Transparent hugepages on RHEL8

[Dridi Boukelmoune]

On Sun, May 24, 2020 at 8:12 AM Geoff Simmons <geoff at uplex.de> wrote:
>
> On 5/24/20 01:29, info+varnish at shee.org wrote:
> > This notes
> >
> > https://varnish-cache.org/docs/trunk/installation/platformnotes.html
> >
> > has a comment about "Transparent hugepages".
> >
> > Does this still apply to EL8?
>
> That's a good heads-up that those docs need to be updated -- they refer
> to RHEL6 and Linux kernel 3.2. If I'm not mistaken, enabling THP by
> default was fairly new at the time, but it's still the default and
> that's old news now, as your settings confirmed (just checked that it's
> also the default on my Debian stretch laptop).
>
> The issue is not really the distro or kernel version, but the use of the
> THP feature, and it's still a problem, probably always will be. AFAICT
> THP does nothing good for Varnish. It's harmless if you're lucky, but it
> can be very disruptive.
>
> I haven't tried it with RHEL8. The doc says that it "is known to cause
> sporadic crashes of Varnish", but while I haven't seen crashes, on RHEL7
> I've seen that the memory usage of the cache process bloats up
> enormously, orders of magnitude larger than the actual size of the cache
> and anything else in Varnish that occupies memory. After disabling THP
> for Varnish (as detailed below), I saw memory usage become much smaller,
> more like what you'd expect from the cache size and other overhead.
>
> There's an explanation for why THP causes that, but suffice it to say
> that THP creates trouble for a variety of apps that manage a lot of
> memory. MongoDB, Oracle, redis and many other projects advise you to
> turn it off. THP is inevitably a problem for the jemalloc memory
> allocator, which is invariably used with Varnish.

I just wanted to react to the "invariably" word here. This is not
accurate, it should read "by default" instead.

See ./configure --help:

> --with-jemalloc use jemalloc memory allocator. Default is yes on Linux, no elsewhere

And considering that jemalloc is not available in el8, but epel8
instead. I suspect Red Hat ships a varnish package that uses glibc
with no custom allocator.

Cheers,
Dridi