On Mon, 4 Dec 2023 17:32:54 +0100 Dmytro Maluka <dmaluka@xxxxxxxxxxxx> wrote: > Add an option to disable transparent hugepages by default, in line with > the existing transparent_hugepage=never command line setting. > > Rationale: khugepaged has its own non-negligible memory cost even if it > is not used by any applications, since it bumps up vm.min_free_kbytes to > its own required minimum in set_recommended_min_free_kbytes(). For > example, on a machine with 4GB RAM, with 3 mm zones and pageblock_order > == MAX_ORDER, starting khugepaged causes vm.min_free_kbytes increase > from 8MB to 132MB. > > So if we use THP on machines with e.g. >=8GB of memory for better > performance, but avoid using it on lower-memory machines to avoid its > memory overhead, then for the same reason we also want to avoid even > starting khugepaged on those <8GB machines. So with > CONFIG_TRANSPARENT_HUGEPAGE_NEVER we can use the same kernel image on > both >=8GB and <8GB machines, with THP support enabled but khugepaged > not started by default. The userspace can then decide to enable THP > (i.e. start khugepaged) via sysfs if needed, based on the total amount > of memory. > > This could also be achieved with the existing transparent_hugepage=never > setting in the kernel command line instead. But it seems cleaner to > avoid tweaking the command line for such a basic setting. > > P.S. I see that CONFIG_TRANSPARENT_HUGEPAGE_NEVER was already proposed > in the past [1] but without an explanation of the purpose. > > ... > > --- a/mm/Kconfig > +++ b/mm/Kconfig > @@ -859,6 +859,12 @@ choice > madvise(MADV_HUGEPAGE) but it won't risk to increase the > memory footprint of applications without a guaranteed > benefit. > + > + config TRANSPARENT_HUGEPAGE_NEVER > + bool "never" > + help > + Disabling Transparent Hugepage by default. It can still be s/Disabling/Disable/ > + enabled at runtime via sysfs. > endchoice The patch adds the config option but doesn't use it?
