Re: Linux 3.19-rc3

[Catalin Marinas <catalin.marinas@arm.com>]

On Sat, Jan 10, 2015 at 09:36:13PM +0000, Arnd Bergmann wrote:
> On Saturday 10 January 2015 13:00:27 Linus Torvalds wrote:
> > > IIRC, AIX works great with 64k pages, but only because of two
> > > reasons that don't apply on Linux:
> > 
> > .. there's a few other ones:
> > 
> >  (c) nobody really runs AIX on dekstops. It's very much a DB load
> > environment, with historically some HPC.
> > 
> >  (d) the powerpc TLB fill/buildup/teardown costs are horrible, so on
> > AIX the cost of lots of small pages is much higher too.
> 
> I think (d) applies to ARM as well, since it has no hardware
> dirty/referenced bit tracking and requires the OS to mark the
> pages as invalid/readonly until the first access. ARMv8.1
> has a fix for that, but it's optional and we haven't seen any
> implementations yet.

Do you happen have any data on how significantly non-hardware
dirty/access bits impact the performance? I think it may affect the user
process start-up time a but at run-time it shouldn't be that bad.

If it is that significant, we could optimise it further in the arch
code. For example, make a fast exception path where we need to mark the
pte dirty. This would be handled by arch code without even calling
handle_pte_fault().

> > so I feel pretty confident in saying it won't happen. It's just too
> > much of a bother, for little to no actual upside. It's likely a much
> > better approach to try to instead use THP for anonymous mappings.
> 
> arm64 already supports 2MB transparent hugepages. I guess it
> wouldn't be too hard to change it so that an existing hugepage
> on an anonymous mapping that gets split up into 4KB pages gets
> split along 64KB boundaries with the contiguous mapping bit set.
> 
> Having full support for multiple hugepage sizes (64KB, 2MB and 32MB
> in case of ARM64 with 4KB PAGE_SIZE) would be even better and
> probably negate any benefits of 64KB PAGE_SIZE, but requires more
> changes to common mm code.

As I replied to your other email, I don't think that's simple for the
transparent huge pages case.

The main advantage I see with 64KB pages is not the reduced TLB pressure
but the number of levels of page tables. Take the AMD Seattle board for
example, with 4KB pages you need 4 levels but 64KB allow only 2 levels
(42-bit VA). Larger TLBs and improved walk caches (caching VA -> pmd
entry translation rather than all the way to pte/PA) make things better
but you still have the warming up time for any fork/new process as they
don't share the same TLB entries.

But as Linus said already, the trade-off with the memory wastage
is highly dependent on the targeted load.

-- 
Catalin