Re: Linux 3.19-rc3

[Catalin Marinas]

On Mon, Jan 12, 2015 at 01:57:48PM +0000, Arnd Bergmann wrote:
> On Monday 12 January 2015 12:18:15 Catalin Marinas wrote:
> > 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().
> 
> If I understand the way that LRU works right, we end up clearing
> the referenced bits in shrink_active_list()->page_referenced()->
> page_referenced_one()->ptep_clear_flush_young_notify()->pte_mkold()
> whenever there is memory pressure, so definitely not just for
> startup.

Yes. Actually, I think the start-up is probably not that bad. For pages
pointing to zero-page, you need to go through the kernel to allocate an
anonymous page and change the pte anyway. If the access was "write", the
page is marked dirty already (same with the access flag, the page is
marked young to avoid a subsequent fault).

So, I guess it's run-time cost of the LRU algorithm, especially under
memory pressure. Harder to benchmark though (we'll see when we get
hardware, though probably not very soon).

> > > > 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.
> 
> Not sure I'm following. Does the A57 core cache partial TLBs or not?

AFAIK, yes (I think A15 started doing this years ago). Architecturally,
an entry at any page table level could be cached in the TLB if it is
valid, irrespective of whether the full translation is valid.
Implementations, however, usually just cache the VA->pmd translation in
the intermediate TLB. Anyway, populating such cache still requires 3
accesses on a 4-level system.

> Even if not, I would expect the page tables to be hot in dcache most
> of the time, possibly with the exception of the last level on
> multi-threaded processes, but then you are back to the difference
> between the page size and the upper levels almost out of the equation.

As I said, I think it's more important with virtualisation where each
guest page table entry address needs to be translated from the guest
address space (IPA) to the physical address via the stage 2 tables.

BTW, arm64 can differentiate between a leaf TLB invalidation (just
changing the pte) and an all-levels one (which affects the walk cache
for a given VA). I have some hacked code to do this in Linux but I
haven't had time to assess the impact properly.

-- 
Catalin
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