Re: [RFC PATCH v2 00/30] 1GB PUD THP support on x86_64

[Zi Yan]

On 5 Oct 2020, at 11:55, Matthew Wilcox wrote:

> On Mon, Oct 05, 2020 at 11:03:56AM -0400, Zi Yan wrote:
>> On 2 Oct 2020, at 4:30, David Hildenbrand wrote:
>>> Yes, I think one important feature would be that we don't end up placing
>>> a gigantic page where only a handful of pages are actually populated
>>> without green light from the application - because that's what some user
>>> space applications care about (not consuming more memory than intended.
>>> IIUC, this is also what this patch set does). I'm fine with placing
>>> gigantic pages if it really just "defragments" the address space layout,
>>> without filling unpopulated holes.
>>>
>>> Then, this would be mostly invisible to user space, and we really
>>> wouldn't have to care about any configuration.
>>
>> I agree that the interface should be as simple as no configuration to
>> most users. But I also wonder why we have hugetlbfs to allow users to
>> specify different kinds of page sizes, which seems against the discussion
>> above. Are we assuming advanced users should always use hugetlbfs instead
>> of THPs?
>
> Evolution doesn't always produce the best outcomes ;-)
>
> A perennial mistake we've made is "Oh, this is a strange & new & weird
> feature that most applications will never care about, let's put it in
> hugetlbfs where nobody will notice and we don't have to think about it
> in the core VM"
>
> And then what was initially strange & new & weird gradually becomes
> something that most applications just want to have happen automatically,
> and telling them all to go use hugetlbfs becomes untenable, so we move
> the feature into the core VM.
>
> It is absurd that my phone is attempting to manage a million 4kB pages.
> I think even trying to manage a quarter-million 16kB pages is too much
> work, and really it would be happier managing 65,000 64kB pages.
>
> Extend that into the future a decade or two, and we'll be expecting
> that it manages memory in megabyte sized units and uses PMD and PUD
> mappings by default.  PTE mappings will still be used, but very much
> on a "Oh you have a tiny file, OK, we'll fragment a megabyte page into
> smaller pages to not waste too much memory when mapping it" basis.  So,
> yeah, PUD sized mappings have problems today, but we should be writing
> software now so a Pixel 15 in a decade can boot a kernel built five
> years from now and have PUD mappings Just Work without requiring the
> future userspace programmer to "use hugetlbfs".

I agree.

>
> One of the longer-term todo items is to support variable sized THPs for
> anonymous memory, just like I've done for the pagecache.  With that in
> place, I think scaling up from PMD sized pages to PUD sized pages starts
> to look more natural.  Itanium and PA-RISC (two architectures that will
> never be found in phones...) support 1MB, 4MB, 16MB, 64MB and upwards.
> The RiscV spec you pointed me at the other day confines itself to adding
> support for 16, 64 & 256kB today, but does note that 8MB, 32MB and 128MB
> sizes would be possible additions in the future.

Just to understand the todo items clearly. With your pagecache patchset,
kernel should be able to understand variable sized THPs no matter they
are anonymous or not, right? For anonymous memory, we need kernel policies
to decide what THP sizes to use at allocation, what to do when under
memory pressure, and so on. In terms of implementation, THP split function
needs to support from any order to any lower order. Anything I am missing here?

>
> But, back to today, what to do with this patchset?  Even on my 16GB
> laptop, let alone my 4GB phone, I'm uncertain that allocating a 1GB
> page is ever the right decision to make.  But my laptop runs a "mixed"
> workload, and if you could convince me that Firefox would run 10% faster
> by using a 1GB page as its in-memory cache, well, I'd be sold.
>
> I do like having the kernel figure out what's in the best interests of the
> system as a whole.  Apps don't have enough information, and while they
> can provide hints, they're often wrong.  So, let's say an app maps 8GB
> of anonymous memory.  As the app accesses it, we should probably start
> by allocating 4kB pages to back that memory.  As time goes on and that
> memory continues to be accessed and more memory is accessed, it makes
> sense to keep track of that, replacing the existing 4kB pages with, say,
> 16-64kB pages and allocating newly accessed memory with larger pages.
> Eventually that should grow to 2MB allocations and PMD mappings.
> And then continue on, all the way to 1GB pages.
>
> We also need to be able to figure out that it's not being effective
> any more.  One of the issues with tracing accessed/dirty at the 1GB level
> is that writing an entire 1GB page is going to take 0.25 seconds on a x4
> gen3 PCIe link.  I know swapping sucks, but that's extreme.  So to use
> 1GB pages effectively today, we need to fragment them before choosing to
> swap them out (*)  Maybe that's the point where we can start to say "OK,
> this sized mapping might not be effective any more".  On the other hand,
> that might not work for some situations.  Imagine, eg, a matrix multiply
> (everybody's favourite worst-case scenario).  C = A * B where each of A,
> B and C is too large to fit in DRAM.  There are going to be points of the
> calculation where each element of A is going to be walked sequentially,
> and so it'd be nice to use larger PTEs to map it, but then we need to
> destroy that almost immediately to allow other things to use the memory.
>
>
> I think I'm leaning towards not merging this patchset yet.  I'm in
> agreement with the goals (allowing systems to use PUD-sized pages
> automatically), but I think we need to improve the infrastructure to
> make it work well automatically.  Does that make sense?

I agree that this patchset should not be merged in the current form.
I think PUD THP support is a part of variable sized THP support, but
current form of the patchset does not have the “variable sized THP”
spirit yet and is more like a special PUD case support. I guess some
changes to existing THP code to make PUD THP less a special case would
make the whole patchset more acceptable?

Can you elaborate more on the infrastructure part? Thanks.

>
> (*) It would be nice if hardware provided a way to track D/A on a sub-PTE
> level when using PMD/PUD sized mappings.  I don't know of any that does
> that today.

I agree it would be a nice hardware feature, but it also has a high cost.
Each TLB would support this with 1024 bits, which is about 16 TLB entry size,
assuming each entry takes 8B space. Now it becomes why not having a bigger
TLB. ;)



—
Best Regards,
Yan Zi

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