough (perhaps
> another option, in addition to "always, never, madvise") that we could just use
> THP in all cases. But the "transparent" could become a sliding scale that could
> go all the way down to "opaque" (hugetlbfs behavior).

Leaving the interface aside, the idea that we could deduplicate redundant parts of the hugetlbfs and THP implementations, without user-visible change, seems promising.